Cambridge Checkpoint Lower Secondary Science TextBook 7 [3 ed.] 9781398300187, 9781398302129

Table of contents :
Cover
Title Page
Copyright
Contents
How to use this book
Introducing science
Biology
Chapter 1 The characteristics of living things
Chapter 2 Identifying species
Chapter 3 Cells
Chapter 4 Microorganisms
Chemistry
Chapter 5 The states of matter
Chapter 6 Atoms and elements
Chapter 7 Elements, compounds and mixtures
Chapter 8 Physical properties of matter
Chapter 9 Chemical reactions
Chapter 10 Acids and alkalis
Physics
Chapter 11 Measurement
Chapter 12 Energy
Chapter 13 Sound
Chapter 14 Electricity
Earth and space
Chapter 15 The Earth in space
Chapter 16 A closer look at the Earth
Glossary
Acknowledgements

Citation preview

To Anita, Sally-Anne and Emma-Louise Cambridge International copyright material in this publication is reproduced under licence and remains the intellectual property of Cambridge Assessment International Education. The Boost knowledge tests and answers have been written by the authors. These may not fully reflect the approach of Cambridge Assessment International Education. Every effort has been made to trace all copyright holders, but if any have been inadvertently overlooked, the Publishers will be pleased to make the necessary arrangements at the first opportunity. Although every effort has been made to ensure that website addresses are correct at time of going to press, Hodder Education cannot be held responsible for the content of any website mentioned in this book. It is sometimes possible to find a relocated web page by typing in the address of the home page for a website in the URL window of your browser. Third-party websites and resources referred to in this publication have not been endorsed by Cambridge Assessment International Education. Hachette UK’s policy is to use papers that are natural, renewable and recyclable products made from wood grown in well-managed forests and other controlled sources. The logging and manufacturing processes are expected to conform to the environmental regulations of the country of origin. Orders: please contact Hachette UK Distribution, Hely Hutchinson Centre, Milton Road, Didcot, Oxfordshire, OX11 7HH. Telephone: +44 (0)1235 827827. Email [email protected] Lines are open from 9 a.m. to 5 p.m., Monday to Friday. You can also order through our website: www.hoddereducation.com ISBN: 978 1 3983 0018 7 eISBN: 978 1 3983 0212 9 © Peter D Riley Ltd 2021 First published in 2005 Second edition published in 2011

This edition published in 2021 by Hodder Education, An Hachette UK Company Carmelite House 50 Victoria Embankment London EC4Y 0DZ www.hoddereducation.com Impression number 10 9 8 7 6 5 4 3 2 1 Year 2025 2024 2023 2022 2021 All rights reserved. Apart from any use permitted under UK copyright law, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and recording, or held within any information storage and retrieval system, without permission in writing from the publisher or under licence from the Copyright Licensing Agency Limited. Further details of such licences (for reprographic reproduction) may be obtained from the Copyright Licensing Agency Limited, www.cla.co.uk Cover photo © Olympixel - stock.adobe.com Illustrations by Barking Dog, Pantek Arts Ltd and Integra Software Services Ltd Typeset by Integra Software Services Pvt. Ltd, Pondicherry, India and DC Graphic Design Printed in Italy A catalogue record for this title is available from the British Library.

Contents How to use this book Introducing science

Biology Chapter 1 The characteristics of living things Chapter 2 Identifying species Chapter 3 Cells Chapter 4 Microorganisms

Chemistry Chapter 5 The states of matter Chapter 6 Atoms and elements Chapter 7 Elements, compounds and mixtures Chapter 8 Physical properties of matter Chapter 9 Chemical reactions Chapter 10 Acids and alkalis

Physics Chapter 11 Measurement Chapter 12 Energy Chapter 13 Sound Chapter 14 Electricity

Earth and space Chapter 15 The Earth in space Chapter 16 A closer look at the Earth

Glossary Acknowledgements

How to use this book To make your study of Cambridge Checkpoint Science as rewarding as possible, look out for the following features when you are using this book: • These aims show you what you will be covering in the chapter.

Do you remember? This will show you the ideas you have learnt before. Think about what you already know before you begin.

Science activity Science activities may be about developing a science skill or making a science enquiry.

Science in context In this box you will find information about how scientists working alone or together have built up our understanding of the world over time, how science is applied in our lives, the issues it can raise and how its use can affect our global environment. Science extra The information in these boxes and any other boxes which have the ‘Science extra’ heading is extra to your course, but you may find

these topics interesting and they may help you with your understanding of the overall chapter topic.

DID YOU KNOW? This is a fact or piece of information that may make you think more deeply about the topic, or that you may share as a fun fact with your family and friends.

Summary This box will show you how much you have learnt by the end of the chapter. This book contains lots of activities to help you learn. Some of the questions will have symbols beside them to help you answer them. Look out for these symbols:

This blue dot shows you that you have already learnt some information to help you with this topic.

If a question has a purple link symbol beside it, you will have to use your skills from another subject.

This star shows where your thinking and working scientifically enquiry skills are being used.

Scientists use models in science to help them understand new ideas. This icon shows you where you are using models to help you

with your ideas in science.

This green dot shows you that you are considering a scientific issue in a context which requires your understanding of some of the science facts you have learnt.

This icon tells you that content is available as audio. All audio is available to download for free from www.hoddereducation.com/cambridgeextras

There is a link to digital content at the end of each unit if you are using the Boost eBook.

CHALLENGE YOURSELF These activities are a challenge! You may have to think a bit harder to get the correct answer.

LET’S TALK When you see this box, talk with a partner or in a small group to decide on your answer.

Work safely This triangle provides you with extra guidance on working safely. Words that look like this are glossary terms, and you will find definitions for them in the glossary at the back of this book. Other key terms that may

not be included in the glossary look like this.

Introducing science Science and scientists

Figure 1 Scientists investigate everything from distant galaxies of stars and the materials here on Earth, to the living things around us.

What do you think of when you think of science? Stars and planets? Explosions? Fizzing liquids? Buzzing insects and deep-sea creatures? Science is all of these things and more. It is not only a topic but a way of studying. Science is such a huge subject to study that it is divided into four sections: • biology – the study of living things • chemistry – the study of the substances from which things are made (known as matter) • physics – the study of how matter and energy interact • Earth and space – the study of the structure and changes in the Earth and the universe. What kind of scientist would you like to be? Here are just a few examples to think about. If you choose to study biology, you might become a botanist (someone who studies plants) or a marine zoologist (someone who studies sea animals).

Figure 2 Marine zoologists often dive with the animals they are studying. They study animals in all the oceans of the world.

If you choose to study chemistry you might become a geochemist (someone who studies rocks to find minerals and oil) or a chemical engineer (someone who makes new substances, or designs huge pieces of equipment – called chemical plants, where vast amounts of chemicals are made).

Figure 3 These chemical engineers are making an investigation.

If you choose to study physics you might become a nuclear physicist, finding out more about how matter is made.

Figure 4 Some physicists use huge equipment like the Large Hadron Collider to find out about the structure of matter.

If you choose to study the Earth and space you might become a geologist, or you might become an astronomer and study the universe.

Figure 5 Astronomers use telescopes linked to computers to make their observations.

DID YOU KNOW? A vulcanologist is a scientist who studies volcanoes.

CHALLENGE YOURSELF

Use the internet to make a list of as many different types of scientist as you can. For each one, write down the name and one sentence describing what the scientist does.

LET’S TALK What kind of scientist would you like to be? Discuss your ideas with your friends and family.

Scientific equipment Scientific equipment is called apparatus. There are many kinds of apparatus; the following are just a few examples. You will meet more as you carry out more investigations in this science course.

General laboratory apparatus Many pieces of apparatus are made of glass because it is transparent. This makes it easy to see what is happening inside. Glass is also easy to clean. The pieces of apparatus that are to be heated are made from borosilicate glass, also known as Pyrex. This is the same as the glass used for casserole dishes that can be safely put in an oven to cook a meal. Figure 6 below shows the diagrams used to represent some common pieces of apparatus, alongside photographs of the apparatus in use.

Figure 6 Some common laboratory apparatus.

Apparatus for supplying heat When scientists want to use heat in an investigation they use a Bunsen burner, or if the laboratory does not have a gas supply they use a spirit burner.

DID YOU KNOW? If something is luminous, it means it gives out light. If something is non-luminous, it means it does not give out light.

1 Draw the apparatus set up as shown in the bottom left photograph in Figure 6 on page viii.

2 Draw the apparatus set up as shown in the bottom right photograph in Figure 6 on page viii.

The Bunsen burner The gas used by many Bunsen burners is methane. As methane burns, it takes part in a chemical reaction with oxygen in the air, and carbon dioxide and water are produced. Both these substances escape into the air as gases. The parts of a Bunsen burner are shown in Figure 7. It is lit and used in the following way: 1 The air regulator (or collar) must be turned to fully close the air hole before the burner is lit. 2 The match should be lit and placed to one side of the top of the chimney before the gas tap is switched on. 3 When the gas tap is switched on, the gas shoots out through the small hole, called the jet, along the rubber tubing and up the chimney of the burner. Not all the carbon in the gas combines with the oxygen straight away and carbon particles are produced which glow brightly in the heat and make the flame yellow. If the flame is used to heat anything, the carbon particles form soot on the surface of the apparatus being heated. The flame is called a luminous flame. It is silent. The carbon in the flame reacts with oxygen in the air and forms carbon dioxide. 4 If the collar is now turned and the air hole is fully opened, air mixes with the gas in the chimney. The gases rush up and form a blue cone of unburnt gas at the top of the chimney. Above the cone, methane completely burns away. The flame made when the air hole is completely open is non-luminous and makes a roaring sound. 5 The size of the flame is controlled by the gas tap on the bench. If the tap is fully open, a large flame is produced. A smaller flame is produced by

partially closing the gas tap.

Figure 7 The Bunsen burner.

Less heat energy is released by the luminous flame than the non-luminous flame, because not all of the carbon reacts with oxygen at once. The hottest part of the non-luminous flame is a few millimetres above the tip of the blue cone of unburnt gas. 3 Why is one type of flame hotter than the other? 4 Why does closing the gas tap a little reduce the size of the flame?

5 What safety precautions should you take when using a Bunsen burner? Explain the reason for each precaution.

DID YOU KNOW? Scientific enquiries begin by asking a question. They begin with words such as ‘how’, ‘when’, ‘what’, ‘where’, ‘why’ and ‘can’. Look for them at the beginning of science activities and some Challenge yourself items in this book.

How do you operate a Bunsen burner? You will need: eye protection, a Bunsen burner, a heat-proof mat, a gas supply with a tap, a flame source such as a match, burning splint or lighter.

Process 1 Push the end of the rubber tube onto the gas tap and check with your teacher that it is secure. 2 Place the base of the Bunsen burner on the heat-proof mat. 3 Turn the collar on the Bunsen burner to fully close the air hole. 4 Place the flame source to one side of the top of the chimney and switch on the gas tap. 5 Observe the flame produced. It should be luminous, yellow and silent. 6 Turn the collar so that the air hole is fully open. 7 Observe the flame. There should be a blue cone of unburnt gas at the top of the chimney and the flame should be non-luminous and making a roaring sound. 8 Ask your teacher to observe you controlling the size of the flame by slowly and carefully half-closing the gas tap and then opening it again.

Be careful Eye protection should be worn.

If carrying a burning splint from a Bunsen burner that is already lit, take great care not to bump into anyone. If you have long hair, tie it back to keep it away from the Bunsen burner. 6 Describe how you felt as you operated the Bunsen burner. Which part did you feel was most difficult? Which parts were you most comfortable doing?

The spirit burner In laboratories without a gas supply, spirit burners can be used. The main components of a spirit burner are the reservoir for the fuel and the wick into which the fuel soaks. The fuel is burnt at the top of the wick and is replaced by more fuel from the reservoir until it runs dry. A reservoir may hold enough fuel for the burner to be lit for an hour. Some spirit burners have an adjustable wick to control the rate at which the fuel burns.

Figure 8 The spirit burner.

DID YOU KNOW? One of the most important tasks for a scientist in an enquiry is measuring. You can learn more about it in Chapter 11 (page 119) and try some measuring activities.

Safety in the science laboratory

Scientific activities often take place in the laboratory. This is also the room in which apparatus is stored for making scientific investigations. School laboratories are busy places. There may be about 30 people doing investigations in a laboratory at the same time. They may be using gas, water, electricity, a wide range of equipment and some hazardous chemicals.

Laboratory rules Despite the large amount of activity, there are fewer accidents in laboratories than in most other parts of a school. The reason for this is that when people work in laboratories, they generally take great care to follow the advice of the teacher and the rules pinned to the laboratory wall.

Figure 9 It’s very important to obey the rules for working in the laboratory, to avoid accidents.

Laboratory rules can be set out in many ways but should cover the same good advice. The example below shows rules for working in the laboratory.

Entering and leaving the laboratory • Do not run into or out of the laboratory. • Make sure that school bags are stored safely.

• Put stools under the bench when not in use. • Leave the benchtop clean and dry.

General behaviour • Do not run in the laboratory. • Do not eat or drink in the laboratory. • Work quietly.

LET’S TALK What are the reasons for each of the laboratory rules? What other rules could you add?

Preparing to do practical work • Tie back long hair and wear lab coats if available. Lab coats should be buttoned up. • Wear safety eye wear when anything is to be heated, or if any hazardous chemicals are to be used.

During experiments • Never point a test tube containing chemicals at anyone, and do not examine the contents by looking down the tube. • Tell your teacher about any breakages or spillages at once. If you are at all unsure of the practical work, check with your teacher that you are following the correct procedure. • Only carry out investigations approved by your teacher, and use the gas, water and electricity supplies sensibly.

LET’S TALK When you are doing practical work, how many of the rules do you follow?

Hazard signs

Some of the substances used in scientific investigations are dangerous if not handled properly. The containers of these substances are labelled with hazard symbols such as those shown in Figure 10 on the next page.

Figure 10 Hazard symbols.

7 What do you understand by the each of the following words? a corrosive b hazardous to the environment c flammable d health hazards e acutely toxic f moderate hazard

Scientific models and representations When you think of the word ‘model’ you probably think about an object that is smaller than the real thing, such as a model car or a model aeroplane. These types of models have three dimensions (3-D) – they have length, width and height.

Figure 11 A three-dimensional model.

LET’S TALK Do you know how the Earth goes around the Sun and how the Moon goes around the Earth? If you do, as a group, make a model of this using a large fruit, a medium- sized fruit and a small fruit, and demonstrate the movements. Let other people look at your model and ask them if they understand what the model shows. In science, a model means something more. A model is used to make something easier to understand. Some scientific models are also in three dimensions like the car and aeroplane. You may have used a 3-D model in science already; for example, to explain and understand how the Earth goes around the Sun and how the Moon goes around the Earth. Living things can be used as 3-D models too. The fruit fly has a short life cycle and many generations can be produced in a laboratory over a year with special care. This means that the way features pass from one generation to another can be studied more quickly than in other animals, especially humans. This can help us understand how features are passed down through generations of humans.

Figure 12 Colonies of fruit flies in a laboratory.

You may also have used living things as models if you have ever set up an aquarium tank and stocked it with plants and animals from a pond, for example In this case, the aquarium tank becomes a living model habitat which is easier to observe than a pond.

Figure 13 These aquariums are models of ponds.

8 Why is it easier to observe an aquarium tank habitat rather than a real pond?

9 Is the aquarium tank habitat just like the real habitat? Explain your answer. Most scientific models, however, are in two dimensions (2-D). These are visual models that are made on paper or computer screens. You have used them in your earlier science courses – they are drawings, diagrams and photographs, charts and graphs. Another visual model that you will meet in this science course is the chemical equation, an example of which is shown below:

All scientific models represent observations that scientists have made. The models are used to communicate ideas about the real thing that has been observed. This ‘real thing’ may be an object such as a tadpole, a phenomenon such as lightning or physical processes such as teeth chewing food.

Figure 14 A tadpole, lightning and someone chewing a mouthful of food.

10 Assess the strengths and weaknesses of the diagrams of laboratory apparatus in Figure 6 (on page viii) and write a short

report. In this science course you will meet many scientific models and will be invited to make them yourself. Sometimes you may be asked to make an accurate visual representation, such as a drawing, or you may be asked to describe the structure of something with a diagram. Each type of model has strengths and weaknesses, and you should evaluate these models to decide what their strengths and limitations are.

DID YOU KNOW? A hypothesis is an idea a scientist has to explain what has been observed. An untestable hypothesis is one that is not specific, such as saying ‘it could be this or that’. It could also use the word ‘better’, which is a matter of opinion and cannot be tested. Or it may feature the influence of something unnatural, like the effect of flying dragons. For example, How familiar will dragons be worldwide?

Science enquiry All the science facts we know today have been discovered by making scientific enquiries. In a scientific enquiry, scientists think and work scientifically. There are three basic stages in a scientific enquiry. They are purpose and planning, carrying out a scientific enquiry and analysis, evaluation and conclusions. In each of these stages there are activities that can be described as scientific activities. They show how scientists think and work scientifically, and they are given in the lists that follow.

Stage 1 Setting up an enquiry • Look at a hypothesis and think about whether it can be tested. • Consider ways in which the evidence gathered in an enquiry can support or contradict the hypothesis. • Use scientific knowledge and understanding to make a prediction about what might happen in an enquiry. • Make sure that the meanings of all hazard symbols are known and take them into account when planning a practical investigation or experiment.

• When planning an enquiry, select from a range of types of investigations, remembering that they do not all involve fair tests, and also think about the variables you need to consider as you plan.

Figure 15 Discussing ideas for an investigation.

Variables in a fair test There are three kinds of variables to consider in a fair test. They are: • the independent variable. This is the variable that is changed by the scientist in the enquiry. • the dependent variable. This is the variable that changes as a result of changing the independent variable. • the control variables. These are the variables that are kept the same by the scientist during the investigation. A variable that does not change in the investigation could also be called a constant variable.

DID YOU KNOW? There is a fourth type of variable. It is called an extraneous variable and is not considered important in an investigation. Two examples of an extraneous variable are the colour of the laboratory

walls and the direction of the wind outside the laboratory when the investigation is taking place inside.

Stage 2 Carrying out an enquiry and recording data • Use your classification skills in testing and observing organisms, materials, objects and phenomena such as biological, chemical and physical processes, and also use these skills for using and making keys. • Select equipment for your enquiry and use each piece properly and safely. • Decide how many observations and measurements need to be taken or repeated to provide reliable data. • Use all measuring instruments to provide accurate and precise measurements, explaining why this is necessary. • Work safely at all times. • Use information from a range of secondary sources, such as books and the internet, but evaluate them with care to make sure they are relevant to your enquiry and are not providing biased information. • Collect enough observations and measurements to make your data reliable and record them in a form that will be easy to examine.

Figure 16 Planning an investigation.

Figure 17 Collecting evidence and taking measurements.

DID YOU KNOW? In science, accuracy means how close a measurement is to the true value of something. If you have a 10 g mass and you record 4.5 g when you weigh it, you are not accurate, but if you record 9.9 g you are. Precision means how close a number of measurements are to each other. If you reweigh the mass four times and get 7.6 g, 7.3 g, 7.9 g and 7.5 g, the numbers are close together, so you are precise but you are not accurate. You should aim to be accurate and precise by recording masses such as 9.9 g, 10 g and 10.1 g.

Stage 3 Examining the results and drawing a conclusion • Compare your predictions with the results of your enquiries and assess their accuracy. • Look through the results to see if they show patterns or a trends, and note them. Look for results (known as anomalous results) which do not fit in with any pattern or trend and note them too.

• Look through the results, interpret what you see and draw a conclusion, but explain how the conclusion could be limited and may not provide a full answer to the enquiry. • Review the investigations and experiments you have made in the enquiry and state how they may be improved. Explain why the changes you may have identified will provide more reliable data for the enquiry. • Communicate your observations and measurements by presenting them clearly and providing a clear interpretation of what you think they show.

Figure 18 Analysing evidence and drawing conclusions.

Science in context Using science Science provides us with an ever-increasing knowledge about our world and the universe. Mathematics is widely used in science and forms part of our scientific knowledge. Engineers use this knowledge in the engineering process to make pieces of technology – from cell phones to satellites – to improve our lives. Science, technology, engineering and mathematics are sometimes brought together under one heading: ‘STEM’. We use increasing numbers of STEM activities to make our homes and cities better places to live in, to improve provision of food and water and to devise ways to keep our environment suitable for life on our planet.

Figure 19 A wind turbine.

The discovery of how wind can turn turbine blades has been linked with how spinning magnets can make electricity, resulting in a piece of technology that can generate electricity from the wind.

Summary • Science is divided into four areas of study. • Items of scientific apparatus are used in investigations. • The Bunsen burner and the spirit burner are pieces of apparatus that supply heat for investigations. • Rules need to be followed for investigation work in the laboratory to be safe. • Hazard signs are used to give information about the danger of using some materials. • Scientists use a range of scientific models and representations. • Science enquiry is divided into three stages.

End of chapter questions 1 You are given a gas supply, a Bunsen burner (or spirit burner), a tripod, gauze, a heat-proof mat, a beaker, a measuring cylinder, a jug of water, a thermometer and a timer. Make a plan to show how you would use these apparatus to find out how fast a Bunsen burner or spirit burner can heat up 100 cm3 to 70 °C with: a a luminous flame b a non-luminous flame. Make a prediction of what might happen. Show your plan to your teacher and, if your teacher approves, try it.

2 Do you remember studying flowers in an earlier science course? If you do, you may have examined the structure of flower as shown in this diagram:

You may use this diagram to help you make a model of a flower using materials of your choice.

Or, you may select a flower from around your school or home and make a model of it using the diagram to help you.

3 How could you measure time with falling water? Invent a piece of technology and make a drawing of it and the materials you need. Show your ideas to your teacher and, if approved, become an engineer and invent your piece of technology.

1 The characteristics of living things In this chapter you will learn: • to identify the seven characteristics of living organisms • the seven characteristics of living things; nutrition, respiration, movement, growth, excretion, reproduction and sensitivity • how many observations and measurements need to be taken or repeated to provide reliable data during an experiment.

Do you remember? • What living things have you already studied in science? Make a list. • How did you know that they were alive? Biology is the study of living things. In this chapter, we are going to look at the features or characteristics that something must have for us to identify it as a living thing.

Living and never lived You can make two groups of things – living things and things that have never lived. The klipspringers in Figure 1.1 below are living things, but the rock they are standing on has never lived.

1 How is a living thing different from something that has never lived?

LET’S TALK If you grouped things into ‘living things’ and ‘things that have never lived’, where would you place a block of wood? A car may have five of the characteristics of life. What are they and how does the car show them?

Figure 1.1 Klipspringers live in parts of the African savannah. They spend the hottest part of the day resting among rocks.

Characteristics of life If something is called a living thing, it must have seven special features. These are called the characteristics of life. These are:

• • • • • • •

movement respiration sensitivity growth reproduction excretion nutrition.

These characteristics are also known as life processes. Non-living objects may have some of these characteristics, but no non-living thing will have all seven of these signs of life.

CHALLENGE YOURSELF The seven characteristics can be represented by the letter sequence ‘MRSGREN’, which you can think of as a person – ‘Mrs Gren’. Remember the letters, close the book and see how many of the characteristics you can quickly name using the MRSGREN sequence.

Figure 1.2 Four of the characteristics of life.

2 Which characteristics of life are shown by the mice in the pictures A–D in Figure 1.2? 3 Does each of the following have any characteristics of life? Explain your answers. a An aeroplane b A computer c A brick.

Looking at characteristics of life Animal life All animals have the same seven characteristics of life but they may show them in different ways. For example, all animals grow, but some have a skeleton on the outside of the body and can grow only when they shed the old skeleton and stretch a new soft skeleton beneath before it sets. Insects and spiders do this by taking in air. Crabs and lobsters stretch their new skeletons by taking in water. Animals with skeletons inside their bodies simply grow larger without having to shed their skeletons.

Figure 1.3 This desert locust is shedding its last skeleton. Here the wings are rolled together, forming an arch on the locust’s back.

Nutrition

All living things need food. Plants make their own food but animals must get it from other living things. Some animals, like humans, eat a wide range of foods, while others eat only a small range of foods. In the rainforest, ticks, lice, leeches and female mosquitoes feed on just one food – blood. They have mouths that can break through skin and suck up their meal. Every animal has a mouth that is specially developed or adapted for that particular animal to feed in a specific way.

Respiration All living things respire, and most of them use oxygen for this. Many animals living on land have lungs, in which they take oxygen from the air. Many aquatic animals have gills, which take up oxygen dissolved in the water.

4 How many different kinds of foods do you eat? 5 How is the mouth of a crocodile adapted for feeding?

Figure 1.4 These leeches are being used to draw out blood as part of a medical operation.

DID YOU KNOW? Insects have holes in the sides of their bodies to let air in. The air goes down tubes to all the parts inside the body.

Figure 1.5 This axolotl lives in Lake Xochimilco in Mexico. Its gills are on the outside of its body, behind its head.

Respiration is the process by which energy is released from food. The released energy is used for life processes such as growth and movement. Respiration takes place inside the cells of living organisms. Respiration is a process in which many chemical reactions take place. During this process, a food called glucose is broken down by some chemical reactions. These reactions release stored energy, and oxygen takes part in other chemical reactions which produce carbon dioxide and water. Respiration should not be confused with breathing, which is the process of moving air in and out of the body.

Movement Let your right arm hang down by your chair. Stick the fingers of your left hand into the skin in the upper part of your right arm (above the forearm). Raise your right forearm and you should feel the flesh in the upper arm become harder. This is muscle, and it is working to move your forearm upwards. Muscles provide movement for nearly all animals. Animals move

to find food, avoid enemies and find shelter. Even when an animal is sitting or standing still, muscles are at work.

DID YOU KNOW? You use 17 face muscles to make a smile and 40 face muscles to make a frown.

How do muscles change your face? You will need: a mirror.

Hypothesis Muscles move the skin on your face. Is this hypothesis testable? If you cannot decide, look back at the Did you know? on page xiv.

Prediction Muscles will change under the skin as you smile and frown.

Process 1 Look in a mirror and place your fingers on your cheeks. 2 Smile and use your fingers to feel if there is any change in the muscles. 3 Frown and use your fingers to feel if there is any change in the muscles. 4 Repeat steps 2 and 3 twice more.

Examining the results How did the cheek muscles feel when you smiled and frowned?

Conclusion

Does your conclusion match the hypothesis and prediction?

Challenge yourself How could you find out scientifically if the muscles in your forehead moved when you smiled and frowned? If your teacher approves your plan, try the investigation.

Sensitivity Animals detect or sense changes in their surroundings through their sensory system, which is made up of sense organs. These are the skin, eyes, ears, tongue and nose. Some animals such as insects and centipedes have long, thin structures on their heads, called antennae, which they use to touch the ground in front of them. The information their brains receive helps them decide if it is safe to move forwards. Like many animals, we use our eyes and ears to tell us a great deal about our surroundings. We use our tongue and nose to provide us with information about food. If it smells and tastes pleasant it may be suitable to eat, but if it smells and tastes bad it could contain poisons. The snake shown in Figure 1.6 appears to be tasting the air when it sticks out its tongue, but it is really collecting chemicals from the air, such as scents. It draws its tongue back into its mouth and pushes the tip into a pit in its nose where the chemicals are detected.

Figure 1.6 This grass snake is collecting chemicals in the air with its forked tongue. Grass snakes live in Europe and north-west Africa.

Growth and reproduction Living things need food for energy to keep the body alive, for materials for growth, and to repair parts of the body that have been damaged. Young animals, like the baby elephants in Figure 1.7, need food in order to grow healthily.

Figure 1.7 Elephants live in large family groups called herds, ruled by an elderly female called a matriarch. African elephants, like these, have large ears, while Asian or Asiatic elephants have smaller ears.

Once the elephants are fully grown, they need food to keep themselves in good health and to produce offspring. If the elephants did not produce offspring, the herd would eventually disappear as the old elephants died. Reproduction is the process that keeps a plant or animal species in existence. Animals usually have either a male or a female reproductive system, although there are many animals (various worms, snails, other invertebrates and even some fish) that are hermaphrodites. This means they have both male and female reproductive organs.

Excretion When food and oxygen are used up in the body, waste products are made. These are poisonous, and if they build up inside the body they can kill it. To prevent this from happening, the body has a way of getting rid of its harmful wastes, called excretion. Wastes are released in urine, sweat and the air that we breathe out. The waste product we release in our breath is carbon dioxide.

DID YOU KNOW? The release of solid wastes from the body is called egestion, not excretion. The solid wastes are called faeces.

Plant life Green plants also have the same seven features of life but they show them in different ways to animals. Plants make food from carbon dioxide in the air and water by using energy from sunlight. Chemicals in the soil are also needed, but in very small amounts. All plant cells respire and gaseous exchange takes place through their leaves.

DID YOU KNOW? In general, plants are known for moving and growing slowly, but the bunchwood dogwood opens its flowers and shoots out its pollen in less than 0.5 milliseconds.

6 How is a green plant’s way of obtaining food different from an animal’s way of feeding? Plants move as they grow and can spread out over the ground. Waste products may also be stored in the leaves. Green plants are sensitive to light and grow towards it. Plants reproduce by making seeds or spores. Plants have reproductive organs, such as anthers and ovaries, that are found in flowers. Some plants can reproduce by making copies of themselves, called plantlets.

LET’S TALK If there are drought conditions, why might a plant produce seeds rather than grow new plantlets? If you look at a plant for a few moments, it does not move and does not seem to be sensitive to its surroundings. In fact, plants are sensitive to their surroundings, but in general take longer to respond and show their sensitivity than animals.

Figure 1.8 The spider plant grows in many moist woodlands in the warmer regions of the world. It makes plantlets on stalks.

Can a plant find light when it is in the shade? To find out if plants need light to live, we can make a model of an environment that has only a small amount of light. You will need: a bean plant, a large cardboard box, cardboard pieces to be made into baffles (see Figure 1.9 on the next page), sticky tape, scissors, a sunny place to store the box for a week or more, a camera.

Hypothesis Plants need light to live and can detect it and will grow towards it. Is this hypothesis testable? If you cannot decide look back at the Did you know? on page xiv.

Prediction If you make it difficult for a plant to find light, it will still find a way to grow towards it if possible.

Investigation and recording data 1 Make a container for the plant as shown in Figure 1.9.

Figure 1.9

2 Water the plant and place it in the box and photograph it. 3 Close the box so that the only light entering it is through the hole in the top.

4 Look in the box every two days and take a photograph each time and check that the soil is damp. 5 Repeat step 4 until you think that the prediction has been tested.

Examining the results Arrange the photographs in the order you took them and compare the plant in each one.

Conclusion Does your conclusion match the hypothesis and prediction? Have you collected enough evidence?

CHALLENGE YOURSELF What are the strengths of the model of a plant’s surroundings as shown in Figure 1.9 on the next page? What are the model’s limitations?

CHALLENGE YOURSELF How could you change the experiment to make it more difficult for the plant to find the light? If your teacher approves your plan, try your investigation.

Science in context Looking for life beyond the Earth Almost everyone has an opinion about alien life in space, but how do scientists go about investigating it? In the 1970s, the Viking missions to Mars took place, and this provided scientists with a chance to devise a scientific investigation to test for signs of life.

In the first stage of the investigation, scientists thought about the link between food and respiration. They reasoned that if living things were present in the soil, they might be detected in the following way. Water containing food could be put into the soil. If living things were present in the soil, they would feed on the food and produce carbon dioxide in respiration. So if carbon dioxide was detected in the soil, living things were present. Apparatus was then designed to dig up the soil and put it in a container where food and water was added. As Mars is much colder than Earth, a heater was built around the container to warm up the soil and make the living things, if they were present, feed and respire faster.

Figure 1.10 A Viking lander on Mars with its digging tool ready to scoop up Martian soil for testing.

When the spacecraft reached Mars, the soil was tested and carbon dioxide gas was detected. It seemed that the gas had been produced by living things, but when the evidence from other investigations taking place there was considered, the scientists concluded that there could be other explanations for the gas being produced. Since then the Mars rovers – vehicles that can move around – have been used to investigate the planet. These include Sojourner, Spirit, Opportunity and Curiosity.

7 Explain the meanings of respiration, sensitivity and excretion. Write a sentence for each word to help you remember what they mean.

8 Write a sentence to show that you know the meaning of these words: – alien – detected – evidence – concluded.

9 Scientists used feeding and respiration as characteristics of life to investigate. How could sensitivity and movement be used to plan an investigation?

10 Did the investigations on Mars show that the gas definitely did not come from living things? Explain your answer.

Have we found alien life? You will need: access to the internet.

What are the recent developments in looking for alien life, such as life on Mars, Europa or planets beyond the solar system? Answer the question by searching the internet for information, then consider the reliability of the source. For example, does the source seem biased in trying to suggest to you that there are aliens, even though it gives you little evidence? When your search is complete, make a presentation of your findings, including any sources of information you think are biased.

LET’S TALK What is your opinion about alien life? Explain your answer.

Summary • There are seven characteristics of living organisms – movement, respiration, sensitivity, growth, reproduction, excretion, nutrition. • Non-living things may have some of these characteristics, but not all seven. • All living things respire.

End of chapter questions 1 What are the signs of life? Use MRSGREN to help you answer. 2 If your skeleton was on the outside, how could you grow bigger? 3 a What does an axolotl have to help it breathe? b Where does it get its oxygen from? 4 5 6 7

Can plants move? Explain your answer. Name two animals that feed on blood. What structures in your body let your body move? Imagine you were a snake. How could you detect chemicals in the air?

8 How does a plant or animal species continue to exist? 9 a What does ‘excretion’ mean? b Do you excrete when you breathe out? Explain your answer. 10 Imagine you were an astronaut on another planet. How could you test the soil on a planet to see if it had living things in it? Now you have completed Chapter 1, you may like to try the Chapter 1 online knowledge test if you are using the Boost eBook.

2 Identifying species In this chapter you will learn: • to sort, group and classify organisms through observation • how to describe a species as a group of organisms that can breed to produce healthy, fertile offspring • how to use your classification skills to test and observe organisms • how to use and construct a dichotomous key to identify different organisms, based on the organism’s traits.

Do you remember? • When scientists study different living things, they compare them, seeing how they are similar and different. Compare a cat and dog as a scientist would. • Plants have four major parts that scientists use when making comparisons. What are they? • How can you tell if an animal is a fish rather than an amphibian? • How can you tell if an animal is a bird and not a mammal? • What are the features of an insect? A species is a group of living things that have a very large number of similarities. The males and females of a species breed together to produce offspring that can also breed. The males and females of different species do not normally breed together but, if they do, they produce offspring that are usually sterile. This means these offspring cannot breed. For example, if a male donkey and female horse mate, the offspring they produce is a sterile animal called a mule. This happens because the donkey and the horse are different species. A major scientific activity taking place on the planet right now is the identification of species to see how many different life forms there are, and also to find out which ones are in danger of becoming extinct

(which means the individuals in a species of living thing die out completely, and there are none left to breed and keep the species in existence).

Making observations The most important skill in identifying species is observation. It is also the most important skill in any scientific enquiry. When you make observations, you look closely and with a purpose. For example, you may look at a plant and just see its flowers and leaves, but if you observed a plant you could study it to find out how the leaves and flowers are arranged on the stem. Leaves can be arranged in many ways; for example, they may grow alternately along a stem or they may be arranged in pairs. Flowers may be arranged singly (individually) or in columns. Look at Figure 2.1. 1 How are the leaves arranged in plants A and B? 2 How are the flowers arranged in plants A and B?

CHALLENGE YOURSELF Take two different plants that might be growing in or around your home, and look for differences and similarities between them. Make a list. Challenge someone else to try, then compare your observational skills.

Figure 2.1 The leaves and flowers in two plants.

Science in context Drawing biological specimens In scientific enquiries, drawings are made from observing specimens. A drawing is a model of the specimen that is being observed. The way that drawing biological specimens developed is explained in the next few paragraphs, together with instructions on how to make your own drawings of biological specimens from now on. Explorers in the seventeenth and eighteenth centuries collected specimens of the plants and animals they found and brought them back to the scientists in Europe, and other countries around the world, for further study. Many of these living things died on the

return journey, and by the time the explorers arrived, the specimens’ remains were decayed and of little use to the scientists. Even when the specimens were kept in preservatives, their colours would be lost, or some other feature would change. To solve the problem of showing how these living things appeared in their habitats, artists accompanied the explorers and drew pictures of the plants and animals that were discovered. The scientists could then use both the specimens and the pictures to help them study and classify the new living things that were being discovered.

Figure 2.2 A seventeenth-century biological drawing.

Biological drawings of specimens are still made today. The size of the specimen is usually indicated in one of two ways. A line may be drawn next to the picture to indicate the length of the specimen, or the drawing may have ‘× 5’ or ‘× 1/2’ next to it. The symbol × means ‘times larger or smaller’; the number gives an indication of size. For example, ‘× 5’ means the drawing is five times larger than the specimen, and ‘× 1/2’ means the drawing is half the size of the specimen.

Figure 2.3 A drawing of a leaf and a fish.

3 Why were artists taken on explorations in the seventeenth and eighteenth centuries? 4 Why do you think artists are used much less in expeditions today? 5 a Why might an organism be drawn × 5? Give an example. b Why might another organism be drawn × 1/2? Give an example. c What is the true size of the living things in these drawings?

Can you make a biological drawing?

You will need: An HB (hard black/medium hard), or number 2 grade pencil, an A4 sheet of paper for each specimen, a selection of specimens such as leaves, entire small plants, snails, slugs, or shellfish such as a prawn or crab, fresh fish (all from a shop) and identification books of the specimens to supply information for labels.

Hypothesis Drawings of biological specimens can provide accurate information about living things. Is this hypothesis testable? Explain your answer.

Investigation Test the hypothesis by reading the following instructions, then selecting two plant and two animal specimens to draw. 1 Instructions for making biological drawings: a Use an HB (hard black/medium hard), or number 2 grade pencil. b Make a large drawing of your specimen. Leave space to add labels. c The finished drawing should have firm clear lines with little or no shading. d Print the words of the labels and draw horizontal lines from each label to the centre of the feature being labelled. e Do not cross one label line with another. f Include an actual size of the specimen, as explained in the section on drawing biological specimens (pages 12–13). g Give your drawing a title. 2 Take each specimen in turn and make a drawing following the instructions listed above.

Work safely Hands should be washed thoroughly after handling living or preserved specimens.

Analysis and evaluation Compare your drawings and specimens.

Conclusion Do you think you made enough observations and measurements to make your drawings? Explain your answer. Does your work support the hypothesis?

6 Look at one of the specimens you have just drawn and write a description of it. Compare your description with the drawing. Which is more useful: a written description or a drawing? Explain your answer.

CHALLENGE YOURSELF We have seen on page xiii that a model can be a drawing. Look at each drawing you have made of a biological specimen and describe its strengths and limitations.

LET’S TALK In a group, show each other your drawings and give praise or helpful comments to each other. Assess the detail in each drawing and make a display of your work.

Observing and classifying

As scientists observe different living things and compare their features, they use this data to split them into groups to make them easier to study. An example of animals that have been split into groups in this way is the vertebrate group, which you will have studied previously. By comparing the animals as you did at the start of this chapter, scientists have set up five groups as Table 2.1 shows.

This division into groups is useful but there are a large number of different animals in each group, so they have to be split up in a similar way to make them easier to study. Scientists have a developed system of dividing living things into groups. It is called a system of classification. This system allows different species of animals to be identified. A species is an animal group whose members have a large number of very similar features and can reproduce together to produce offspring which can also breed.

DID YOU KNOW? Members of different species very rarely breed together and if they do, they produce offspring which cannot breed. These offspring are described as sterile. The offspring of members of the same species can breed and are described as fertile. Science extra: Orders We can see how this system of classification works by examining how the mammal class is divided into species. First the class is divided up into smaller groups called orders. The members of each

order have so many features in common that they look alike and are easy to group. There are 19 orders of mammals. An order is made up of smaller groups called families. The members of the different families look similar but there are differences. This can be seen by looking quite closely. The members of a family have differences between them and are split up into smaller groups called genera (singular: genus). The differences between members of each genus are found by looking very closely. For example, if you look at dolphins A, B and C in Figure 2.4, you will see that A seems to have more features in common with B than with C. Because of this, dolphins A and B are placed in one genus and C is placed in a separate genus.

Figure 2.4 Members of the dolphin family.

A Dusky dolphins live in coastal waters off New Zealand, Africa and South America. B The white-sided dolphin lives in the open water of the northern Pacific Ocean and around the coasts of Japan and North

America. C Bottle-nosed dolphins live in all the oceans and seas of the tropical and temperate regions of the world. Dolphins A and B have some small differences between them, so although they are very similar, they are each part of a different species. A species is the smallest group of this system of classification. This system of classification is used in the construction of keys, which makes identifying species easier and quicker than going through all the groups each time a species is to be identified.

7 How are dolphins A and B different from C?

8 How are dolphins A and B different from each other?

DID YOU KNOW? The smallest bat is the bumble bee bat which grows up to 33 mm long and has a mass of 2 g.

Science in context International names of living things For hundreds of years, people have explored different lands and brought back plants and animals from the places they visited. Explorers tried to sort their specimens into groups to make their work easier, but they all used different names.

Then, Carl Linnaeus (1707–1778) had an idea. He lived in Sweden but travelled all over western Europe collecting and studying plants. He made close observations of his specimens and worked out a way of describing how one living thing was different from another. He began by putting very similar living things in the same group. He then looked at all the specimens in the group and sorted them out into smaller groups in which all the individuals were alike. This group is called the species.

Figure 2.5 Carl Linnaeus, dressed for a journey into the Scandinavian countryside to look for plants.

Linnaeus then decided to give each group, and species, a name. Each name described some feature of the specimen. He further decided that the two names put together would become the scientific name of the living thing. At the time, scientists in every country learnt and used Latin and Greek so he used words from these two languages so that every scientist could understand them. These words are still used today. For example, the species name of the African clawed toad is Xenopus laevis (the group name is Xenopus). Xenopus is made from two Greek words – xenos

meaning ‘strange’ and pous meaning ‘foot’. The words refer to the toad’s webbed hind foot, each toe of which is capped with a dark, sharp claw. The word laevis is Latin for ‘smooth’ and refers to the toad’s smooth skin.

Figure 2.6 Xenopus laevis lives in wetlands in South Africa.

9 Why was there a need to group living things as explorers brought them back from their travels? 10 Why were Latin and Greek words used to name living things? 11 Which scientific activities led Linnaeus to think of his idea for classification?

LET’S TALK Why were the common names or local names not used in the naming of plants and animals by scientists?

Keys The observations of living things and their classification have led scientists to produce keys called dichotomous keys. The word ‘dichotomous’ describes something that divides into two. An example is a twig. As you

move along the twig it sends out a side branch, and if you move along that you may see that the side branch also divides into two, and the second side branch may also divide into two, and so on. A key allows a scientist to quickly identify a living thing as belonging to a group such as a class or even a species. 12 In a key about fish, the species are identified by the length of their bodies and by their body mass. Is this key reliable? Explain your answer. There are two kinds of key – spider keys (whose name refers to the way the key spreads out like the legs of a spider) and numbered keys (which take you step by step to the identification of the specimen you have found). Keys are made by considering features in a species that do not vary. Features that might change, due to growth, for example, are not used.

Spider key On each ‘leg’ of the spider is a feature that is possessed by the living thing named below it. An example is shown below in Figure 2.7. A spider key is read by starting at the top in the centre and reading the features down the legs until the specimen is identified.

Figure 2.7 A spider key of leaves from broadleaved trees.

How can a spider key be used to identify molluscs and annelids? You will need: paper, a pen and Figure 2.8.

Hypothesis The features of molluscs and annelids can be used to make a spider key to identify them. Is this hypothesis testable? Explain your answer.

Plan and investigation 2 Look at the features of all the animals, choose a feature that they all have and put this at the top of the key. 2 Select another feature to divide them into two groups. 3 Select another feature of the animals in each group to separate them further.

Figure 2.8 Molluscs and annelid worms.

Examining the results Compare your key with the hypothesis.

Conclusion What do you conclude? Was the hypothesis correct?

Numbered key You work through a numbered key by reading each pair of statements and matching the description of one of them to the features you see on the specimen you are trying to identify. At the end of each statement there is an instruction to move to another pair of statements or to the name of a living thing. Table 2.2 shows a simple numbered key. It can be used to identify molluscs that live in freshwater habitats such as rivers, lakes and ponds.

13 a Identify the molluscs a–f in Figure 2.9 using the numbered key in Table 2.2 on the previous page. In each case, write down the number of each statement you used to make the identification. For example, specimen a is identified by following statements 1a, 2b, 3a. It is a freshwater limpet.

Figure 2.9 Some freshwater molluscs.

b Why should another feature in addition to size be added to the statements in part 7 of the key?

How can a numbered key be used to identify arthropods? You will need: paper, a pen and Figure 2.10.

Figure 2.10 Arthropod specimens.

Hypothesis The features of arthropods can be used to make a numbered key to identify them. Is this hypothesis testable? Explain your answer.

Plan and investigation 1 Look back at the example of a numbered key on page 19 to see how it is set out.

2 Begin by separating the butterfly, which has six legs, from the others. 3 Look at the four other arthropods and find a way to separate them in a numbered key.

Examining the results Compare your key with the hypothesis.

Conclusion What do you conclude? Was the hypothesis correct?

CHALLENGE YOURSELF Look at the spider key for the leaves (page 18) and the numbered key for freshwater molluscs (page 20), and answer the following questions. 1 Which key identifies the larger number of living things? 2 If both keys featured the same number of living things, which key would need the larger amount of space? 3 Give an advantage of a numbered key. 4 Give an advantage of a spider key. 5 Which is the better one to use in a poster? Explain. 6 Which is the better one to use in a pocket book for fieldwork? Explain.

Science in context Exploring for new species The search for new species is taking place all the time, all over the world. In 2019, Sonali Garg and SD Biju were exploring the Western Ghats of India when they discovered a new species of frog (the Mysticellus franki, shown in Figure 2.11). It was in a puddle by the roadside.

Figure 2.11 Mysticellus franki.

In the past, scientists collected specimens and preserved them for further study. You can see them in museums of natural history, but they are also kept in collections at research institutes and univesities. Quan Li, working at the Kunning institute of Zoology at the Chinese Academy of Sciences, was studying a collection of specimens of preserved flying squirrels when he noticed that one seemed quite different from the others and he believed it to be a new species. He organised a research team to explore the Mount Gaoligong in Yunnan Province, Southwest China, where the original specimen had been found. The team successfully located more flying squirrels which did prove to be a new species – the Biswamoyopterus gaoligongensis, shown in Figure 2.12 – as Quan Li suspected.

Figure 2.12 Biswamoyopterus gaoligongensis.

14 An ecologist says the specimen she has just collected is a new species, but another ecologist says it is actually a species that is already known, but is just has a slightly different colour. How would you try to settle the issue?

Science in context Identifying endangered species An endangered species is one which exists in a habitat in such small numbers that, if it fails to breed, it may become extinct. In the past, the identification of species involved collecting specimens from the habitat and often killing and preserving them for later study back in the laboratory. Today, if an endangered animal species is identified using keys, techniques which allow the animals to stay in their natural habitats are used. These techniques include very detailed photography, recording the animal’s sounds (for example, mating calls), collecting its DNA from its surroundings (for example, from water and soil) or taking swabs from its skin or mouth to check its DNA. DNA is the chemical found in the cells of living things which is passed from one generation to another and can be used to identify each species.

Summary • Identifying and classifying is a type of scientific enquiry that is used to group organisms. • A species is a group of organisms that has a large number of similarities, and can breed to produce healthy, fertile offspring. • A dichotomous key is used to observe and classify different organisms, based on the organism’s traits.

End of chapter questions 1 What is a species?

2 Why did scientists in the seventeenth and eighteenth centuries make drawings of the specimens of living things they found in distant lands? 3 What advice would you give to someone who wanted to make a biological drawing? 4 How did the work of Carl Linnaeus help scientists identify different species? 5 Are the preserved specimens found in museums of any use in science today? Explain your answer. 6 What techniques are used in addition to keys to help identify endanged species today? Now you have completed Chapter 2, you may like to try the Chapter 2 online knowledge test if you are using the Boost eBook.

3 Cells In this chapter you will learn: • that all organisms are made of cells • that microorganisms are usually made up of only one cell – they are single-celled • to give reasons for classifying viruses as living or non-living • to identify and describe cell structures • how the structures of cells are related to their functions • about the similarities and differences between the structures of plant and animal cells • how cells can be grouped together to form tissues, organs and organ systems.

Do you remember? In your previous science course, you will have learned about some systems of the human body. These systems are made up of a number of organs working together. • The digestive system is made up from a number of organs. What are they? • What is the main organ of the circulatory system and what does it do to the blood? • What are the main organs of the respiratory system? What do they do and where are they found? In this chapter we are going to find out what organs are made from.

What is a cell?

You may have met the word ‘cell’ when studying electricity, and found that it is a source of electricity, sometimes called a ‘battery’ when there is more than one cell. In biology the word ‘cell’ means something very different. It can be described as the basic unit of life and is a very small structure from which all organisms are made. It has all the characteristics of life that we examined in Chapter 1.

DID YOU KNOW? The average human adult body may be composed of up to 40 trillion cells. Nobody is sure of the exact number.

Science in context The discovery of cells The discovery of cells began by the careful dissection of the human body to reveal the organs. These careful dissections were first carried out over 2000 years ago, in Greece, by two doctors called Herophilus and Erasistratus. Their work encouraged others living nearly 400 years ago to investigate further. In the sixteenth century, scientists in Europe began studying the human body by dissection. This work has continued ever since. Marie-François X Bichat (1771–1802) was a French doctor who examined many bodies after they had died. This is what is known as a post-mortem examination. In the last year of his life, he carried out 600. He cut up the bodies of dead people to find out how they had died. From this he discovered that organs were made of layers of materials. He called these layers ‘tissues’ and identified 21 different kinds. For a while, scientists thought that tissues were made of simple non-living materials. In 1665, long before Bichat was born, an English scientist named Robert Hooke (1635–1703) used a microscope to investigate the structure of a very thin sheet of cork, which is the outer covering (called the bark) of a type of oak tree. He discovered that it had tiny compartments in it. He thought of them as rooms and called them

cells, after the small rooms in monasteries – which are the buildings where monks live, work and meditate.

Figure 3.1 This painting by Rembrandt shows Dr Nicolaes Tulp making a dissection in Holland in 1632.

Bichat did not examine the tissues he had found under a microscope because most of those made at that time did not produce very clear images. When better microscopes were made, scientists investigated pieces of plants and found that, like cork, they also had a cell structure. The cells in Hooke’s piece of cork had been empty, but other plant cells were found to contain structures. A Scottish scientist called Robert Brown (1773–1858) studied plant cells and noticed that each one had a dark spot inside it. In 1831, he named the spot the nucleus, which means ‘little nut’.

Figure 3.2 Here are the compartments in cork that Hooke saw using his microscope. He called them cells.

Matthias Schleiden (1804–1881) was a German scientist who studied the parts of many plants, and in 1838 he put forward a theory that all plants were made of cells. A year later Theodor Schwann (1810–1882), another German scientist, stated that animals were also made of cells. The ideas of Schleiden and Schwann became known as ‘cell theory’. It led other scientists to make more discoveries about cells and showed that tissues are made up of groups of similar cells. 1 Where did Bichat get his idea that organs were made from tissues? 2 Who first described cells and where did the idea for the word come from? 3 Who named the nucleus and what does the word mean? 4 What instrument was essential for the study of cells? 5 How could cell theory have been developed sooner? 6 Arrange these parts of a body in order of size starting with the largest: – cell

– organ – tissue – organ system. 7 Which scientific activity did Bichat perform to think up his idea that organs were made from tissues? 8 Which was the final scientific activity that led Schleiden and Schwann to set up cell theory?

LET’S TALK How do you feel about being made up from trillions of tiny cells all working together to keep you alive?

The microscope The microscope is a laboratory instrument used to observe very tiny objects. It does this by producing a highly magnified view of the object. Most laboratory microscopes give a magnification up to about 200 times, but some can give a magnification of over 1000 times. The microscope must also provide a clear view, and this is achieved by controlling the amount of light shining onto the specimen.

DID YOU KNOW? Biologists use the word ‘specimen’ to describe an object like a plant, an animal or a part of one, such as a group of cells. The objects that they place on microscope slides are called ‘specimens’.

How do we see cells? You will need:

a microscope and light source, and a prepared slide with a specimen on it (the specimen could be like the one shown in Figure 3.3 on the next page). The microscope is used to see very tiny objects such as cells. It does this by making a highly magnified view of them.

Figure 3.3 A very thin slice through human skin which has been stained to make the cells easier to see.

Figure 3.4 shows the main parts of a microscope. Many older microscopes (like the one shown in Figure 3.4) have a mirror for collecting light, but microscopes made today have an illuminator instead of a mirror, as shown in Figure 3.5. The illuminator has an LED lamp inside it and a light control on the side.

Figure 3.4 The main parts of a microscope.

Figure 3.5 A modern microscope.

Process

1 The objective lenses have different magnifying powers. This is shown on their sides, such as × 5, × 10 or × 20. Select the lowest-power objective lens and click it into position under the ocular tube. 2 Put the slide with the specimen on it onto the stage. Make sure that the specimen is in the centre of the hole on the stage. 3 Carefully place the stage clips on the slide without moving it. 4 If the microscope has a mirror and lamp, look down the eyepiece to see that the specimen is in a circle of light. If it is not, move the mirror slowly, as instructed by your teacher, to point the mirror at the lamp from a slightly different direction. 5 Look at the side of the microscope and turn the focusing knob so that the objective lens moves closer to the slide but does not touch it. 6 Look down the eyepiece and turn the focusing knob slowly, so that the objective lens moves up and away from the specimen on the slide. As you do this, keep looking at the specimen through the eyepiece until you can see it clearly. The microscope is now focused on the specimen. 7 Describe what you see, either in words or by making a drawing.

LET’S TALK The microscope is a very important piece of scientific equipment. Did you find it easy to use? What was the most difficult task in seeing the specimen? What did you think of the specimen when you saw it magnified under the microscope?

CHALLENGE YOURSELF Look at the microscopes in your laboratory and use Figures 3.4 and 3.5 to identify their parts. Learn to point at each part and say its name without looking at the figures. What would you do to see a more powerful magnification of the specimen than in the enquiry? Check your plan with your teacher

and, if approved, try it. Science extra: The magnification of the microscope Above the stage is the ocular tube. This has an eyepiece lens at the top and one or more objective lenses at the bottom. The magnification of the two lenses is written on them. An eyepiece lens may give a magnification of × 5 or × 10. An objective lens may give a magnification of × 10, × 15 or × 20. The magnification provided by both the eyepiece lens and the objective lens is found by multiplying their magnifying powers together.

9 What magnification would you get by using an eyepiece of × 5 magnification with an objective lens of × 10 magnification?

10 If you had a microscope with × 5 and × 10 eyepieces and objective lenses of × 10, × 15 and × 20, what powers of magnification could your microscope provide?

Basic parts of a cell Nucleus The nucleus is the control centre of the cell. It controls all the life processes which take place to keep the cell alive.

Cytoplasm Cytoplasm is a watery jelly that fills most of each animal cell. It can move around inside the cell. The cytoplasm may contain stored food in the form

of grains. Most of the chemical reactions that keep the cell alive take place in the cytoplasm.

Figure 3.6 An animal cell.

Cell membrane The cell membrane has tiny holes in it called pores that control the movement of chemicals in or out of the cell. Dissolved substances such as food, oxygen and carbon dioxide can pass through the cell membrane. Some harmful chemicals are stopped from entering the cell by the membrane.

Mitochondria Mitochondria are tiny structures in the cytoplasm. In each mitochondrion, chemical reactions take place which release energy from food for life processes in the cell. Mitochondria are shown in Figure 3.6, but not in all of the other cell diagrams, to allow different structures to be seen more clearly, but even when they are not shown, it is important to remember that mitochondria are in the cytoplasm, helping to keep the cell alive.

DID YOU KNOW? DNA are letters which stand for a chemical found in every plant and animal cell. DNA controls how living things grow and develop their life processes, and can sometimes be found in the surroundings close to where a plant or animal lives.

11 Imagine that you are looking down a microscope at a slide labelled ‘cells’. You can see a coloured substance with dots in it and lines that divide the substance into rectangular shapes. What are: a the dots b the lines? c What is the coloured substance? 12 How does the cell membrane protect the cell?

DID YOU KNOW? The largest human cell is the ovum or egg cell. It is about 1 mm across and can be seen without a microscope. You were once that size!

The parts of a plant cell

Figure 3.7 A typical plant cell.

Cell wall The cell wall is found outside the membrane of a plant cell. It is made of cellulose, which is a tough material that gives support to the cell.

Chloroplasts

Chloroplasts are found in the cytoplasm of many plant cells. They contain a green pigment, which traps a small amount of the energy in sunlight. This energy is used by the plant to make food. Chloroplasts are found in many leaf cells and in the stem cells of some plants.

Sap vacuole The vacuole is a large space in the cytoplasm of a plant cell that is filled with a liquid, called cell sap, containing dissolved sugars and salts. When the vacuole is full of cell sap, the liquid pushes outwards on the cell wall and gives it support. If the plant is short of water, the support is lost and the plant wilts. 13 Name two things that give support to a plant cell. 14 Would you expect to find chloroplasts in a root cell? Explain your answer. 15 Why do plants wilt if they are not watered regularly? The features of animal and plant cells can be compared by setting them out in a table, as Table 3.1 shows. Table 3.1 Features of animal and plant cells.

Feature

Animal cell

Plant cell

nucleus

present

present

cytoplasm

present

present

cell membrane

present

present

mitochondria

present

present

cell wall

absent

present

chloroplasts

absent

present

sap vacuole

absent

present

LET’S TALK

Imagine that you are a cell in an animal. How would you describe yourself? If you were a plant cell how would you be different? Spend a little time jotting down your ideas then read them out to the group.

Looking at plant cells with the microscope In the next enquiry you will learn how to make a wet mount – placing a specimen in water for examination – and repeat the skills you have learnt in the previous scientific enquiry.

What makes a leaf green? You will need: a microscope and light source, a slide, a cover-slip, a small beaker of water, a dropping pipette, a pair of forceps, a mounted needle, a moss plant growing on a stone or log (or another suitable green plant you can find locally), an HB (hard black/medium hard) or a number 2 grade pencil and an A4 sheet of paper.

Hypothesis The green colour of the moss leaf is made by a pigment in the chloroplasts. Is this hypothesis testable? Explain your answer.

Prediction Where will the green colour in the moss cells be found? Make a prediction.

Investigation

1 Use a pipette to place a drop of water on a microscope slide.

2 Use a pair of forceps to pull a leaf off a moss plant (or other suitable green plant).

3 Lower the leaf gently into the drop of water.

Figure 3.8

4 Lower a cover-slip over the leaf in the water with a mounted needle. 5 Examine the slide for air bubbles. They will appear as circles with dark walls. If you find any, gently raise the cover-slip with the mounted needle and lower it again carefully as you check to see that no more bubbles form. 6 Set up the slide on the microscope and examine it under the lowpower lens and make a drawing of what you see. You do not need to draw a circle; just draw about six to ten cells on the paper. 7 Repeat step 6 with the medium- and high-power lenses. Draw fewer cells if necessary.

Work safely Hands should be washed thoroughly after handling living or preserved specimens. Microscope mirrors should not be turned to catch direct sunlight.

Examining the results Compare your drawings with Figure 3.7.

Conclusion

Compare your drawings with the hypothesis and prediction and make a conclusion.

Adaptation in cells The word adaptation means the change of an existing design for a particular task. You learnt about the basic structures of animal and plant cells in the last section, but many cells are adapted, which allows them to perform a more specific task and become what we call specialised. Here are some common examples of the different types of plant and animal cells that have become specialised.

Adaptations in plant cells Root hair cells Root hair cells are plant cells that grow a short distance behind the root tip. The cells have long, thin extensions that allow them to grow easily between the soil particles. The shape of these extensions gives the root hair cells a large surface area through which water can be taken up from the soil.

Figure 3.9 A root hair cell.

16 What changes have taken place in the basic plant cell to produce a root hair cell?

17 Why would it be a problem if root hair cell extensions were short and stubby?

Can you see root hairs on germinating seeds? You will need: a soaked bean seed, a damp paper towel, a dish, a hand lens (magnifying glass), a ruler, some A4 paper and an HB (hard black/medium hard) or a number 2 grade pencil.

Hypothesis As the root hairs grow out from the root surface, they may be able to be seen. Is this hypothesis testable? Explain your answer.

Prediction A germinating seed produces a root which searches for water. It may produce the root hair cells along its length to help it in its task.

Investigation 1 Wrap a soaked bean seed in a damp paper towel and put the towel on a plate. 2 Keep the plate and towel in a warm place. It does not have to be in the light. 3 Unwrap the towel carefully every day to look at the bean for signs of germination and root production, then wrap up the bean carefully again. Make sure that the towel stays damp. 4 Repeat step 3 until a root begins to appear, then wrap up the bean less tightly so the root has room to grow inside the towel. 5 Repeat step 3 for a few more days. Take care not to damage the root as you unwrap and wrap up the towel.

6 Look for root hairs with a hand lens by taking the following steps: a Hold the hand lens close to your eye. b Pick up the bean with its root and bring them close to your hand lens, until you can see the root clearly. 7 Repeat step 6 once a day for a few days. If you see changes in the root, write them down and make drawings of the root.

Work safely Hands should be washed thoroughly after handling living or preserved specimens.

Examining the results Compare your written description and drawings.

Conclusion Compare your descriptions and drawings with the hypothesis and prediction and make a conclusion.

Palisade cells Palisade cells have a shape that allows them to pack closely together in the upper part of a leaf, near the light. They have large numbers of chloroplasts in them to trap as much light energy as possible. 18 How is a palisade cell different from a root hair cell? Explain these differences. 19 Why are there different kinds of cells?

Figure 3.10 A palisade cell.

Adaptation in animal cells Skin cells There are a number of different types of skin cell, and skin cells have more than one function. However, one of the main functions of skins cells is to protect the surface of the body. After they have formed by cell division below the skin surface, they rise up and form a layer of dead cells which stop water and microorganisms entering the body. If you stay in the water in the swimming pool for a long time, you may notice sometimes that when you dry yourself, part of your skin flakes off. These flakes are made of dead skin cells. You are losing skin cells all the time, but in a much smaller way. As your clothes rub against your skin they pull off tiny flakes, which pass into the air and settle in the dust. A small part of the dirt that is swept up at the end of a school day comes from the skin that students have left behind. In the 1870s, it was discovered that dyes could be made from coal tar – a thick black liquid which is produced when coal is burnt to make the fuels coke and coal gas – which would stain different parts of the cell. Cell

biologists found they could stain the nucleus and other parts of the cell different colours to see them more easily.

How can skin cells be examined? You will need: a microscope and light source, clear sticky tape, scissors, a pair of forceps, a microscope slide, chemical-resistant gloves, a bottle of methylene blue stain, safety glasses, some A4 paper and an HB (hard black/medium hard) or a number 2 grade pencil.

Hypothesis The outer surface of the skin is covered with dead skin cells which might be easily removed by sticky tape. Is this hypothesis testable? Explain your answer.

Prediction Make your own prediction based on the hypothesis.

Investigation 1 Cut off a 2 cm length of clear sticky tape. 2 Put on a glove to pick it up as this will prevent fingerprints forming in the glue. 3 Press the tape onto the underside of your other wrist with the glue side touching the skin. 4 Use the forceps to lift the tape from your skin and place it on a microscope side with the glue side touching the slide. 5 Put on the other glove and use the pipette to take up a small volume of methylene blue solution from the bottle. 6 Use the forceps to lift one end of the tape a little and insert the end of the pipette under it and, carefully and gently, squeeze the

pipette so that a drop of the methylene blue solution goes under the tape. 7 Use the forceps to carefully lower the tape back onto the slide and check that air bubbles do not form. They appear as circles with dark walls. If bubbles form, gently lift the tape and lower it again with the forceps. 8 Place a cover-slip over the tape. 9 Examine the tape under the lowest-power lens of the microscope. Move the slide around to find objects stained blue. If you find them, put them in the centre of your field of view and use the medium-power lens and then the highest-power lens to examine them. Write down what you see and make a drawing.

Work safely Microscope mirrors should not be turned to catch direct sunlight. Be careful when handling methylene blue as it can be an irritant or dangerous when in contact with skin or when swallowed.

Examining the results Compare your written description and drawings with Figure 3.6.

Conclusion Compare your evaluation with the hypothesis and prediction and make a conclusion.

Red blood cells Red blood cells travel through all the arteries, veins and capillaries in the body. They are disc-shaped but their centres dip inwards. This structure is called a biconcave disc. It is also flexible, which allows the cells to bend and fold to pass through the smallest of capillaries. It does not have a

nucleus so that it can be completely filled with a red substance, called haemoglobin, which carries oxygen to all the other cells in the body.

Figure 3.11 Red blood cell.

20 If the red cell had a nucleus, how would this affect the oxygen supply to the other body cells?

Figure 3.12 The nervous system

DID YOU KNOW? Nerve cells are grouped together in the body to form the nerves, spinal cord and brain (known collectively as the nervous system), as Figure 3.12 shows.

Nerve cells

Nerves are made from nerve cells or neurones, which have long thread-like extensions. These nerve cells are connected to other nerve cells in the spinal cord. The nerve cells in the spinal cord are then connected to nerve cells in the brain. A nerve cell can conduct tiny electrical signals through its body. In the nerve cell shown in Figure 3.13, the signals are collected from other nerve cells (perhaps in a sense organ or a muscle) by the fibres on the cell body, and are transported through the nerve fibre to the projections at the far end. Once they reach these places, they set off another electrical signal in the next cell. Electrical signals are moved around the body in this way, from sense organs to the brain, and from the brain down the spinal column, and along nerves to the muscles to make them move.

Figure 3.13 Nerve cell.

Ciliated epithelial cells Cells that line the surface of structures are called epithelial cells. Cilia are microscopic hair-like extensions of the cytoplasm. If cells have one surface covered in cilia, they are described as being ciliated.

Ciliated epithelial cells line the throat. Air entering the throat contains dust that becomes trapped in the mucus of the throat lining. The cilia wave to and fro and carry the dust trapped in the mucus away from the lungs. The previous three examples of cells are found on the inside of the human body. The next scientific enquiry gives you the chance to see some of your own cells from inside your body.

Figure 3.14 Ciliated epithelial cells.

21 Smoking damages the cilia lining the breathing tubes. What effect might this have on breathing?

How can cheek cells be examined? You will need: a microscope and light source, a sterile cotton bud, a slide, a cover-slip, a mounted needle, a small beaker of water, a bottle of methylene blue stain, a dropping pipette, some A4 paper and an HB (hard black/medium hard) or a number 2 grade pencil.

Hypothesis Skin cells become loose and flake off. The inside of the mouth is lined with a moist skin-like tissue, so the cells there may also flake off. Is this hypothesis testable? Explain your answer.

Prediction

Make your own prediction based on the hypothesis.

Investigation

1 You will be given a sterile cotton bud by your teacher. Use one end of the cotton bud to wipe the surface inside your cheek.

2 Wipe that end of the cotton bud onto the middle of a microscope slide. 3 Give the cotton bud back to your teacher as it needs to be disposed of safely.

4 Use a pipette to put two drops of methylene blue solution on the sample.

Figure 3.15

5 Use a mounted needle to lower the cover-slip over the sample. 6 Examine the slide under the microscope. Look out for air bubbles, which have dark walls and may spoil your view of the cells. 7 Examine the slide under the lowest-power lens of the microscope. Move the slide around to find objects stained blue. If you find them, put them in the centre of your field of view and use the medium-power lens and then the highest-power lens to examine them. Make drawings of what you see at each magnification. 8 At the end of the experiment, give your slide back to your teacher as it needs to be disposed of safely.

Work safely Hands should be washed thoroughly after this experiment and swabs should be safely disposed of. Microscope mirrors should not be turned to catch direct sunlight.

Be careful when handling methylene blue as it can be an irritant or dangerous when in contact with skin or when swallowed.

Examining the results Compare your drawings with Figure 3.6.

Conclusion Compare your evaluation with the hypothesis and prediction and make a conclusion.

Cells, tissues, organs and organisms Cells in a living thing are arranged into groups. The cells in a group are all the same kind and perform a special task in the life of the organism. This group of cells is called a tissue. Different tissues join together to make a larger group of cells called an organ. All the special tasks performed by the cells in the different tissues in the organ help the organ to keep the body alive. Organs can form groups called organ systems. The organs in a system perform a vital task in the survival of the body – related to the seven life processes (see page 2). All the organs and organ systems in a living thing form a larger group – the body of the living thing – which is known as a living organism. Science extra: Plants as living organisms

Figure 3.16 This section through a leaf shows the different tissues that work together. The cells in the epidermis form a protective surface like tiles on a roof.

The cells which form the outer covering of a plant are broad and flat and join together to form a tissue called the epidermis. In a leaf, they cover a layer of palisade cells, which form a tissue called palisade mesophyll. The purpose of this tissue is to collect light and make food. Beneath this tissue are rounder-shaped cells with gaps between them, which form the spongy mesophyll tissue. Their task is to help bring water to the leaf in order to make food. Water evaporates from their surfaces and is replaced by water drawn up in tube-like xylem tissue, which forms much of the midrib (the line of thicker tissue running down the centre of the leaf, giving it support) and many of the veins in the leaf. Specialised pairs of cells in the lower epidermis of a leaf can bend to make an opening through which water vapour can escape from the spongy mesophyll. These openings are called stomata (singular: stoma).

The tissues in the leaf work together and form an organ – the leaf – which makes food for the plant. Epidermis and xylem tissues join with other tissues to make other organs of the plant such as the root, stem, bud and flower. Together all the organs make up the organism – the plant. 22 Make a chart to show how different types of plant cells form tissues, which in turn form an organ. You could make a table of three columns with the headings ‘Cells’, ‘Tissues’ and ‘Organ’ to help you and make one or more drawings in each column. Give your chart a title, such as ‘The tissues of a leaf’.

Science extra: Animals as living organisms Animal bodies have many organs. If we look at one organ (the stomach), we can see how it is built up from different kinds of cells, which form tissues (Figure 3.17).

Figure 3.17 The stomach wall is made of many different tissues of cells.

The inner surface of the stomach is lined with a tissue of epithelial cells that secrete mucus, which helps the food slide by. There are

cavities in the lining called gastric pits where tissues of gland cells secrete digestive juices to break down the food. The epithelial and gland cells are supported by a layer of fibres made from fibremaking cells. All these tissues form the structure called the stomach, and this in turn is connected to the other digestive organs to form the digestive system. This system is very closely linked to other systems such as the circulatory system, where blood vessels in the intestines take away absorbed food and carry it around the body. The group of closely linked organ systems form the organism – the animal. 23 What is the connection between a cell and an organism?

CHALLENGE YOURSELF Can you make models of cells from everyday materials such as cardboard, plastic, cloth, string, wire, or clear plastic sheets? Challenge yourself to make a model of: • a palisade cell • a nerve cell • a tissue of ciliated epithelial cells. Display your cells with some information about what they do. Assess people’s reactions to them. What are the strengths and limitations of each model?

Microorganisms Microorganisms are organisms that usually have a body made from only one cell. They can only be seen with a microscope unless a huge number of them join together to make a colony like green slime, or a larger body like a fungus. There are huge numbers of different microorganisms and they are classified into different kingdoms.

DID YOU KNOW? Your body is the home to trillions of microorganisms. They live all over your skin, up your nose, inside your mouth and along your digestive system.

Figure 3.18 A highly magnified view of a group of bacteria.

Science extra: Bacteria Bacteria are found almost everywhere – in air and water, on the surfaces of plants, animals and rocks, and inside living things too. Bacteria are single-celled and usually reproduce by dividing in two. If they have enough warmth, moisture and food, some bacteria can reproduce by fission once every 20 minutes. Some bacteria feed on the insides of living bodies, where they cause disease. Diphtheria, whooping cough, cholera, typhoid, tuberculosis and food poisoning are all diseases caused by different kinds of bacteria.

Bacteria can divide quite rapidly, but when conditions become dry and hot – unsuitable for feeding and dividing – some bacteria form spores. They can survive inside spores for a long time. They break out of the spores when they find warmth, food and moisture again.

Figure 3.19 Bacterium (left) and bacterial spore (right).

DID YOU KNOW? A chromosome is a structure in a cell which contains instructions to keep the cell alive, and passes on the instructions to future generations of cells.

24 If some bacteria have enough warmth, moisture and food they can divide into two every 20 minutes. If one of these bacteria divided like this for six hours, how many bacteria would it produce?

Viruses Scientists are not certain that viruses are living things, as they do not show any of the characteristics of life. Figure 3.20 shows the basic structure of a virus. The spike helps the virus to enter cells where it can start to replicate. The outer protective covering of

the virus is called the envelope. The DNA or other nucleic acid is the replicating part of the virus. It makes copies of itself to form new viruses. The coat around the replicating part of the virus keeps it safe.

Figure 3.20 The basic structure of a virus.

Viruses and the characteristics of life If we examine a virus to see if it possesses the seven characteristics of life that we studied on page 2, we find the following facts: viruses cannot move on their own; they depend on air or water currents outside the body, and the movement of liquids in the bodies of plants and animals. Viruses do not respire. They are sensitive to changes in temperature and to certain chemicals. A virus does not grow; when the body forms, it stays the same size. Viruses replicate, but they need a cell in which to replicate (see Figure 3.21). Viruses do not excrete, nor do they take in food for nutrition.

Viruses and living tissue Viruses do not have a cell structure. They can be stored like mineral specimens for many years without changing. They do not feed, respire or excrete. When they are placed on living tissues, they enter the living cells and replicate, which means they make copies of themselves. They destroy

the cells in the process and may cause disease. Each kind of virus attacks certain cells in the body. For example, the cold virus attacks the cells that line the inside of the nose. The destructive action of the cold virus on the cells in the nose makes the nose produce excess mucus. Other viruses cause influenza, chicken pox, measles and rabies, and can lead to the development of AIDS (which stands for Acquired Immunodeficiency Syndrome – a condition in which the body no longer has the ability to fight infections). Some viruses are capable of spreading quickly around the world, such as Coronavirus (COVID-19).

DID YOU KNOW? A disease which affects a large number of people in a city or a country is called an ‘epidemic’. A disease which affects a huge number of people in many countries and continents across the world is called a ‘pandemic’.

Figure 3.21 How a virus replicates. In this diagram, the virus is shown as a coil and, when it is outside a cell, it has a coat, shown as an oval around it. Not all viruses have a coat when they are outside a host cell.

DID YOU KNOW? Replication and reproduction are not the same thing. Replication refers to the exact copying of something. Reproduction is a characteristic of life in which living things produce offspring that are not exact copies, but have some variation in their features.

LET’S TALK Are viruses living things? Explain your answer.

Summary • All organisms are made of cells; microorganisms are usually made up of only one cell – they are single-celled. • Every cell structure includes a nucleus, cytoplasm, mitochondria and a cell membrane. • Plant cells are different to animal cells; in a plant cell there is a cellulose cell wall, chloroplasts and a sap vacuole. • Plant and animal cells do have some similarities, such as a nucleus, cytoplasm, cell membrane and mitochondria. • A cell’s structure is related to its function; cells are adapted to do specific tasks in the body and the life of the organism. • Cells are grouped together into tissues; tissues are grouped together to make organs, and organs can form groups called organ systems. • Viruses have some but not all of the characteristics of life.

End of chapter questions 1 Give a very simple description of a cell. 2 a Who investigated a thin sheet of cork with a microscope? b What observation did he make that made him think of the word ‘cell’? 3 a When you begin to use the microscope, which objective lens should you select first – the one with the lowest power or the one with the highest power? b When you put a slide on the stage, where should you make sure the specimen is placed? c When you look at the side of the microscope to turn the focusing knob should you i aim to bring the objective lens closer to the slide carefully, or ii move it away from the slide? 4 Which part of a cell:

5 6 7 a b

a has pores that control the movement of chemicals? b is made from cellulose? c contains green pigment? d is made from watery jelly? e contains cell sap? Which of the cell parts in Question 4 are found only in plant cells? State two ways in which a root hair cell is different from a typical plant cell. Where do you find palisade cells in a plant? Do they have a large number of chloroplasts?

8 In the 1870s, what did scientists start to use so they could see the inside of cells more clearly? 9 How is a tissue different from an organ? 10 a What is a microorganism? b Why can viruses be considered i living things? ii non-living things? Now you have completed Chapter 3, you may like to try the Chapter 3 online knowledge test if you are using the Boost eBook.

4 Microorganisms In this chapter you will learn: • about decomposers and their ecological roles • how to construct and interpret food chains and food webs which include decomposers • whether the sciences can have an impact on the environment. As you learned in Chapter 3, a microorganism is a single-celled organism, which means it has a body made from one cell. Microorganisms are sometimes called microbes.

Do you remember?

Figure 4.1 A food web in a grassland ecosystem.

A group of scientists observed some living things in and around a pond and presented their discoveries in the form of a model, shown in Figure 4.1.

1 What does Figure 4.1 represent? 2 What do the arrows represent? 3 Identify three food chains shown in the scientists’ model and write them down. 4 For each food chain from your answer to Question 3, write down if each organism is a prey animal or a predator or both. 5 What organisms are the producers in every food chain? 6 What is the source of energy for every food chain?

Food chains In Chapter 1 it was stated that plants make their own food from carbon dioxide in the air and water in the ground, by using energy from sunlight. They also need other chemicals and they take them in from the soil when they take in water. These chemicals are called minerals and are needed for all the life processes in a plant. When an animal eats a plant, it takes in the minerals in the plant’s tissues and uses them for its own life processes. Predators also take in the minerals from their prey and use them for their life processes too, so the minerals pass up through a food chain. The passage of energy, food and minerals through a food chain and back into the habitat is shown simply in Figure 4.2.

Figure 4.2 The arrows show the passage of energy and minerals through an ecosystem.

The diagram in Figure 4.2 is a model that helps us understand how minerals are related to food chains. Its strength is how it shows the minerals recycling through the habitat, but it has a limitation. It does not show how the minerals leave the food chain. A key word in the diagram is ‘decay’. This is a process in which the dead bodies of plants and animals break down and fall apart. The living things that help in this process are called decomposers.

Decomposers Some decomposers belong to groups of living things we have already studied – invertebrates such as annelid worms and the young stages of flies in the insect group, called maggots. These decomposers use the dead remains of plants and animals for food. The most important decomposers are microorganisms. They begin to feed and break down dead bodies of plants and animals as soon as they are no longer alive, and continue until the bodies are completely broken down and their minerals are returned to the soil. Most microorganisms, such as bacteria, have a body made of only one cell, so cannot be seen without using a microscope. 1 What does compost add to the soil which will help plants grow? Figure 4.3 shows a compost heap containing dead plants, and fruit and vegetable waste from meals. This mixture is forming the food for earthworms, but there are other organisms in the compost breaking it down. At the end of the process, a material called compost is made at the bottom of the heap, which can be mixed with soil in which plants are to be grown. In this way, decomposers play an important ecological role in returning minerals back to the soil and the food chain.

Figure 4.3 Decay in a compost heap.

LET’S TALK What do you think would happen in an ecosystem if all the microorganisms died off due to disease or poison?

What happens to waste when it is turned into compost? You will need: internet access, fruit and vegetable waste, grass cuttings, straw, cut flowers that have died, a camera, and materials for building a compost bin such as wood, chicken wire or plastic sheeting.

Hypothesis Dead plant material forms the food of decomposers. As the decomposers feed, the plant material will break down into smaller pieces. Is this hypothesis testable? Explain your answer.

Prediction It may take many weeks (have a guess) for the decomposers to break down the waste into smaller pieces.

Plan and investigation 1 Use the internet to find out how to make a compost bin. You do not have to follow the instructions, but it may give you ideas for the design of your compost bin using materials you can find easily. Make one side of the bin removable so you can check on how the compost is breaking down (in step 5). 2 Draw a diagram of your bin and a flow chart of how you would assemble it. Check your ideas with your teacher and, if approved, make your bin. 3 Fill your bin with vegetable waste (you may do this over a few days as waste material becomes available), water it and, if you have made a lid, put it on. 4 Look at the condition of the compost every week. You may see decomposers on the surface but they will also be inside the compost. Make a note of the condition of the surface of the compost and photograph it. 5 After a few months (or however long you decide) remove the side of the compost bin and photograph the compost.

Work safely Hands should be washed thoroughly after practical work.

Examining the results Read through your notes and look at your photographs in sequence and compare them.

Conclusion

Draw a conclusion from your analysis and evaluation. 2 How could you find out if your compost bin produced compost more quickly than a compost heap? You have already met the bacteria that are very important in decomposition, but there are other groups of microorganisms too, as you will see in the red box below.

Science extra: Microorganisms There are some microscopic plants which have a body made from only one cell (see Figure 4.4). They are called algae.

Figure 4.4 Some single-celled algae.

They have all the characteristics of a plant, such as a cell wall and chloroplasts, and make their own food using sunlight, so they do not help in decomposing in an ecosystem. Algae can grow together in huge colonies of single cells and make a green surface on the sides of trees.

Blue-green algae Blue-green algae live in seas, oceans and lakes. They also grow on wet rocks at the sides of stream and rivers, at the tops of rocky

seashores, and may occur widely in the soil. When they occur in huge numbers you can see them with the naked eye.

Figure 4.5 Blue-green algae growing in water may make the surface look as if oil or paint has been spilt on it.

There are some microscopic organisms which have a body made from one cell, and they have features that are also found in animals, such as having cell bodies with a cell membrane, and finding food rather than making their own. Many eat other microorganisms and some have a surface covered with microscopic hairs (see Figure 4.6 on the next page), which they use like oars to swim through water in their habitats in soil and ponds.

Figure 4.6 The cell wall of a ciliate protozoan is covered in tiny hairs called cilia, which it uses like oars to row itself through the water.

One example of these animal-like microorganisms is a decomposer called Amoeba (see Figure 4.7). It has a flexible cell membrane, which it can shape into projections to wrap around tiny parts of dead plants and animals.

Figure 4.7 The structure of an Amoeba can be seen clearly here.

Fungi It may seem surprising to describe fungi as microorganisms, since a large number of fungi, such as the mushroom, can be easily seen without a microscope.

Figure 4.8 This is a ‘fairy ring’ of mushrooms. The parts of a fungus which produce spores (known as the fruiting bodies) have been made at the edges of a discshaped mycelium in the soil.

However, at one stage in their life cycle, mushrooms cannot be seen, and they behave like other microorganisms. This is the spore stage, which occurs when fungi reproduce. Fungal spores are carried by air and water currents, can travel long distances, and can survive in hot, cold and dry conditions until they find suitable conditions for the fungus to grow. We have seen that bacteria are microorganisms that are important decomposers, but there is another group that we can investigate, called moulds. They are a kind of fungus.

What is needed for mould to survive? You will need: two slices of bread, two plates, two clear-plastic bags a with a few pinpricks in them, sticky tape, a small beaker of water and a camera.

Hypothesis Living things need food and water to survive. Bread provides food for fungi to grow, but damp bread also supplies water, and fungi may survive better there. Is this hypothesis testable? Explain your answer.

Prediction Make your own prediction from the hypothesis.

Investigation 1 Pour a little water over one of the slices of bread to make it damp. 2 Leave the two slices on plates for an hour to give spores in the air time to settle on them. 3 Put a slice of bread in each plastic bag. 4 Use the sticky tape to seal the bags. 5 Leave the bags in the laboratory for a week but look at them every day and photograph them. Do not open the plastic bags. If mould is found, count the colonies every day and note their colours. 6 At the end of the experiment, give your plastic bag to your teacher as it needs to be disposed of safely.

Work safely Hands should be washed thoroughly after this experiment. This is not a suitable activity for students with allergies. Do not open the bag containing mouldy bread. Ensure that the mouldy bread is safely disposed of after the experiment.

Examining the results Set out the photographs in sequence and examine them for signs of mould growth. Compare your photographs with the hypothesis and prediction.

Conclusion Draw a conclusion from your analysis and evaluation.

DID YOU KNOW? A colony is a group of similar organisms living very closely together. Microorganisms can be seen without a microscope if they form a large number of individuals in a colony.

CHALLENGE YOURSELF Where does mould grow?

Does mould grow on all kinds of foods such as fruit and vegetables, cheese and biscuits? Plan an investigation and, if your teacher approves, try it.

DID YOU KNOW? Broth is a liquid which contains nutrients that can be used as foods by microorganisms. A sterile broth is one which does not have any living microorganisms in it.

Science in context Louis Pasteur Louis Pasteur (1822–1895) was a French scientist who made many discoveries about microorganisms. When he began his work, many people believed that living things could spring up from non-living materials. Their belief was based on observations they had made, such as how maggots appeared in rotten meat. This idea was called spontaneous generation. They also believed that some forms of this emerging new life made liquids like wine and broth turn bad.

Figure 4.9 Louis Pasteur at work in his laboratory.

Pasteur investigated how broths turned bad in the following way. As he knew that excess heat could kill living things, he boiled some broth in flasks to kill anything that might be living in it at the start. He then heated the necks of the glass flasks until they were soft, and pulled them out into a long, thin, curving tube called a swanneck. The broths in the flasks did not go bad.

Figure 4.10 The first stage of Pasteur’s swan-necked flask experiment.

Then Pasteur broke open one of the flasks and exposed the broth to the open air. The broth went bad. This made Pasteur conclude that there was something in the air that was making the broth go bad. In the swan-necked flasks where the broth did not go bad, this ‘something’ might have settled in the bend of the neck and therefore not reached the broth. To test his idea he tipped a swannecked flask so that some of the broth went into the bend where dust and other particles may have collected, and then he tipped it back again. The broth in this second flask then went bad.

Figure 4.11 Further stages of Pasteur’s swan-necked flask experiment.

Pasteur concluded that whatever was causing the change could be carried by air currents, but it must be heavier than air as it settled in the bend in the swan neck. He described these tiny objects as spores. Pasteur went on to show that the souring of wine and milk was due to microorganisms and not spontaneous generation. He developed a process to help stop these liquids going bad quickly, which was named after him – pasteurisation. In one of the main forms of pasteurisation used today on milk, called ‘high-temperature shorttime’ (HTST) treatment, the milk passes through pipes surrounded by hot water. During this time, the temperature of the milk is raised to 72 °C for 16 seconds, then cooled to 4 °C. This process kills mould spores, yeasts and bacteria that cause the milk to turn sour. After realising that broths were made bad by microorganisms, Pasteur reasoned that microorganisms might cause disease in animals and in humans. When silk farmers asked him to explain why their insects were dying, Pasteur performed investigations which showed that the disease was due to microorganisms. He later went on to discover that chicken cholera was due to a

microorganism attack and found that anthrax, a disease of cattle and humans, was also due to microorganisms. This last discovery showed that there was a link between microorganisms and human diseases. Pasteur believed in and gave great support to a developing theory called germ theory. This theory says that many diseases are caused by microorganisms invading the body. Pasteur’s work led many scientists afterwards to work to discover the causes of diseases and how to prevent them. 3 In Pasteur’s first investigation, how did he make his test fair? 4 Gases mix freely in the air. How did Pasteur’s first experiment show that it was not a gas that caused the broth to go bad? 5 How do you think Pasteur’s experiments with broth affected the way people thought about spontaneous generation? 6 How has the work of Pasteur helped the way we live today? 7 What piece of evidence did Pasteur use when he decided to boil the broths? 8 What prediction do you think Pasteur might have made before breaking open one of the flasks? 9 What made Pasteur think of the idea that microorganisms might cause disease in animals and humans? 10 What evidence produced by Pasteur supports germ theory?

Microorganisms in food chains and food webs At the beginning of the chapter, we examined a food web and picked out food chains in it. We looked at how food and energy passed through a food chain and discovered that, in addition to the food chains of large organisms like plants and animals, there were food chains in which microorganisms were involved. These food chains also provide a vital function to all the other large organisms in food chains. They recycle chemicals called minerals. In this recycling process, the minerals are released from plants and animals and enter the soil, where plant roots can take them up and use them to grow and stay healthy.

Figure 4.12 shows the position of microorganisms in a food chain.

Figure 4.12 Decomposers in a food chain.

11 Does adding a microorganism to a food chain turn it into a food web? Explain your answer. 12 Describe the path of nutrients from their plants to the egret and back to the plants.

13 A scientific researcher has read that some microorganisms are harmful and that microorganisms are found in soil. He writes an article in a newspaper advising gardeners to use chemicals to kill the microorganisms in their gardens, in order to protect garden wildlife. What would you put in an article to show that this advice is wrong? Decomposers are equally important in food webs. Figure 4.13 is a simple food web showing how each of the larger organisms link to the microorganisms.

Figure 4.13 The path of minerals that are released as decomposers feed as they pass through a food web.

14 Construct two food chains using the food web shown in Figure 4.13. 15 Explain how the decomposers in Figure 4.13 contribute to the food web. 16 Create your own food web using animals you know locally. 17 Using your local food web, construct a simple food chain.

Can you handle data to make a food chain and a food web? Scientists construct food chains and webs from the observations they have made about living things in a habitat. 1 Here are some observations about living things in a habitat. Use them to construct a food chain.

– Rabbits feed on grass. – Foxes feed on rabbits. – Microorganisms feed on dead plants, rabbits and foxes. 2 Here are some more observations about living things in the same habitat. Use them to construct a food web from the chain you have just made. – Mice feed on grass. – Foxes eat mice. – Owls eat mice. – Owls eat rabbits. – Microorganisms feed on dead mice, foxes and owls.

Science in context Soil microbiology As microorganisms are so important in recycling the materials from which living things are built, a great deal of scientific research is carried out on them. In addition to providing support for ecosystems, they also provide support for farmers growing crops and providing healthy plants for farm animals to eat. Many countries around the world have soils that are in poor condition due to a number of factors, including climate change, frequent ploughing up of the soil which reduces organic matter (the dead plant and animal remains on which microorganisms feed), and the overuse of chemical fertilisers. Most countries have associations, such as the Soil Science Society of Pakistan, which bring together the research of soil scientists, and distribute the findings to farmers to help them improve their crops and livestock.

Figure 4.14 A lecture at the Soil Science Society of Pakistan.

Summary • Decomposers, such as maggots, earthworms and some microorganisms, have an ecological role; they use the dead remains of plants and animals for food. • Food chains and food webs can be used to help us understand the processes involved and can include microoganisms as decomposers. • Science can have an impact on the environment.

End of chapter questions 1 2 3 4

What is a microorganism? What do decomposers feed on? What happens to a body when it is decomposed? When something has completely decomposed, what passes into the soil to be used by plants? 5 a Use the following information to produce a food web.

– A frog feeds on butterfly caterpillars. – A butterfly caterpillar feeds on plants. – A snake eats frogs. – A grasshopper feeds on plants. – A frog eats grasshoppers. – A hawk eats snakes. – A mongoose eats snakes. b Complete the food web by adding decomposers to it.

Now you have completed Chapter 4, you may like to try the Chapter 4 online knowledge test if you are using the Boost eBook.

5 The states of matter In this chapter you will learn: • to describe and compare the three states of matter – solid, liquid and gas • what a vacuum is • to test, observe and classify some common gases.

Do you remember? • • • •

What is the difference between a solid and a liquid? What is the difference between a liquid and a gas? What is the difference between a solid and a gas? Name five gases.

What is everything made from? Scientists have asked this question for thousands of years and gradually they came up with an answer. Everything is made of matter.

Matter everywhere To help you think of the world in terms of the three states of matter, think about going on a school hike. As you walk along, you move across the solid earth. Your body pushes through a mixture of gases we call air. If it rains as you walk along, droplets of liquid fall from the sky.

Figure 5.1 The three states of matter on a school hike.

LET’S TALK Select one state of matter and imagine that it has been removed from the world. List things that could not exist if it was absent. Do the same for the other two states of matter in turn. Would it be possible to live in any of these three imaginary worlds?

Not only do you move through a world made from the three states of matter, you are made from them too. You have solid bones that are moved by solid muscles, as liquids flow through your arteries, veins and intestines. When you breathe in, air (a mixture of gases) fills your windpipe and your lungs.

Comparing the states of matter You can tell one state of matter from another by examining their properties. • A solid has a definite mass, a definite shape and its volume does not change. It does not flow and it is hard to compress (squash) it. • A liquid has a definite mass and its volume does not change. It is hard to compress but it flows easily. The shape of the liquid varies and depends on the shape of the container holding it. • A gas has a definite mass but its volume can vary and it takes up the shape of the container holding it. It flows easily and it is easy to compress.

Figure 5.2 This picture shows the three states of matter.

1. 1 Make a table listing the properties of the three states of matter. 2. 2 How are all three states of matter: 1. a similar 2. b different? 3. 3 Identify the three states of matter that make up a glass of fizzy drink.

CHALLENGE YOURSELF Look at the world around you as you travel between school and home, and describe it in terms of solids, liquids and gases. What are the solids, liquids and gases in the vehicles you see on the road?

The particle model of matter You will have learnt earlier in your science courses that scientists believe that the behaviour of matter – solids, liquids and gases – can be explained by thinking of them as being made of particles. These particles are so tiny that they cannot be seen. This idea about matter being made of particles is really a model to explain how matter behaves, and it is called the particle model. The properties and behaviour of the three states of matter can be explained by the particle model in the following way.

Particles in the three states of matter

In solids, strong forces hold the particles together in a three-dimensional structure. In many solids, the particles form an orderly arrangement called a lattice. The particles in all solids move a little. They do not change position, but vibrate backwards and forwards about one position. In liquids, the forces that hold the particles together are weaker than in solids. The particles in a liquid can change position by sliding over each other. In gases, the forces of attraction between the particles are very small, and the particles can move away from each other and travel in all directions. When they hit each other or the surface of their container, they bounce and change direction.

Figure 5.3 Arrangement of particles in a solid, a liquid and a gas.

1. 4 According to the particle model, why do liquids flow but solids do not? 2. 5 How is the movement of particles in gases different from the movement of particles in liquids?

You can see that the particles in solids and liquids touch each other, but the particles move around separately in gases. The particle model is set out in the form of diagrams on this page, but sometimes a physical model made out of objects can make the model easier to understand. You can make a physical model of the particle model in the following activity.

Modelling the three states of matter

You will need: a small tray that is about 1 cm deep, and a collection of marbles that can make three rows when the tray is tipped, (for activity b) a cardboard box (such as a chocolate box), (for activity c) a clean, transparent bottle with a screw top and small balls of modelling clay, or dried peas or chickpeas.

Use the tray and marbles to show how particles behave in: 1. a a solid 2. a a liquid 3. a a gas.

1. 6 All models have strengths and limitations. What are the strengths and limitations of this model in showing: 1. a the motion of particles in solids, liquids and gases? 2. b the arrangement of particles in solids, liquids and gases? 3. c how a liquid can flow but a solid cannot? 4. d the contact and separation of the three states of matter? 2. 7 The cardboard of a chocolate box is softer than a metal or wooden tray. Could this be used to understand the vibrations of particles in a solid? Test the idea and draw a conclusion. 3. 8 All particles move in three dimensions, but gas particles move the most. Could small modelling clay balls or dried peas or chickpeas in a transparent plastic bottle with a top on be a better way to model the particles in a gas? Test the idea and draw a conclusion.

A vacuum A vacuum is a space in which there are no particles of matter. Space is often described as a vacuum, but it still has a few particles in it that are spread out over great distances from each other. This means that it is not a perfect vacuum, but a partial vacuum. For example, a vacuum cleaner sucks air particles and dust out of the carpet, but it does not create a perfect vacuum in the carpet, as there are still some air particles present. Because space is a vacuum, this means there is no air resistance to slow objects down – meaning if you threw a ball in space it would theoretically travel in a straight line forever!

Science in context A scientific showman Otto von Guericke (1602–86) was the mayor of Magdeburg in Germany for 35 years. He was also interested in science. He was keen to discover if a vacuum could really exist, and he made an air pump to test his ideas. He used his pump to draw air out of a variety of vessels. When he tried barrels, he found they collapsed. He eventually found that hollow copper hemispheres, joined together to make a globe, were much stronger. Aristotle had believed that if a vacuum could exist then sound would not be able to pass through it. When von Guericke put a bell in one of his vessels and removed the air, he discovered that a ringing bell could not be heard (see page 144).

Figure 5.4 Otto von Guericke’s experiment to pump air out of a barrel, creating a vacuum.

Testing common gases

Science extra: Preparing the gases Gases can be prepared by chemical reactions that are done in some of the apparatus we saw on page viii.

Figure 5.5 A solid chemical to make the gas is placed in the flask, then a stopper is put into the flask neck. In the stopper is a funnel. A liquid chemical is poured down the funnel and into the flask. The solid and liquid chemicals mix and a chemical reaction takes place, which produces a gas. As the chemical reaction continues, more gas is produced, and it pushes air out of the delivery tube and the gas jar. The gas collects in the gas jar and can be used for experiments. Later in your science course, we will look more closely at chemical reactions, but for now we need to be able to perform tests on the three common gases.

Which gas is it: oxygen, hydrogen or carbon dioxide?

You will need: a test tube with a stopper of each gas prepared by your teacher in apparatus like that shown in Figure 5.5, a test tube rack to hold the test tubes, a small candle (tea light) burning away from the test area, three wooden splints and some safety glasses.

Hypothesis The three gases are three separate substances and may have different properties. Is this hypothesis testable? If you cannot decide, look back at the Did you know? on page xiv. Prediction The three gases may show different properties when presented with a burning substance. Process 1. 1 Pick up a splint at one end and put the other end in the candle flame to light it. 2. 2 Blow out the flame on the splint but make sure it is still glowing. 3. 3 Remove the stopper from the test tube with oxygen gas and dip the glowing end of the splint into the test tube. Record what you see and hear. 4. 4 Repeat step 1 with the second splint but keep the flame burning. 5. 5 Remove the stopper from the test tube with hydrogen gas and dip the burning end of the splint into the test tube. Record what you see and hear. 6. 6 Repeat step 1 with the third splint but keep the flame burning. 7. 7 Remove the stopper from the test tube with carbon dioxide gas and dip the burning end of the splint into the test tube. Record what you see and hear.

Work safely Eye protection should be worn for this experiment.

Examining the results Compare your observations about what occurred when the splint was placed into each of the three test tubes. Conclusion Compare your analysis of your data with your hypothesis and prediction and make a conclusion.

DID YOU KNOW? Hydrogen is the most common chemical in the universe. It forms stars and changes to helium inside them. As it does so, heat and light travel out into space.

The results of the three experiments can be summarised in a table, as follows. Table 5.1 Gas oxygen hydrogen carbon dioxide * carbon dioxide

Test glowing splint burning splint

Result splint produces a flame ‘pop’ sound

burning splint

splint stops burning

bubble through calcium hydroxide solution

solution changes from clear to cloudy

* This test is discussed on pages 103–4.

Summary 1. • There are three states of matter – solid, liquid and gas – and they can be described using the arrangement, separation and movement of their particles. 2. • A vacuum is a space where there are no particles of matter. 3. • Chemical tests can be made to identify some common gases.

End of chapter questions 1. 1 How can you tell if something is: 1. a a solid? 2. b a liquid? 3. c a gas? 2. 2 Describe what you could do if you were a particle in: 1. a a solid 2. b a liquid 3. c a gas. 3. 3 What is the difference between a perfect vacuum and a partial vacuum? 1. 4 a When Otto von Guericke tried to make a vacuum in a barrel, what happened to the barrel? 2. b How did Otto improve his experiment? 3. c How did he test Aristotle’s idea about sound in a vacuum? 1. 5 How could you demonstrate to the class how to identify: 1. a hydrogen? 2. b oxygen? 3. c carbon dioxide?

Now you have completed Chapter 5, you may like to try the Chapter 5 online knowledge test if you are using the Boost eBook.

6 Atoms and elements In this chapter you will learn: • how all matter is made of atoms, and that atoms are what make one element different to another • how all substances have chemical properties and physical properties that we can use to identify them • that the periodic table displays all known chemical elements.

Do you remember? • What do you know about the particles of matter in a solid, a liquid and a gas? • What happens to the particles in a solid when it melts? • What happens to the particles in a liquid when it evaporates? One of the main activities in chemistry is breaking down substances to discover what they are made of. During the course of this work, chemists have discovered that some substances cannot be broken down into simpler substances by physical changes or chemical reactions. These substances are called elements. An element is made from one particular type of atom which has its own particular properties. Every element is made from a different type of atom. This gives each element specific chemical and physical properties. 1 Why do elements have different properties?

Science in context The discovery of the elements

Before 1669, the following elements had already been discovered: carbon, sulfur, iron, copper, arsenic, silver, tin, antimony, gold, mercury and lead. Some had been known for thousands of years, although they had not been recognised as elements. The order in which the other elements were discovered is shown in Table 6.1. This table uses mainly European historical data, but it is known that people in China and Islamic countries also studied chemistry, so some of the elements could have been discovered by them at an earlier date.

2 Which of these elements have you met before in your study of science? For each one, describe how you came to know about it.

CHALLENGE YOURSELF 1 How many elements are there? Use the internet to find out.

2 Use the internet to find out about the physical and chemical properties of carbon. 3 Use the internet to find out the physical and chemical properties of another element of your choice. Be sure to include the web reference in your answer so that your teacher can check it.

The chemical properties of elements A chemical property is one which is measurable once the substance undergoes a chemical reaction. You will look at chemical reactions and chemical properties more in Chapter 9. When some elements take part in a chemical reaction they produce a colour. You can see the colours produced by these chemical properties of elements in a firework display.

DID YOU KNOW? Compounds are a group of atoms of two or more elements joined together (see pages 77–8). Science extra: Fireworks Probably the most spectacular use of the properties of elements is in fireworks. Compounds of some elements are used to make fireworks. When they are heated they produce light of various colours (see Table 6.2). Table 6.2 The colours produced by elements when heated.

Element

Colour

aluminium

silvery-white

barium

apple green

calcium

orange

caesium

blue

copper

green

Element

Colour

lithium

red

magnesium

white

sodium

golden-yellow

potassium

lilac

rubidium

violet-red

strontium

red

Figure 6.1 A dazzling fireworks display.

3 Iron and titanium are two elements that are used in compounds to make sparks, and zinc is used in compounds to make smoke. Which other elements could you use in compounds to make a firework that produces green and red sparks and finishes with blue smoke?

Science extra: Flame tests A flame test can be used to identify certain elements. Use the flame test for the following activity, in order to investigate four elements.

Science extra: How can you identify sodium, potassium, calcium and copper? You will need: a nichrome wire, solid salts of sodium, potassium, calcium and copper labelled by your teacher to conceal their identity, a Bunsen burner or spirit burner, a heat-proof mat and eye protection.

Process 1 Light the Bunsen burner or spirit burner and set it on a small flame. 2 Take one end of the nichrome wire and dip it into the sample marked A. 3 Hold the nichrome wire so that the end with the sample on it is in the Bunsen flame. 4 Note the colour of the flame that is produced. 5 Repeat steps 2–4 with the three other samples.

Work safely Eye protection should be worn for this experiment.

Examining the results Compare the colours produced by the samples with the information in Table 6.2.

Conclusion Identify the samples.

The physical properties of elements A physical property of an element is one which can be observed without altering the element through any form of chemical reaction such as whether it is a solid, liquid or gas, and if it is solid, whether its surface is shiny or dull. There are only two elements that are liquid at room temperature and standard pressure. They are mercury and bromine and these liquids are stored in bottles that have hazard symbols on them. Eleven elements are gases under normal conditions. All the others are solids. Each element has its own special properties. For example, sodium is a soft, silvery-white metal with a melting point of 98 °C and a boiling point of 884 °C, and chlorine is a yellow-green gas with a melting point of –101 °C and a boiling point of –34 °C.

LET’S TALK Without looking back at the introduction of this book, make a simple sketch of all the hazard warning symbols that you can remember and say what they mean.

Substances and elements A substance is any form of matter such as a gas, liquid or solid. It may be made of the atoms of just one element, like pure gold, which is made from only gold atoms. Most substances are made from one or more elements joined together. Carbon dioxide is an example as it is made from one atom of carbon joined to two atoms of oxygen. The carbon dioxide you breathe out is made from a huge number of these groups of three atoms.

LET’S TALK How would you describe an atom? Where are atoms found? How can you tell one element from another? A substance has chemical and physical properties too. These depend on the elements from which it is made. A candle, for example, is made from a

substance called wax. This is made from carbon and hydrogen atoms joined together. When the wax is very hot it takes part in a chemical reaction with oxygen in the air and produces two substances – carbon dioxide and water (water is made from two atoms of hydrogen and one atom of oxygen). The elements which make up wax give it the following physical properties – it is a solid with a smooth shiny surface which starts to flow like a liquid when it gets warm. These chemical and physical properties of a substance can be used to identify it. 4 How could you recognise wax by its chemical and physical properties?

Science in context How scientists developed symbols for chemicals The first chemists were called alchemists. Two of their main activities were investigating materials in an attempt to find a way to make gold or a medicine that would extend the human lifespan. They wrote down details of their investigations using symbols to represent the substances they used or produced. The use of symbols saved them time. Figure 6.2 shows a few of the alchemists’ symbols. Some of the symbols were deliberately mysterious as the alchemists really wanted to keep their work a secret – just in case they discovered how to make gold or a medicine that would make them live forever!

Figure 6.2 Alchemists’ symbols.

The alchemists also gave many of the substances a number of different names, again to increase secrecy, and this led to confusion when the science of chemistry began properly. At this point, it was decided that each substance used in an investigation,

or produced from it, should be clearly identified by one name only so that reports of investigations could be clearly understood. In 1787, Antoine Lavoisier and three other scientists set out the names of all the substances used in chemical investigations in a 300-page book. In 1813, Jöns Jakob Berzelius introduced the symbols we still use today to represent the elements. Each element was identified by the first letter of its name. If two or more elements began with the same letter, another letter in the name was also used. Some of the symbols are made from old names for the elements. Iron, for example, had an old name of ferrum and the symbol ‘Fe’ comes from it. Silver was known as argentum and its symbol is ‘Ag’. Sodium was known as natrium and potassium was known as kalium, from Latin (and other languages) and the symbols for sodium and potassium were made from these names. The symbol for sodium is ‘Na’ and the symbol for potassium is ‘K’. The elements have received their names from a variety of sources. Some elements, such as chlorine (from the Greek word meaning ‘green colour’) and bromine (from the Greek word for ‘stench’), are named after their properties. Other elements are named after places. The places may be as small as a village (strontium is named after Strontian in Scotland) or as large as a planet (uranium is named after the planet Uranus). A few elements, such as einsteinium, are named after people. 5 Why do some elements have two letters for their chemical symbol and others have only one? 6 Why isn’t the symbol for silver ‘S’ and the symbol for potassium ‘P’? 7 How did some of the elements get their names?

The first twenty elements in the periodic table

After a large number of elements had been identified, scientists began arranging them into order based on their properties, such as mass, and a table called the periodic table was produced. It is called the periodic table because, as you move along the rows, elements with certain properties occur periodically, which means they occur at regular intervals. For now, we just need to know that such a table exists and is very widely used. For example, there may be a copy of the periodic table on the wall of your laboratory. Figure 6.3 on the next page shows part of the periodic table. If you look closely at the table you will see that in each square at the top left hand side is a number. It is called the atomic number. Atoms have a structure too, as you will see next year, but for now you just need to know about one feature of their structure. It is called the proton and is found in the centre of the atom. Each element has a certain number of protons and this is indicated by the atomic number. Here are some examples: hydrogen has just one proton so its atomic number is 1. Carbon has six protons so its atomic number is 6. Calcium has twenty so its atomic number is twenty. The first twenty elements in the periodic table are given in Table 6.3. Note that you read down the left-hand column first, then sodium follows neon and you read down the right-hand column.

Figure 6.3 Part of the periodic table.

Summary • All matter is made of atoms; atoms are what make one element different from another. • All substances have chemical properties and physical properties that we can use to identify them. • The periodic table displays all of the known chemical elements in order of increasing atomic number.

End of chapter questions 1 What cannot be broken down into simpler substances by physical changes of chemical reactions? 2 Name any two elements that had already been discovered before 1669. 3 What is a substance? 4 What is an element made from? 5 The physical properties of an element are the properties you can see. State a physical property of: a bromine b mercury

6 7

8 9

c sodium d chlorine. A firework contains calcium, copper and magnesium. What colours would you expect to see when the firework is lit? In the periodic table, elements are represented by their symbols. What is the element represented by: a C? b O? c Na? d K? Why was it decided that each substance used or produced in an investigation should have only one name? Table 6.4 shows the main elements making up the Earth’s crust, the air and the human body. Elements in quantities less than 1% are not shown, but may be present in very small amounts.

a Arrange the elements in the human body in order, starting with the most plentiful.

b Arrange the elements in the Earth’s crust in order, starting with the most plentiful. c Which element is found in large amounts in the Earth’s crust, the air and the human body? d How does the amount of calcium in the Earth’s crust compare with the amount in the human body? 10 What does the atomic number of an element tell you about the structure of its atoms? Now you have completed Chapter 6, you may like to try the Chapter 6 online knowledge test if you are using the Boost eBook.

7 Elements, compounds and mixtures In this chapter you will learn: • the differences between elements, compounds and mixtures, and that an alloy is a mixture of two or more elements • how alloys have different chemical and physical properties from their constituent substances • to use the particle model to explain the difference in hardness between pure metals and their alloys • to use the particle model to represent elements, compounds and mixtures • how to consider the strengths and weaknesses of using models in science.

Do you remember? • What is a substance? Give some examples you have studied. • Have you mixed substances? If you have, give some examples of what you did and what happened. • What do you know about dissolving?

Elements, atoms and compounds In Chapter 5 we saw how the particle model could be represented. In Chapter 6 we learnt that elements are made of atoms and that an atom can be represented as a particle. Also, we learnt that each element is made of a particular kind of atom. We can now take these ideas further by considering how the particles are arranged in an element, a compound and a mixture.

In Figure 5.3 (page 61) we saw how the atoms of an element are arranged when it is in the form of a solid, a liquid and a gas. Notice that all the particles are the same. In Figure 7.1 on the next page, we can see how the elements join together to make a compound, by showing atoms of two different elements joined together. In a compound, one atom of one element may join with one atom of another element (as in Figure 7.1a), or one atom of one element may join with two atoms of another element (as in Figure 7.1b). Compounds can be made by the atoms of more elements joining together. This figure just shows two very simple arrangements. Figure 7.1 Atoms join together to form compounds.

A mixture is composed of two or more separate substances. The substances may be different elements, an element and a compound, or two or more compounds. In Figure 7.2, the circles are used to represent two different substances. Figure 7.2 Two different mixtures of A and B.

Mixing elements and making compounds Chemists deal with elements, compounds and mixtures in their investigations. In the last chapter we looked at elements, and in this chapter we will look at what happens when we mix two elements and then make them into a compound. We will also look at different types of mixtures. Each element has its own particular properties. Sulfur, for example, is yellow, and if shaken with water it will tend to float. Iron is silvery-grey and magnetic, and produces hydrogen when it is placed in hydrochloric acid. If these two elements are mixed together, a grey-black powder is produced. The colour depends on the amount of sulfur mixed with iron. Although the two elements are close together, their properties do not change. If a magnet

is passed over the mixture, iron particles leap up and stick to it. If the mixture is shaken with water, the sulfur will still tend to float. Figure 7.3 Silvery-grey iron (left) and yellow sulfur (centre) mix to form a grey-black powder (right).

If the mixture of iron and sulfur is heated, using the apparatus shown in Figure 7.4, a chemical reaction takes place. The pipe clay triangle rests on the tripod and supports the crucible containing the mixture above the Bunsen burner flame. This heats the mixture, and the atoms of iron and sulfur join together and form a compound called iron sulfide. It does not have the yellow colour of the sulfur or the magnetic properties of the iron. It has its own properties – it is a black, non-magnetic solid. Thus, when elements form compounds, they no longer display their own special properties. Instead, compounds have properties which differ from the elements that formed them.

LET’S TALK How would you explain to someone that an element is different from a compound? How would you explain to someone how a compound is different from a mixture?

Figure 7.4 When iron and sulfur react together, they form a product called iron sulfide.

LET’S TALK What safety precautions would you need to take if you were to heat iron and sulfur as shown in Figure 7.4?

Science extra: Chemical names of compounds Chemical names can seem complicated, but there are rules for how the names are built up. Here are just a few examples.

The first part of the name is usually the name of an element in the compound. The second part of the name usually has part of the name of a second element in the compound. If the elements of the joined pair are not connected to any other elements, the suffix -ide may be added, as in ‘iron sulfide’. Another example is the chemical name for common salt. This is a compound made from sodium and chlorine, and it is called ‘sodium chloride’. If more than one atom of an element joins to an atom of the first named element, the prefix di- (for two) or tri- (for three) is added. An example of this is carbon dioxide. The di- part of the name tells you that there are two atoms of oxygen joined to a carbon atom, and the -ide part tells you that there are no other elements in the compound. If there are two elements joined to the first named element, a name may be made up from their three names (sodium hydroxide), or the second part of the name may be made up from their two names (hydroxide). The suffix -ate is used to indicate that the second named element is also joined to some oxygen atoms. For example, calcium carbonate means that the compound contains calcium, carbon and oxygen. Figure 7.5 The ‘limelight man’ provided light in theatres before electric light was used. He directed the flame of a mixture of the gases oxygen and hydrogen onto a piece of calcium oxide, which produced a powerful white light in the heat.

1 Which elements are present in the following compounds? a calcium oxide b sulfur dioxide c potassium hydroxide d calcium sulfate e copper carbonate Table 7.1 Some of the compounds that you may meet in your science course.

Compound

Description

calcium oxide

• a white solid used to produce limelight (see Figure 7.5) • commonly called quicklime

carbon dioxide

• a colourless gas produced in respiration

copper oxide

• a black pigment used to colour pottery

sulfur dioxide

• a colourless gas produced when fossil fuels are burnt • produces acid rain

calcium hydroxide

• a white solid used in the treatment of sewage and drinking water • used to make calcium hydroxide solution (limewater) for laboratory tests

potassium hydroxide

• a colourless solid used in soapmaking • makes an alkaline solution used in laboratories

sodium hydroxide

• a colourless solid used in soapmaking • makes an alkaline solution used in laboratories

sodium chloride

• a white crystalline solid • also known as common salt

Compound

Description

potassium chloride

• a white crystalline solid used in making fertiliser

calcium chloride

• a white solid which absorbs water vapour from the air • used to keep material dry

calcium sulfate

• a white solid • used to make plaster of Paris

Figure 7.6 This broken limb is protected by a plaster of Paris cast to help it heal.

copper sulfate

• can form a white powder or bright blue crystals

Figure 7.7 Copper sulfate crystals.

calcium carbonate Figure 7.8 Pinnacles Desert in Australia is a limestone landscape.

copper carbonate Figure 7.9 The mineral malachite is made from copper carbonate.

• a white solid • a component of snail shells and egg shells • present in large amounts in limestone • a blue-green solid which can form the mineral called malachite

Different types of mixtures

A mixture is composed of two or more separate substances. The composition of a mixture may vary widely. One mixture of two substances, A and B, might have a large amount of A and a small amount of B. Another mixture might have a small amount of A and a large amount of B.

Modelling mixtures You will need: three pieces of differently coloured clay.

Plan Using Figure 7.2 (page 78) for reference, but keeping the different coloured clay balls a similar size, make models of the following and photograph them: a a mixture of air showing four times as many nitrogen particles as oxygen particles b a mixture of water particles and oxygen particles to represent a water sample with a small amount of dissolved oxygen c the mixture of air and helium around a deflating helium balloon, where small amounts of helium leak through the balloon wall.

Science extra: Types of mixture There are many different types of mixture, as the following examples show. Table 7.2 Different types of mixture.

Type of mixture

Examples

solid mixed • soil contains clay, silt and sand with a solid

Type of mixture

Examples

solid mixed • clay and water – the clay particles are suspended with a in the water and make a mixture called a liquid suspension • if the solid dissolves, a solution is made (see page 87) solid mixed • smoke with a gas liquid • milk is made from tiny droplets of oil in water – this mixed with type of mixture is called an emulsion a liquid • some paints are also emulsions gas mixed with a gas

• air contains nitrogen, oxygen, carbon dioxide and many other gases

liquid • mist is tiny droplets of water mixed with air mixed with • a suspension of liquid droplets in a gas is called an a gas aerosol gas mixed with a liquid

• bubbles of a gas trapped in a liquid form foam • foams can be used for shaving products and for giving protection from the Sun

Metal alloys Not all the metals we use are single elements. Some of them are a mixture of metals called an alloy. Bronze is an alloy which is a mixture of copper and tin. It makes a ringing sound when struck and is used to make bells (see Figure 7.10 on the next page) and cymbals. When a small amount of tin is added to copper, the mixture (bronze) is much harder than copper. This was an early material that was so strong and useful, archeologists named the time during which it was the main metal in use as ‘the Bronze Age’.

Figure 7.10 These bronze bells are used by Buddhist monks.

Brass is an alloy of copper and zinc. It is strong and corrosion resistant, and is used to make the pins in electrical plugs. Its shiny surface makes brass a suitable metal for making ornaments. Steels are very widely used alloys based on the metal iron. Carbon steels are made by mixing the metal iron with a small amount of the non-metal carbon. They are used to make many items, from springs to car bodies. Stainless steel is an alloy of steel and chromium. It does not rust as easily as other steels and is used to make knives, forks, spoons and kitchen sinks. The reason that alloys are useful is because of the arrangement of their particles. In pure metals, particles are arranged in organised layers. This means that when a force is applied to the pure metal these layers may slide over each other, distorting the metal (see Figure 7.11). Figure 7.11 The structure of pure metals means that particle layers can be easily rearranged if a force is applied, making pure metals malleable.

When an alloy is created, the particles of the second substance disrupt these organised layers, making it harder and introducing new properties. This is why alloys are generally much stronger than pure metals (see Figure 7.12). Figure 7.12 The structure of an alloy means that it is more difficult to rearrange particle layers, making alloys less malleable than pure metals.

Modelling an alloy You will need: a plastic box without a lid, forty small marbles and six large marbles.

Hypothesis

In a pure metal, all the atoms are the same and can move easily over each other. This makes the metal soft. If the atoms of another metal are mixed with the atoms of the first metal, they stop the atoms of the first metal flowing so freely and make the alloy harder. Is this hypothesis testable? If you cannot decide, look back at the Did you know? on page xiv.

Prediction The marbles are used in the model to represent metal atoms. When all the marbles are the same as in atoms of a pure metal, they will flow more easily than when a few larger marbles, representing the metals of the second atom, are added.

Process 1 Place the small marbles in the box, then tip the box towards you so the marbles form rows piled up on each other. 2 Gently move the box from side to side so that there is some movement in some of the rows of marbles. 3 Place six large marbles in different places among the small marbles. 4 Tip the box towards you again and move it gently from side to side and note the movement of the marbles.

Examining the results Compare your observations at steps 2 and 4.

Conclusion Compare your evaluation with the hypotheses and prediction and draw a conclusion. How satisfactory was the model? What could you do to make a better model?

Science in context Alloys in our world

The first alloy to be used by humans was an alloy of nickel and iron found in meteorites. These alloys were not made by humans, but formed as the planets were formed in the solar system. People simply picked up the alloys and shaped them into useful tools, such as arrowheads and knives. Some metals are found in the Earth’s crust in a pure form. These metals are called native metals, and copper, silver and gold are examples. These were also shaped into useful items, from tools to jewellery. In time, as people worked with metals, they learned how to extract other metals (such as tin and zinc) from their compounds in rocks, called ores. At the time, copper was the most widely used metal, so these metals were mixed with it to form alloys, which had different properties from the copper, and could be used in other ways. The first alloy was bronze, made from a mixture of copper and tin. The next alloy to be made was brass, made from copper and zinc. Over time, more alloys were made; today, most of the metals we use are alloys. Table 7.3 contains some examples.

The most widely used alloy is steel. It is formed by mixing iron with the non-metal carbon. The amount of carbon used to make the alloy affects its strength. Steels with a very small amount of carbon are soft, and can be pulled into wires and rolled into thin sheets to make cans. Steels with more carbon are stronger and are used to make car body panels. Steels with more carbon still are very hard and are used for making saws and drills. Steels can also have other metals added to them to give them other properties. For example, stainless steel is made from an alloy of steel, nickel and chromium and does not corrode, so it is used to make cutlery (knives, forks and spoons) and kitchen sinks. 2 Why is aluminium bronze particularly useful for making boats? 3 Why is duralumin particularly useful for making aircraft? 4 If stainless steel did not have its particular property, what might happen to cutlery? (Remember, cutlery is washed and cleaned many times after meals.)

Solutions The most common mixture used in chemistry investigations is the solution. A solution is made when a substance, called a solute, mixes with a liquid, called a solvent, in such a way that the solute can no longer be seen, although if it is coloured it may give its colour to the solution. This type of mixing is called dissolving. The solute may be a solid, a liquid or a gas. Figure 7.13 Mixtures.

5 6 7 8

What type of mixture is the muddy water in a flood? What type of mixture is a sandstorm? What is the difference between a solvent and a solute? What is the difference between a substance that is soluble in water and one that is insoluble in water?

Different solvents Water has been called the universal solvent because so many different substances dissolve in it. However, there are many liquids used as solvents in a wide range of products. Ethanol is used in perfumes, aftershaves and glues. Propanone is used to remove nail varnish and grease. Gloss paint is dissolved in white spirit. White spirit is a liquid made from a mixture of chemicals found in oil that is taken out of the ground. Substances that dissolve in one solvent do not necessarily dissolve in another. Salt dissolves in water but not in ethanol, but white sugar dissolves in both. A solute does not take part in a chemical reaction when it dissolves, so it can be recovered by separating it from the solvent.

Summary • Elements in a mixture keep their own properties.

• Elements in a compound lose their own properties. • A mixture of metals is called an alloy; they have different chemical and physical properties from their constituent substances. • The particle model can be used to represent elements, compounds and mixtures. • The particle model can be used to explain the difference in hardness between pure metals and their alloys.

End of chapter questions 1 When the atoms of two or more elements join together, what kind of substance do they form? 2 What is a mixture composed of? 3 a Name two properties of the element sulfur. b Name three properties of the element iron. c When iron and sulfur are made into a mixture, what happens when a magnet is passed over it? d When the iron and sulfur mixture is added to water, what happens? e What happens to the atoms of iron and sulfur when they are heated together? f What substance is formed when the atoms are heated? g Name two properties of this new substance. 4 a What is an alloy? b Name an alloy and state what it is made of. 5 What is the alloy that is used to make a magnets b aircraft bodies? 6 What is the difference between a suspension and a sediment? 7 If you put a solid substance in water and stirred it up, how could you tell if the substance was a soluble

b insoluble? Now you have completed Chapter 7, you may like to try the Chapter 7 online knowledge test if you are using the Boost eBook.

8 Physical properties of matter In this chapter you will learn: • about the two main groupings of elements as metals and non-metals • the common differences between metals and non-metals, referring to their physical properties.

Do you remember? • • • • •

What are the properties of a solid? What are the properties of a liquid? What are the properties of a gas? Can a solid change into a liquid? Explain your answer. Can water change into a gas? Explain your answer.

In Chapter 5 we looked at the three states of matter, then in Chapter 7 we discovered that matter was made from atoms and elements and that the elements could be arranged into a table, called the periodic table. To make them a little easier to study, scientists also divide elements into two groups – metals and non-metals.

Metals and non-metals

Figure 8.1 Part of the periodic table showing the positions of metals, non-metals and metalloids.

DID YOU KNOW? There are a small group of elements which have some properties of both metals and non-metals, and they are called metalloids.

As the number of elements is so large, scientists divide them into two groups – metals and non-metals. We are familiar with elements that are metals and can often recognise them by just one of their properties – they have a shiny surface.

Figure 8.2 Cooking pans are made of metal. Elements that are non-metals do not shine. They have dull surfaces. Two examples of non-metals are: • carbon, which you might see as charcoal in a barbecue • sulfur, a yellow substance that is used to make a wide range of items from car tyres to medicines.

Figure 8.3 The surface of sulfur does not shine like the surface of metals. However, an element must have other properties if it is to be put in the metal group or the non-metal group. The properties that scientists use to classify elements into metals and non-metals are shown in Table 8.1. Table 8.1 The properties of metals and non-metals.

Property Metal surface shiny physical state at usually solid room temperature can be shaped by pressing and strength stretching without breaking easily melting point usually high boiling point usually high conduction of good electricity

Non-metal dull solid, liquid or gas solids are usually soft or brittle usually low usually low very poor

1. 1 Name all the metals you know.

Most people have a good idea of what a metal is and can usually name a few different kinds. However, many people would find it difficult to identify a non-metal element, even though we are surrounded by them. The air is a mixture of non-metal elements. Most of it (78%) is made up of nitrogen and one-fifth (20%) is made up of oxygen. Chlorine is a non-metal element that is used worldwide to purify water by killing harmful microorganisms in it, thus making it fit for drinking. Chlorine is often used to keep the water clean in swimming pools. Another non-metal element that is widely used is phosphorus. Red phosphorus is used on the tip of matches and helps to produce a flame when the match is struck. As the phosphorus is rubbed against the matchbox, its temperature rises and the heat causes the non-metal to burst into flame.

Figure 8.4 Chlorine is used in many swimming pools to keep the water clean. Nearly all metals and non-metals have the properties shown in Table 8.1. For example, one property that all metals have is that they conduct

electricity. You can test whether a material conducts electricity or not by using the electrical circuit shown in Figure 8.14 (page 97). If the sample conducts an electric current, the bulb lights up.

1. 2 You are given a dull solid in the laboratory. How could you make some simple tests, without using heat, to see if it is a metal or a nonmetal?

Investigating everyday materials Anything that is made of matter is sometimes called a substance. In science it is usually called a material. People usually think of the word ‘material’ when they think about cloth, such as the clothes you are wearing now. But in science, the word ‘material’ is used to mean any substance around you. The wood in a table or desk is a material. So is the water coming from a tap, or the air around you. Humans have been selecting materials from the world around them from the earliest times and using the properties of these materials to help them survive. For example, the waxy coating of a large leaf has been used to make a waterproof shelter in a shower of rain for thousands of years.

Figure 8.5 The smooth sides of some of these iodine flakes shine like pieces of metal.

Figure 8.6 These children in a rainforest are using leaves for shelter.

Figure 8.7 What materials could you use to help you survive on a desert island?

1. 3 Imagine you have been cast away on a desert island (see Figure 8.7). What materials would you select to help you survive? Explain your answer.

Science in context Graphene Most of the materials we use are natural materials such as wood or clay, manufactured materials such as metals which are released from their ores, or plastic which is made from oil. Scientists are always working on these materials to improve their properties to make them more useful to us. Professor Andre Geim and Professor Kostya Novoselov were two scientists working on materials at the University of Manchester in England. They spent their Friday evenings working on experiments that were sometimes not linked directly to the work they did during the rest of the week. These experiments arose from the scientists developing ideas and testing them, which could be described as a playful approach to science.

Figure 8.9 Professor Andre Geim and Professor Kostya Novoselov. The professors knew that a material called graphene existed, that it was one atom thick and made from carbon atoms. Graphite is composed of carbon atoms arranged in layers, and the scientists set to work to see if the layers could be separated to produce graphene. They began by applying sticky tape to a piece of graphite, and then pulling it off. Flakes of graphite were

stuck to the tape but some were thinner than others. They took these thinner flakes and continued to separate them with sticky tape until they had a material just one carbon atom thick. Graphene is much stronger than steel, but has very little weight and is extremely flexible. It is transparent, is an electrical and thermal conductor and is 1 million times thinner than a human hair. Professors Geim and Novoselov made their discovery in 2004 and were awarded the Nobel Prize in Physics in 2010. Today there are many scientists working on graphene to produce it in larger quantities, explore its properties and develop it for many uses in our lives.

1. 4 Use the internet to find out about: 1. a the uses of graphene today 2. b the possible uses of graphene in the future.

LET’S TALK The Friday evening experiments developed from these scientists’ ideas and curiosity. Are there any subjects in science that you wish you had more time to study and perform experiments in? If there are, perhaps you can find others who share your ideas and, with a teacher’s help and approval, you could set up an after-school science club of your own.

The physical properties of materials There are a number of physical properties that materials may have. Here is a list of these properties, with some examples of materials that possess them.

• Surface appearance: Shiny, dull, rough or smooth. • Rigid: A rigid material cannot be bent or squashed. Rock has been used as building material from early times because it is rigid. • Flexible: A flexible material can be bent or squashed, but when the pushing or pulling force is removed it springs back to its original shape. Flexible pieces of wood have been used in the form of hunting bows to launch arrows at animals for thousands of years, and are still used today in some parts of the world. • Hard or soft: Most materials have a hard surface, which can be due to them being rigid. Materials such as sponge feel soft because of their flexibility.

Figure 8.9 This castle in Aragón, Spain, was built from rock hundreds of years ago.

1. 5 Look at the surfaces of the materials around you. Which are shiny, dull, rough or smooth? 2. 6 What would happen to the castle in Figure 8.9 if the rocks suddenly lost their rigidity?

Which material is the hardest?

You will need: some cardboard, some metal objects, a few different sorts of wood, some rock (sandstone, limestone) and some plastic (PVC, polystyrene).

Hypothesis You can find out if one material is harder than another by scratching one with the other and looking for scratch marks. Is this hypothesis testable? Explain your answer. Prediction 1. 1 A harder material will leave a scratch on a softer material. 2. 2 Materials can be sorted out into a list according to their hardness. Process 1. 1 Take a pair of materials and scratch one on the other. Find out which leaves the deeper mark. 2. 2 Repeat step 1 with every combination of the materials you have. Examining the results Compare how each material scratched others. Conclusion Compare your evaluation with the hypothesis and predictions. What do you conclude?

• Malleable: A malleable material can be shaped by hammering or by pressing, without the material cracking. It stays in its new shape after the shaping process has ended. Metals such as gold, silver and copper are malleable. They can be made into wires and bent to form jewellery such as necklaces, bangles and earrings. • Brittle: A brittle material breaks suddenly if it is bent or hit. You can snap a biscuit or a chocolate bar because they are brittle. • Absorbent: An absorbent material has holes in its surface through which water can pass, and it also has spaces inside where the water can collect. Absorbent cloths and papers are used to wipe up spills in kitchens and laboratories. Some kinds of rocks, such as sandstone or

limestone, are absorbent – there are gaps, called pores, between the rocky grains and water can pass through them or fill them up to make an underground store of water called an aquifer.

Figure 8.10 Gold is the most malleable of the metals used to make jewellery.

CHALLENGE YOURSELF 1. 1 Is the shell of a hen’s egg as brittle as the shell of a duck or goose egg? What investigation could you make to find out? You do not have to try the experiment, just plan it. 2. 2 Devise an investigation to compare the absorbent properties of different brands of paper towels. You may try this investigation if your teacher approves it.

• Waterproof: A waterproof material does not let water pass through it. There are two kinds of waterproof material – water-resistant materials and water-repellent materials. – A water-resistant material is made of fibres. There are holes between the fibres through which water could pass. However, the fibres are coated in silicones, which make the water gather up into large droplets that cannot pass through the material. Waterresistant materials are used to make umbrellas, outdoor jackets and trousers. – A water-repellent material does not have any holes in it through which water can pass. Water-repellent material is used to make rain boots, for example.

CHALLENGE YOURSELF Design an investigation to compare the waterproof property of a range of materials. What should you do to make your test fair? When you have collected your materials, predict which will be the most and least waterproof. Check your investigation plan with your teacher and, if it is approved, carry it out, analyse and evaluate your data, and draw conclusions.

• Transparent: A transparent material is one that lets light pass through it without the light rays being scattered. This means that you can see objects clearly through transparent materials. You are looking through a transparent material at these words. Air is a transparent material and so is water, although we tend to think of transparent materials as being solids such as glass and plastic. • Translucent: A translucent material also lets light pass through it but the light rays are scattered. This means that you cannot see objects clearly. Translucent glass is used in bathroom windows. • Opaque: Opaque materials do not let light pass through them. Most materials are opaque but one material that is used especially for its opaque property is curtain fabric. Curtains prevent light entering a bedroom on a sunny morning and prevent people outside seeing into homes in the evening. Opaque materials are also useful in making sunshades. • Heat conductor: A material that is a heat conductor allows heat to pass through it. The particles from which the material is made pass heat energy along, from one particle to the next. • Heat insulator: A material that is a heat insulator does not let heat pass through it, because its particles do not pass heat easily from one to the next. Good insulators are also known as bad conductors.

Figure 8.11 In countries with long periods of sunny weather, parasols and canopies provide shade.

1. 7 Using a metal spoon, a plastic spoon, a wooden spoon and a piece of aluminium foil, a bowl of hot water and some butter and a knife, how could you find out which material is the best conductor of heat?

Figure 8.12 As heat is conducted along the metal rod, it glows red. The blacksmith holds the hot metal using a thick fabric glove – the fabric is an insulating material. • Electrical conductor: A material that is an electrical conductor allows an electric current to flow through it. • Electrical insulator: A material that is an electrical insulator does not allow an electric current to flow through it.

Figure 8.13 On this pylon, the metal cable conducts electricity between the power station and the city, while the glass insulators stop the current travelling to the pylon’s metal frame.

LET’S TALK What are the properties of a) your skin, b) a bone in your body?

Property profiles

A description of a material in terms of its properties is called its property profile. You can make a property profile of a material by testing it for each of the properties in the list that starts on page 94.

1. 8 Can metals, plastics and pottery be both rigid and flexible? Explain your answer.

When testing for flexibility it is important to consider the shape of the material. For example, a block of wood or plastic is not flexible, but a strip of the same material, such as a wooden or plastic ruler, is flexible.

DID YOU KNOW? A crocodile clip has the appearance of the teeth and jaws of a tiny crocodile, and is used to connect together components in an electrical circuit.

A solid can be tested to find out if it conducts electricity by using a circuit like the one shown in Figure 8.14. The solid to be tested is secured between the pair of crocodile clips and the switch is closed. The bulb lights up if the solid conducts electricity. By using this circuit, metals and the non-metal carbon in the form of graphite, are found to conduct electricity. Other non-metals such as plastic and pottery do not conduct electricity.

Figure 8.14 A circuit for testing conduction of solid materials.

Comparing the physical properties of materials If you make the property profiles of a few materials, you can make a table like Table 8.2 and see which materials could be grouped in the ‘absorbent group’, the ‘brittle group’ and so on.

CHALLENGE YOURSELF This is an opportunity to use your creativity to devise a series of tests to check the properties of materials in the table. Work out a test including the equipment needed for each property and, if your teacher approves them, try them.

Science extra: Challenge yourself Using the information of your studies of materials, select one of the activities below, work out a plan and, if your teacher approves, try it. 1. 1 Make a material for clothing for exploring a wet, very cold environment. 2. 2 Make a container in which soft fruit can be dropped from a certain height without being damaged. 3. 3 Make a turbine which will turn in the lightest of winds. 4. 4 Make a larger turbine and a support that can lift a 5 g weight on a string.

Summary 1. • The two main groupings of elements can be described as metals and non-metals. 2. • The common differences between metals and non-metals can be described by referring to their physical properties.

End of chapter questions 1. 1 How might you be able to tell a metal from a non-metal? 2. 2 Here are some items you may find in a kitchen: – metal saucepan – wooden spoon – glass measuring jug – cotton cloth. Here are some properties of materials: – absorbent – good conductor of heat – good heat insulator – transparent. 1. a Match each item with a property of the material from which it is made. 2. b How does this property of the material make the item useful when you are making a meal? 1. 3 a From which element is graphene made? 2. b How is the structure of graphene different from the structure of graphite? 3. c Name three properties that make graphene a very useful material. 1. 4 How many different materials are your shoes made from? What are the properties of each material? How are these properties useful?

CHALLENGE YOURSELF People living by a lake want to grow extra plants quickly. One idea is to make floating greenhouses. Make a model of a floating greenhouse. Test how it can ride waves on the lake. Make sure the greenhouse still floats when the plants grow up.

Now you have completed Chapter 8, you may like to try the Chapter 8 online knowledge test if you are using the Boost eBook.

9 Chemical reactions In this chapter you will learn: • to make predictions of likely outcomes for a scientific enquiry based on scientific knowledge and understanding • what happens in a chemical reaction • to observe, identify and state whether a chemical reaction has taken place • why a precipitate forms during a chemical reaction • to use a particle model to describe chemical reactions.

Do you remember? • If you have seen a chemistry set, describe what is in it. • If you have used a chemistry set, describe the experiments you performed and what happened. • If you have done some experiments previously with chemicals, describe what you did and what happened. • What are the substances called that interact in a chemical reaction, and what are the substances called that are produced?

LET’S TALK What do you understand by the word ‘chemical’? In Chapter 5 we examined how substances could exist in different states – solids, liquids and gases – the three states of matter. You already know from your previous studies how a substance can change from one state to another. These studies related to the physical properties of the substances and the physical changes that took place when the substances changed state.

In this chapter we are going to look at chemical reactions. The word ‘chemical’ is often used in everyday life to describe the substances found in cleaners used in the home to remove grease and stains. However, in science the word ‘chemical’ is used to describe the atoms and compounds that form any substance. In Chapter 6 we discovered that substances are made of atoms and elements and that the atoms could join together into groups called compounds. In this chapter we are going to look at what happens to these atoms and compounds when the substances they make take part in activities with other substances and new substances are produced. These changes are not physical changes but chemical changes, and they are brought about by chemical reactions.

Modelling a chemical reaction A chemical reaction can be modelled in two dimensions by using coloured circles to represent the atoms. A hydrogen atom can be represented by a white circle and an oxygen atom can be represented by a red circle, as Figure 9.1 shows. Figure 9.1 a A representation of a hydrogen atom; b A representation of an oxygen atom.

Hydrogen atoms occur naturally in pairs. They form a compound of two hydrogen atoms. This can be represented by two white circles joined together, as Figure 9.2 shows. Figure 9.2 A representation of a compound of two hydrogen atoms.

Oxygen atoms also occur naturally in pairs. This can be represented by two red circles joined together, as Figure 9.3 shows. Figure 9.3 A representation of a compound of two oxygen atoms.

Models of atoms can be used to show what happens to them when a chemical reaction takes place. In Chapter 5 you tested gases to identify

them. One of the gases was hydrogen and when you tested it with a lighted splint it popped. At the same time, the pairs of hydrogen atoms in the test tube also reacted with the pairs of oxygen atoms in the air, and water was produced. This reaction can be modelled as Figure 9.4 shows. Figure 9.4 The reaction between hydrogen and oxygen.

The model in Figure 9.4 shows that the pairs of hydrogen atoms separate, and the pair of oxygen atoms separate, then two atoms of hydrogen join with one atom of oxygen to form the compound we call water. A chemical reaction can be modelled in three dimensions by using balls to represent atoms, as you will discover when you try the following activity.

Modelling a chemical reaction You will need: a lump of white modelling clay and a lump of red modelling clay, a camera.

Process 1 Use Figure 9.4 to help you with this activity. 2 Take some white clay from the lump and make four small balls to represent hydrogen atoms. 3 Take some red clay from the lump and make two slightly larger balls to represent oxygen atoms. 4 Arrange the atoms into their pairs. 5 Split up the atoms and rearrange them to make the groups which represent water. 6 Set up the pairs of hydrogen and oxygen atoms again by repeating step 4. 7 Ask a friend to make a video recording of you rearranging the atoms as in step 5. You should explain what is happening as you rearrange the atoms.

8 Look back at your video and assess how well you explained the chemical reaction. What are the model’s strengths and limitations?

CHALLENGE YOURSELF When carbon burns in oxygen, a chemical reaction takes place which produces carbon dioxide. The size of an oxygen atom is approximately 0.3 nanometers (nm). The size of a carbon atom is approximately 0.1 nanometers (nm). Therefore, an oxygen atom is about 3 times larger than an atom of carbon. Make a 2-D model of this reaction using red circles for oxygen atoms and a black circle for a carbon atom. Make a 3-D model of this reaction using modelling clay and ask a friend to make a video of you explaining the chemical reaction. What are the strengths and limitations of your model and your presentation? When chemical changes occur, it is possible to observe chemical properties of substances (you will remember from Chapter 6 that physical properties can be observed without chemical reactions). Examples of these chemical properties include whether a substance is an acid or an alkaline, how reactive or stable a substance is, it’s flammability, and whether a substance is toxic or not.

When a candle burns The physical changes A candle is made from a solid called wax. It has a thread running through it called the wick. The top of the wick sticks out at the top of the candle. When a flame is brought near it, the wick starts to burn. As it does so, it heats the wax around the top of the candle and the wax changes into a liquid. This liquid wax around the wick is drawn up inside the wick by a force acting between the wax and the thread. This movement of the wax is called capillary action and it takes the molten wax to the top of the wick, where it is so hot that the wax changes into a gas called wax vapour.

Figure 9.5 A burning candle.

The chemical changes When the hot wax vapour mixes with oxygen, a chemical reaction takes place which produces a chemical change. The result of the chemical reaction is that water and carbon dioxide are formed. The chemical reaction in the flame of a burning candle can be investigated by setting up the apparatus shown in Figure 9.6. Figure 9.6 Testing the products of a burning candle.

The suction pump is attached to a water tap and is connected to the tube coming out of the calcium hydroxide (limewater) tube by a rubber tube. When the tap is turned on and the water runs away down the sink, it takes air out of the rubber tube and this is replaced by air passing into the apparatus around the upturned thistle funnel. This generates a current of air through the apparatus. The liquid in the U-shaped tube can be tested with cobalt chloride paper. This paper changes from blue to pink in the presence of water. The limewater turning cloudy shows the presence of carbon dioxide. 1 What physical process has taken place to produce liquid droplets in the delivery tube and the U-shaped tube?

What happens when a birthday candle burns? You will need: a thistle funnel, a long delivery tube, three pieces of rubber tubing, a test tube of calcium hydroxide (limewater), a stopper with delivery tubes like the one on the right in Figure 9.6 (with tops cut

short), a suction pump, a tap and sink, a birthday candle, modelling clay to hold the candle in place, a lighter, a Bunsen burner or spirit burner (perhaps), clamps and stands.

Plan and investigation 1 Look at the apparatus in Figure 9.6 on the previous page and assemble it so that air flows from the candle through the test tube of limewater to the tap. Make sure there is a gap between the candle and the thistle funnel for the air to flow easily. 2 Show your setup to your teacher and, if approved, try to collect chemicals from the burning candle. 3 Predict where you would expect water to collect and explain your answer. 4 Light the candle, turn on the tap and check that bubbles are flowing through the limewater. 5 Run the experiment until the limewater turns cloudy.

Examining the results How will you present your results, and can you explain your observations?

Conclusion Did your apparatus detect both the presence of carbon dioxide and water?

LET’S TALK When a liquid is frozen it changes into a solid but if it is warmed it turns back to a liquid again. You can say that the change is reversible because the original material can easily be obtained again. Is the chemical change taking place when a candle burns like that – a reversible change?

Science in context Classifying chemicals

The study of any scientific subject needs to begin with a classification of objects and materials. In chemistry, this was begun in the eighth century CE by Jābir ibn Hayyān, also known as Geber. He experimented with all the materials around him and found that he could classify them as follows: • Materials that would turn into a gas or vapour when heated, he described as ‘spirits’. • Copper, tin, iron and lead he described as ‘metals’. • Material which could be ground up to a powder – that is, nonmalleable substances. From early classification work like this, chemistry moved on to the study of chemical reactions. You will meet Geber again in Chapter 10, where his work on acids also helped to provide a basis for their study. 2 Could water be classified as a spirit? Explain your answer. 3 Name three other metals that could be put in Geber’s metal group. 4 Could sugar be classified as a non-malleable substance? Figure 9.7 shows a particle model representing condensation; it is the same as evaporation. It can be used to represent condensation of water in the apparatus of the burning candle experiment. Figure 9.7 Particles evaporating from a liquid surface.

Precipitates A precipitate is a solid substance that forms in a liquid. It does not dissolve in it, and particles of the precipitate may make the liquid cloudy, then settle at the bottom of the liquid. A precipitate can form when a chemical reaction takes place between two soluble compounds in a solution. This reaction results in a compound forming that is insoluble, so its solid particles enter the solution.

5 Make a drawing showing how condensation takes place using water particles. When the carbon dioxide from a burning candle passes into limewater it dissolves in it. Limewater is made from a compound called calcium hydroxide, which dissolves in water. When carbon dioxide enters the limewater, a chemical reaction takes place between the carbon dioxide and the calcium hydroxide that are dissolved in the water, and the compound calcium carbonate is produced. This compound is not soluble in water, so it forms a solid in the liquid and forms a precipitate. The precipitate makes the limewater cloudy. Many chemical reactions produce precipitates.

Signs of a chemical reaction In the candle experiment, you will have seen that the limewater in the test tube turns cloudy because of the presence of carbon dioxide and formation of calcium carbonate. The formation of a precipitate is one of the signs we can observe that show a reaction has taken place. The formation of a gas, such as carbon dioxide, is another. We call the substances formed during a reaction the products.

LET’S TALK Do chemical changes take place in cooking? Explain your answer. Other signs that a reaction has taken place include substances changing colour, or the reduction or loss of the substances that are taking part in the reaction. We call the substances taking part in the reaction the reactants. Observing or measuring a loss of reactants is a good way of determining whether a reaction has taken place. Understanding the likely results of a chemical reaction means you can make improved predictions and take focused observations.

Summary

• Predictions of likely outcomes for a scientific enquiry can be made based on scientific knowledge and understanding. • We can make observations to determine whether a chemical reaction has taken place. • A chemical reaction is a process which results in a chemical change. • Models can be made to describe the chemicals taking part in chemical reactions, and the chemicals being produced by chemical reactions. • Some substances do not dissolve in water; they are insoluble and form solid particles in the water, called a precipitate.

End of chapter questions 1 2 3 4 5 6 7 8 9

What is the word ‘chemical’ used for in science? What is a candle made from? In a candle, what is the wick? What are the signs that a chemical reaction has taken place when hot wax vapour and oxygen meet at the top of a candle? When investigating the substances produced by a burning candle, what is cobalt chloride paper used for? How is calcium hydroxide (limewater) used when investigating substances produced by a burning candle? How did Geber help in developing the study of chemistry? What is the difference between a reactant and a product? a When is a precipitate produced? b How would you recognise a precipitate in a test tube?

Now you have completed Chapter 9, you may like to try the Chapter 9 online knowledge test if you are using the Boost eBook.

10 Acids and alkalis In this chapter you will learn: • how to define the terms ‘acid’ and ‘alkali’ • how to use indicators to identify acidic, alkaline and neutral solutions • how neutralisation moves the pH of an alkali down towards seven • what happens in a chemical reaction.

Do you remember? • Water is a solvent. What does this mean? • Is water a solvent for every substance? Explain your answer by describing an investigation you may have carried out previously. • What do you think an acid is? • Can you name any acids? If you can, write them down.

Early acids and alkalis People have known since the time of the Ancient Egyptians and Greeks that some substances taste sour and some feel slippery. Vinegar is probably the best example of a sour-tasting liquid that early peoples would have known about. Early examples of slippery substances included potassium-rich chemicals, called potash, found in the ashes of burnt wood, sodium bicarbonate (soda) made from the evaporation of some solutions, and calcium oxide (lime) made from the burning of seashells. Scientists developed the word acid from acidus, which is the Latin word for ‘sour’. The word alkali was developed from al-qaliy, which is an Arabic word meaning ‘the ashes’. As we saw in Chapter 9, a great deal of early investigative work in chemistry was done in Islamic countries, starting about 1200 years ago.

Probably the greatest chemist at this time was Ja¯bir ibn Hayya¯n, who was also known as Geber. He worked on many investigations, which resulted in him devising new apparatus and discovering different kinds of acids. Figure 10.1 Scientists in Geber’s time showed their discoveries to others and built up a vast amount of knowledge about chemistry.

1 Which pieces of apparatus shown Figure 10.1 are similar to those we use today? Look at Figure 6 on page viii to help you answer. 2 How is the source of heat shown in Figure 10.1 different from the laboratory heat sources we use today?

Acids Most people think of acids as corrosive liquids that fizz when they come into contact with solids, and burn when they touch the skin. This description is true for many acids, and when they are being transported, the container holding them has the hazard symbol shown in Figure 10.2. Figure 10.2 The hazard symbol for a corrosive substance.

Some acids are not corrosive and are found in our food. They give some foods their sour taste. Many acids are found in living things. Table 10.1 on the next page shows some acids found in plants and animals. Figure 10.3 Animals and plants that produce acids. Table 10.1 Acids found in plants and animals.

Acids with plant origins

Acids with animal origins

Acids with plant origins

Acids with animal origins

• citric acid in orange and lemon juice • tartaric acid in grapes • ascorbic acid (vitamin C) in citrus fruits and blackcurrants • methanoic acid in nettle stings

• hydrochloric acid in mammalian stomachs • lactic acid in muscles during vigorous exercise • uric acid in urine • methanoic acid in ant stings

3 Look at Table 10.1 and state the organ systems in your body where three of the acids are found.

The acid in vinegar Ethanoic acid is found in vinegar and is produced as wine becomes sour. The wine contains ethanol and also has some oxygen dissolved in it from the air. Over a period of time, the oxygen reacts with the ethanol and converts it to ethanoic acid. This chemical reaction happens more quickly if the wine bottle is left uncorked.

LET’S TALK Name three properties of acids. Are they dangerous or useful? Explain your answer with examples. 4 Why does wine go sour faster if the cork is removed from the bottle?

CHALLENGE YOURSELF Examine food packaging (tins, bottle, packets and bags) from the kitchen in your home and look for the word ‘acid’. Make a list of the foods that use acids in their manufacture or preservation. How frequently are you eating acids?

Figure 10.4 A bottle of hydrochloric acid and its hazard symbols.

5 Acids in the laboratory are stored in labelled bottles, as shown in Figure 10.4. What do the hazard symbols tell you about the acid?

Alkalis Sodium hydroxide solution and potassium hydroxide solution are examples of alkalis that are used in laboratories. Calcium hydroxide, also called slaked lime, is used in many industries to make products such as bleach and whitewash. A weak solution of calcium hydroxide is used in the laboratory, where it is known as limewater. It is used to test for carbon dioxide gas. If this gas passes through limewater, a chemical reaction takes place between the carbon dioxide and the calcium hydroxide to produce calcium carbonate, which is insoluble and forms a precipitate which turns the lime water cloudy or milky. Figure 10.5 Limewater is clear, but a precipitate of calcium carbonate forms in it when carbon dioxide is bubbled through.

A concentrated solution of an alkali is corrosive and can burn the skin. The same hazard symbol as the one used for acids (Figure 10.2) is used on containers of alkalis when they are transported. Even dilute solutions of alkali, such as dilute sodium hydroxide solution, react with fat on the surface of the skin and change it into substances found in soap. Many household cleaners used on metal, floors and ovens contain alkalis and must be handled with great care. Figure 10.6 Alkalis used in the home.

6 Why should alkalis be treated with care?

Science in context

Detecting acids and alkalis The work of Geber (see page 104) helped other scientists to set up investigations on acids many centuries later. Robert Boyle was an Irish scientist who lived just over 300 years ago. He studied acids and alkalis and decided to try and find an easy way to identify them. He knew that in France, workers who made silk clothes dyed them with the juices of plants, and he began testing plant juices to see if they would solve his problem. When Boyle tested acids and alkalis with the juice from red cabbage, he found a way to identify them easily. When acid is added to red cabbage juice, it turns from purple to red. When alkali is added, the juice turns from purple to green. He also found that juices from violets (a flower with purple petals) turned purple with acid and greenish yellow with an alkali, but his discovery about the colour change in litmus, a juice from a lichen, went on to be used in chemistry laboratories around the world. Figure 10.7 Robert Boyle and his assistant at work in his laboratory.

Litmus is used as a solution or it is absorbed onto paper strips. Litmus solution is purple but it turns red when it comes into contact with an acid. Litmus paper that is used for testing for acids is blue. The paper turns red when it is dipped in acid or when a drop of acid is put on it. When an alkali comes into contact with purple litmus solution, the solution turns blue. Litmus paper that is used for testing for an alkali is red. When red litmus paper comes into contact with an alkali, it turns blue. 7 What question do you think Boyle asked himself when he learnt about the work of the French dyers? 8 Which scientific skill did Boyle use in his tests on plant juices? 9 When Boyle tested a liquid with red cabbage juice, it turned the indicator from purple to red.

a What was the liquid? b What colour would it turn Boyle’s violet juice? 10 When Boyle tested a liquid with litmus solution, it went blue. a What was the liquid? b What colour would it turn violet juice? c What colour would it turn red cabbage juice? 11 Compare the laboratory of Geber (Figure 10.1) with that of Boyle (Figure 10.7). a Do any pieces of apparatus in the different laboratories look similar? b What differences do you notice?

Can you make an indicator? You will need: some chopped red cabbage, a Bunsen burner or spirit burner, a heat-proof mat, a tripod, gauze, a sieve, two beakers, a stirrer, a dropping pipette, shallow glass dishes, such as petri dishes, eye protection, and test samples (namely: lemon juice, lime juice, vinegar, sodium bicarbonate (baking powder), sodium carbonate (washing soda), hydrochloric acid and sodium hydroxide solutions at concentrations approved by your teacher for laboratory use).

Investigation 1 Set up the heatproof mat, Bunsen burner or spirit burner, tripod and gauze. 2 Half-fill a beaker with water, place it on the gauze and heat it with the Bunsen burner or spirit burner until the water boils. 3 When the water boils, turn off the heat and put the chopped cabbage carefully into the beaker and stir the water and cabbage together.

4 Leave the mixture until the water has cooled down, then place a sieve over a second beaker and carefully pour the mixture through the sieve into the empty beaker. 5 Put the test samples in watch glasses or petri dishes, and use the dropping pipette to collect some coloured cabbage water from the beaker and place drops of it on each sample.

Examining the results Have you made an indicator? Explain your answer. Is there anything you could do to improve your experiment or results?

Work safely Take care with sodium hydroxide and washing soda; they are irritants. Eye protection should be worn for this experiment. Make sure that the tripods have cooled down before clearing them away.

Can plants be used as indicators? There are many non-poisonous fruits which are colourful, and some leaves like the cabbage leaf are coloured too. The petals of many flowers are also coloured.

Hypothesis The colourful parts of non-poisonous fruits, leaves and petals can be used to make indicators.

Prediction

Make a prediction about each plant part you test based on the hypothesis.

Plan, investigation and recording data Construct an experimental procedure to test your hypothesis and prediction and show them all to your teacher. If approved, try your investigation, then collect, analyse and evaluate your data and draw one or more conclusions. Do you have enough evidence to support your conclusions?

Using indicators There are over 20 indicators that scientists use. There is even a plant that can be used as an indicator as it grows in the soil – the hydrangea. The colour of its flowers can be affected by alkali in the soil. Hydrangeas have pink flowers when they are grown in a soil containing lime (calcium hydroxide, an alkali) and blue flowers when grown in a lime-free soil. The colour of the flowers can be used to assess the alkalinity of the soil that the plant is in.

LET’S TALK What is an indicator and why is it useful?

Figure 10.8 Pink and blue hydrangeas.

Science in context The pH scale After indicators had been found to identify acids and alkalis, scientists wanted to know how to compare the strengths of acids and alkalis. In 1909, a Danish scientist called Søren Sørensen invented a scale called the pH scale to do just that. The letters p and H stand for the ‘power of hydrogen’, because this is an element

that is found in acids which takes an active part in their chemical reactions. Figure 10.9 The pH scale (top) and universal indicator (bottom).

The pH scale runs from 0 to 14. On this scale, the strongest acid is 0 and the strongest alkali is 14. A strong acid has a pH of 0–2, a weak acid has a pH of 3–6, a weak alkali has a pH of 8–11 and a strong alkali has a pH of 12–14. A solution with a pH of 7 is neutral. It is neither an acid nor an alkali. Today, an electrical instrument called a pH meter is used to measure the pH of an acid or alkali accurately and, for general laboratory use, the pH of an acid or an alkali is measured with universal indicator. This is made from a mixture of indicators. Each indicator changes colour over part of the range of the scale. By combining the indicators, a solution is made that gives different colours over the whole of the pH range (see Figure 10.9). 12 Look at Sørensen’s laboratory in Figure 10.10. How is it different from the laboratories of Geber and Boyle? Figure 10.10 Søren Sørensen at work in his laboratory.

13 Here are some measurements of solutions that were made using a pH meter: – solution A: pH 0 – solution B: pH 11 – solution C: pH 6 – solution D: pH 3 – solution E: pH 13 – solution F: pH 8 a Which of the solutions are: i acids

ii alkalis? b If the solutions were tested with universal indicator paper, what colour would the indicator paper be with each one? c Fresh milk has a pH of 6. How do you think the pH would change as it becomes sour? Explain your answer. 14 Here are some results of solutions tested with universal indicator paper: – sulfuric acid: red – metal polish: dark blue – washing-up liquid: yellow – milk of magnesia: light blue – oven cleaner: purple – car-battery acid: pink Arrange the solutions in order of their pH, starting with the lowest. 15 Identify the strong and weak acids and alkalis from the results shown in Questions 13 and 14. 16 Look at pages 108–109 about acids and predict whether nitric acid is a strong or a weak acid. Explain your answer.

How is universal indicator used to test acids and alkalis? You will need: a test tube rack with four test tubes labelled A–D; in A is a sodium hydroxide solution, in B is hydrochloric acid, in C is enthanoic acid and in D is an ammonia solution, and a bottle of universal indicator solution with a dropper.

Investigation 1 Add a few drops of the indicator solution to each test tube in turn and note down the pH of the solutions.

2 List the solutions in order of increasing pH.

Work safely Take care with sodium hydroxide; it is an irritant. Eye protection should be worn for this experiment.

Neutralisation When an acid reacts with an alkali, a process called neutralisation occurs, in which a salt and water are formed. Sodium hydrogen carbonate is a white solid. It is not an alkali, but dissolves in water to produce an alkaline solution. It also takes part in neutralisation reactions with acids, but produces another substance as well as a salt and water: carbon dioxide. Sodium hydrogen carbonate is also called sodium bicarbonate. It has several uses in neutralisation reactions. Some of these are described in the next section. Neutralisation reactions will work both from acidic pH to a neutral pH and from an alkali pH to a neutral pH. You should see this movement on the pH scale upwards as you add an alkali to an acid until the pH reaches pH 7. Likewise, as you add an acid to an alkali you will see the pH measurement move down the scale until it reaches pH 7. 17 Why are alkalis sometimes described as the opposite of acids? 18 How are acids and alkalis similar? 19 In a laboratory, hydrochloric acid is added to potassium hydroxide until the pH measure is pH 7. What would you expect to see on a pH meter if you continued to add more hydrochloric acid?

Using neutralisation reactions Insect stings A bee sting is acidic and may be neutralised by soap, which is an alkali. A wasp sting is alkaline and may be neutralised with vinegar, which is a weak acid.

Curing indigestion Sodium bicarbonate is used in some of the tablets that are made to cure indigestion. Indigestion is caused by the stomach making too much acid as it digests food. When a tablet of sodium bicarbonate is swallowed, the chemical dissolves to make an alkaline solution, which neutralises the acid in the stomach and cures the indigestion.

Baking a cake Baking powder contains a mixture of a solid acid and sodium bicarbonate. When the baking powder is mixed with water and flour to make a cake, the acid and the sodium bicarbonate dissolve in the water and take part in a neutralisation reaction. The carbon dioxide gas forms bubbles in the mixture and makes it rise to give the cake a light texture. Figure 10.11 The acid or alkali used in an insect sting is delivered by a sharp point at the tip of the insect’s abdomen.

A model volcano In the past, you may have made a model volcano. To do this, you add a tablespoon of sodium bicarbonate, called baking soda, to an empty plastic drinks bottle and then build a mound of sand around the bottle so that it looks like a conical volcano. Finally, you add red dye to half a cup of vinegar, then pour the vinegar into the bottle. Moments later, a red froth emerges from the top of the bottle and flows down the cone of sand, like lava flowing down a volcano (Figure 10.12). The model looks impressive!

It does not illustrate how lava is formed, but it does show the power of a neutralisation reaction – between the baking soda and the vinegar. Figure 10.12 The ingredients (left) for making a model volcano (right).

Fighting a fire The soda–acid fire extinguisher contains a bottle of sulfuric acid and a solution of sodium bicarbonate. When the plunger is struck or the extinguisher is turned upside down, the acid mixes with the sodium bicarbonate solution and a neutralisation reaction takes place. The pressure of the carbon dioxide produced in the reaction pushes the water out of the extinguisher and onto the fire.

Improving crop growth Acidity in the soil affects the growth of crops and makes them produce less food. Lime (calcium hydroxide) is used to neutralise acidity in soil. When it is applied to fields, it makes them appear temporarily white. 20 Describe how naturally acidic rainwater can affect a mountain of limestone. 21 A sample of acid rain turned universal indicator yellow. What would you expect its pH to be? Is it a strong or a weak acid?

Summary • Some acids are made by living things. • Sodium hydroxide and potassium hydroxide are examples of alkalis. • We use indicators to identify acidic, alkaline and neutral solutions: an acid turns blue litmus paper red; an alkali turns red litmus paper blue. • The pH scale is used to measure the degree of acidity or alkalinity of a solution, and acidity and alkalinity are chemical properties.

• When an acid reacts with an alkali, a neutralisation reaction takes place. • Neutralisation reactions have a wide range of uses.

End of chapter questions 1 a Name two acids made by plants. b Name two acids produced by animals. 2 Wine takes part in a chemical reaction with a gas in the air to make ethanoic acid. a What is the gas in the air? b What is the common name for ethanoic acid? 3 What property do indicators have that helps in the study of acids and alkalis? 4 A solution is shown to be pH 1 with universal indicator paper. What colour will it turn a purple litmus solution? 5 A hydrangea has pink flowers. You collect some soil from around it and test it with litmus solution. What colour would you expect the litmus solution to turn? 6 What are the products when a neutralisation reaction occurs? 7 Write an account entitled ‘The acids and alkalis in our lives’. Now you have completed Chapter 10, you may like to try the Chapter 10 online knowledge test if you are using the Boost eBook.

11 Measurement In this chapter you will learn: • to take accurate and precise measurements • why accuracy and precision are important • how to collect and record observations and/or measurements in an appropriate form • to describe trends and patterns in results, including any anomalous results.

Do you remember? • What do you use to measure the length of something? What units do you measure in? • What do you use to measure weight? What units do you measure in? • What do you use to measure time? What units do you measure in? • What do you use to measure temperature? What units do you measure in?

Phenomena Physics is the scientific study of how matter and energy interact. These interactions can produce the colours of the rainbow in a shower or the roar of the wind in a hurricane. At a greater distance, the interactions of matter and energy in the Sun produce light and heat, while inside our eyes, stores of energy in light are transferred to electricity, which passes to our brain and allows us to see.

Figure 11.1 The white light in sunbeams is split by the water in rain droplets to produce an arch of coloured bands in the sky.

Every event in the universe, from your next breath, to a star exploding, is an interaction of matter and energy, so physics is really a part of all the other scientific subjects, rather than a separate one. All the information we gather with our senses (such as the presence of light) and events (such as the formation of a rainbow) are called phenomena (singular: phenomenon). So physics can also be described as the science of investigating phenomena.

Launching a rocket Sometimes we can observe several phenomena in an event, as the following example of launching a rocket shows. Light from the rocket engines can be seen immediately by the distant spectators as the rocket begins to rise from the launch pad. When the roar of the rocket engines reaches the spectators, it nearly deafens them. The rocket’s speed increases every second as it rises into the sky.

Figure 11.2 Three, two, one, lift off! A multistage rocket leaving the launch pad.

The rocket is divided into parts, called stages. Each stage has fuel tanks and rocket engines. When the fuel is used up in one stage, that stage will separate from the rocket and fall back towards Earth. As the stage rushes back through the atmosphere, it will become so hot that it will burn up. When the last stage has separated, only a small spacecraft will remain in orbit around the Earth, or set off across the solar system.

LET’S TALK After reading about the rocket launch, a person asked, ‘Why was light from the rocket seen before the sound of the rocket was heard? Why did the stages fall back to Earth when they separated from the rocket? Why did the stages burn up in the atmosphere?’

What explanations can you give to answer these questions?

Fooling our senses Occasionally what we detect with our senses fools us into thinking we are seeing something else. When this happens we are fooled by an optical illusion. All of our senses can be fooled into seeming to detect something that is not really present. This means that, instead of relying on our senses, we need to make more accurate observations of phenomena in science. We do this by taking measurements. 1 Look at the three lines in Figure 11.3a and write down their letters in order of length, starting with the longest. Repeat the exercise with the lines in Figure 11.3b. When you have finished, check your answers by measuring the lines. What does this tell you about your senses and the need to take measurements? a

Figure 11.3a

b

Figure 11.3b

2 A girl puts her left hand in a bowl of cold water and her right hand in a bowl of hot water. After a minute she puts both hands in a bowl of warm water. How do you think the left hand and the right hand will feel in the bowl of warm water?

CHALLENGE YOURSELF Plan an experiment to answer Question 2. Show your plan to your teacher and, if it is approved, try it. What answer does your experiment provide?

Length, mass and time Three things that are measured in many investigations are length, mass and time. Any measurement is made in units – for example, a common unit of length is the centimetre (cm). There is an international system of units that is used by scientists across the world. This is known as the Système International d’Unités. The units in this system are known as SI units.

Measuring length The standard SI unit of length is the metre. Its symbol is m, and, as with all symbols for SI units, no full stop is placed after it. The metre is divided into smaller units for measuring smaller lengths or distances, and large numbers of metres are made into bigger units to measure longer lengths or distances. Table 11.1 shows some of these other SI units. Table 11.1 Units of length.

Unit

Symbol

kilometre

km

metre

m

1m

centimetre

cm

0.01 m

millimetre

mm

0.001 m

micrometre

μm

0.000 001 m

nanometre

nm

0.000 000 001 m

Measuring mass

Number of metres 1 000 m

The standard SI unit of mass is the kilogram, whose symbol is kg. The other SI units of mass used in investigations are shown in Table 11.2. Table 11.2 Units of mass.

Unit

Symbol

Number of kilograms

megatonne

Mt

tonne

t

kilogram

kg

1 kg

gram

g

0.001 kg

milligram

mg

0.000 001 kg

1 000 000 000 kg 1 000 kg

Figure 11.4 Measuring mass.

Measuring time The standard SI unit of time is the second, and its symbol is s. Other units of time used in investigations are shown in Table 11.3. Table 11.3 Units of time.

Unit

Symbol

Number of seconds

day

d

86 400 s (or 1440 min, or 24 h)

hour

h

minute

min

3 600 s (or 60 min) 60 s

Unit

Symbol

Number of seconds

second

s

1s

millisecond

ms

0.001 s

3 a What is the mass of the objects on each of the balances shown in Figure 11.4? b Which balance was the easiest to read?

4 a If you saw someone commit a crime, how might you describe to a detective the appearance of the criminal and what happened? b Do mass, length and time feature in your answer? If they do, say where they occur.

Estimating quantities At the beginning of an investigation, it may be useful to estimate the quantities that are going to be used or the time that is going to be taken for certain observations. At this stage of the investigation, accuracy is not essential – that comes later. 5 Estimate the following quantities and then check your answers by measurement. a the length of your index finger (the first finger on each hand, next to the thumb) and the length of your thumb b the height of your chair c the distance between you and a door d the mass of: i this book ii your school bag and its contents e the time it takes you to:

i count the first 50 words under the heading ‘Accuracy of measurements’ ii say those 50 words iii write down those 50 words 6 a How could you estimate the thickness of a page of this book? b Write down your estimate of the thickness of a page of this book and compare it with the estimates made by others. Are all the estimates the same? Explain what you discover.

Accuracy of measurements On page xv of the Introduction, we saw that accuracy and precision are important in collecting data in scientific enquiries. Here we take a closer look by considering an instrument that is frequently used in physics enquiries: the stop-watch. Your accuracy when making a measurement depends on the measuring instrument – how well it has been made and calibrated (compared with the standard), and how well the scale on the instrument has been constructed. A stop-watch that only measures time by seconds cannot be used to time events to a tenth of a second, for instance. Your accuracy also depends on how well you use the measuring instrument. Care in setting up the device is needed. For example, you must place a ruler accurately when measuring length (both ends are important), reset a stopwatch before repeating a timing, and make sure that a balance is set at zero before a mass is put on it. If a balance is used with a scale which is read by looking at the position of a pointer, your eye should be positioned directly in front of the pointer.

Figure 11.5 It is important to position your eye correctly, looking horizontally, when taking readings.

7 In Figure 11.5, how would looking at the pointer from positions A and B affect the accuracy of the measurement?

Precision When you make measurements with an accurate instrument (one that can measure the dimensions you need), you also need precision. This means that when you take a measurement, you should take it a few times to check that the measurement stays the same. If it does, your measurement is precise and can be used as a reliable source of data. If the measurement varies then it is not precise and cannot be used as a reliable source of data. Measurements must be taken until they are the same. When this happens, the measurements become precise and reliable. Try the following enquiry to test your ability to make precise measurements.

Can the speed of a liquid be measured precisely?

You will need: a slope made from a long, smooth piece of plastic or polished wood, a measuring cylinder, a bottle of smooth tomato sauce, a stopwatch, some blocks or books to hold up one end of the slope, and a ruler.

Plan • Decide how steep to make the slope. • Decide on the volume of liquid to use. • Decide on how long you will time the flowing liquid – for a certain distance down the slope or for the whole slope. • Decide how many times you will test the liquid to try and achieve a precise result. • Prepare a table in which to record your results. • Show your plan to your teacher and, if approved, try it.

Investigation and recording data Try your investigation and record your measurements. You may take more measurements if you need to, if your teacher approves.

Examining the results Look through your measurements to find a precise speed for the tomato sauce to flow. Identify any anomalous results (they should only occur in the early part of your investigation, as your ability to make precise measurements improves).

Conclusion 1 Compare the data you have collected with the question to be answered and draw a conclusion. 2 Is your conclusion limited in some way? Explain your answer. 3 What improvements could be made to your investigation? Explain the changes that you suggest. 8 Why are precise measurements important in science?

Science in context Material-testing laboratories Many of the materials we use every day have been tested in material-testing laboratories. Each material is tested for many different properties, but equipment is used to measure properties such as strength and electrical conductivity. The data provided by these measurements help the manufacturers select materials which are particularly suitable for making their product. Figure 11.6 shows a material-testing laboratory, where the properties of materials for making textiles (clothes and furnishings) are tested.

Figure 11.6 A material-testing laboratory.

There are many tests that are made on materials. One test is about measuring the strength of a material by pulling on it until it tears and measuring the force that produced the tear. 9 Why do you think it is necessary to test a material to see the force needed to tear it?

10 Why do you think accuracy and precision are important in the measurement of the force that tears the material?

Heat and temperature The hotness or coldness of a substance is measured by taking its temperature. The temperature of a substance is measured on a scale that has two fixed points. The most widely used temperature scale is the Celsius scale. Its two fixed points are 0 ºC (the melting point of ice or freezing point of water) and 100 ºC (the boiling point of water). In between these two fixed points, the scale is divided into 100 units or degrees. The scale may be extended below 0 ºC and above 100 ºC; laboratory thermometers usually have a scale reading from −10 ºC to 110 ºC. A thermometer compares the temperature of the substance in which the bulb is immersed with the freezing point and boiling point of water. That is, it compares the hotness or coldness of a substance. 11 How much hotter is: a 45 ºC than 30 ºC b 20 ºC than −15 ºC? 12 Why are two fixed points needed for a temperature scale rather than just one? The lowest possible temperature, known as absolute zero, is −273 ºC. Temperatures can go as high as millions of degrees Celsius.

Liquids in thermometers Two liquids that are commonly used in thermometers are mercury and alcohol. Mercury has a freezing point of −39 ºC and a boiling point of 360 ºC. Alcohol has a freezing point of −112 ºC and a boiling point of 78 ºC.

Figure 11.7 The Celsius scale of temperature.

If the bulb of the thermometer is placed in a hot substance, the liquid inside moves up inside the thermometer tube. The level it reaches depends on the hotness of the substance, and its temperature can be read from the scale. If the bulb is then placed in a cold substance, the liquid inside goes back down

the tube. The level at which it settles depends on the coldness and, again, its temperature is read from the scale.

LET’S TALK When you take the temperature of the liquid, should you quickly dip the bulb of the thermometer into the liquid and then take it out, leave it for a minute and then take the temperature? Explain your answer. 13 Which thermometer, one containing mercury or one containing alcohol, could be used in a polar region where the temperature reaches below −40 ºC? Explain your answer. 14 Which type of thermometer could be used to measure the boiling point of water? Explain your answer.

Summary • Information we gather with our senses and via events is called phenomena; phenomena are investigated in physics. • Investigations are carried out by measuring length, mass and time in an appropriate form. • It is important to be accurate and precise with measurements so that data in results are reliable. • Results can show trends and patterns from which we can draw conclusions, but can also contain anomalous results.

End of chapter questions 1 2 3 4 5 6

What is physics the scientific study of? What is the standard SI unit of length? What is the standard SI unit of mass? What is the standard SI unit of time? What is the most widely used temperature scale? What is the lowest possible temperature?

7 How could you fool someone’s senses? 8 A model rocket was launched, and measurements of its height and horizontal distance from the launch pad were taken, as shown in Table 11.4. Table 11.4

Horizontal distance from launch pad/m

Vertical distance from launch pad/m

0

0

1

4

2

8

3

11

4

13

5

14.2

6

15

7

15.5

8

15

9

13

10

10

11

0

a Plot the flight path of the model rocket from these measurements. b What was the vertical distance from the launch pad when the rocket stopped rising? c What was the horizontal distance from the launch pad when the rocket stopped rising? d What was the horizontal distance from the launch pad when the rocket hit the ground?

e If the rocket had been 10 m above the ground when its horizontal distance from the launch pad was 2 m, where do you think the rocket would have landed? Now you have completed Chapter 11, you may like to try the Chapter 11 online knowledge test if you are using the Boost eBook.

12 Energy In this chapter you will learn: • about transfers of energy that are a result of an action or process • how energy can dissipate and, in doing so, become less useful • that gravity is a force of attraction between any two objects how the size of the gravitational force is related to the mass of the object.

Do you remember? • Where do you find energy? • Do you need energy? Explain your answer. • Name something else that needs energy and explain your answer.

Thinking about energy In everyday language, we use the word ‘energy’ in many different ways. Just look at the examples in Figure 12.1.

Figure 12.1 Different ways of using the word ‘energy’.

LET’S TALK Think about conversations where you might use the word ‘energy’. What do you mean when you use the word? The scientific way of thinking about energy is that it is a property of something that lets it move or take part in some sort of action like a chemical reaction.

Stores of energy

One way to classify energy is to think about it as being stored in different ways. Some people talk about different types of energy, but it is important to remember that if we say, for example, that electrical energy in a circuit is changed to light and heat energy, it is not the energy that is changing, but how we can see, feel or measure it. Energy can be classified into two groups: • potential energy, which depends on the position of something • movement energy, which is also called kinetic energy.

Potential energy There are different examples of potential energy.

Gravitational potential energy Gravity is a force of attraction between any two objects in the universe. It exists between large objects such as stars and planets, and even between very small objects like two ants walking across the ground. The size of the force of gravitational force varies, and is related to the mass of the object. A star has a great deal of mass, and its force of gravity pulls on planets around it and keeps them in their orbits. An ant has a very tiny mass and a very tiny force of gravity. The force is also so small that its pull does not affect other ants or any objects around it. The force of gravity of humans also does not affect anything around us. 1 If you are holding this book, or if it is resting on a table or desk, why does it possess potential energy? 2 If you held a stone over the mouth of a well and then let it go, what would happen to the stone? Explain your answer. If one object has a large mass and another object has a small mass, the force of gravity of the large mass is greater than the force of gravity of the small mass. This difference in the strength of the forces may result in the object with the small mass being pulled towards the object with the large mass. All the objects on Earth have much less mass than the planet, and the much larger gravitational force of the Earth pulls them towards the centre of the planet.

If an object is in position above the surface of the Earth, it possesses stored energy called gravitational potential energy. Examples of objects with this type of gravitational potential energy are plates on a table, books on a shelf, a child at the top of a slide and an apple growing on a branch. Each of these objects is supported by something, but if the support is removed they will move quickly to the Earth’s surface and their potential energy will be released and changed into other forms of energy.

Figure 12.2 When the objects fall, their gravitational potential energy reduces.

Strain energy Strain energy is also called elastic potential energy. Some materials can be easily squashed, stretched or bent, but they spring back into shape once the force acting on them is removed. They are called elastic materials. When their shape is changed by squashing, stretching or bending, they store energy in a form which will allow them to return to their original shape. A spring stores strain energy when it is stretched or squashed. Gases store strain energy in them when they are squashed. For example, when the gas used in an aerosol is squashed into a can, it stores strain energy, some of which is used up when the nozzle is pressed down and some of the gas is released in the spray.

Figure 12.3 Places where strain energy can be stored and released.

3 Look at Figure 12.3. When is elastic potential energy stored and when is it released in: a a toy glider launcher b the elastic cords or springs beneath a sun lounger c a diving board?

Chemical energy Energy can be stored in the chemicals from which a material is made. Food, fuel and the chemicals in an electrical cell (or battery) are examples of stored chemical energy. Chemical energy can be released in various processes. For example, carbohydrates are a store of chemical energy in food. During respiration, carbohydrate is broken down into carbon dioxide and water. The energy that is released in this process is used by your body to keep you alive. Another example is the energy released by the burning of a fuel is used to heat homes, to heat water to produce steam for generating electricity in power stations, and for the production of new materials.

Figure 12.4 Energy is stored in all of these objects.

Kinetic energy The word ‘kinetic’ comes from a Greek word meaning ‘motion’. Any moving object has kinetic energy. The object may be as large as a planet or as small as an atom and because of its motion it can do work. When an object with kinetic energy strikes another object, a force acts on them both that will distort the second object or set it moving. For example, if you move your foot and kick a stationary ball, the ball moves away.

Figure 12.5 The kinetic energy of the demolition ball is transferred to the building and breaks up its structure.

Sound energy Sound energy is produced when an object is made to vibrate. For example, sound energy is released into the air when a guitar string is plucked or a drum skin is struck with a stick. Sound energy does not just pass through gases, such as air, but through liquids and solids too. 4 Look out of a window and make a list of everything you can see that has a store of kinetic energy.

Electrical energy Electric current is the movement of electric charges through a conductor, such as copper or graphite. The electric charges possess electrical energy (from the store in the battery) and carry it to the working parts of a circuit. This may be a lamp, for example, where the energy is transferred into a store of light and heat energy.

5 In what ways is electrical energy put to work in your home?

Internal energy Internal energy is also called thermal energy. All substances are made up of particles. They possess a certain amount of energy, which allows them to move. When a substance is heated, this movement increases. For example, the particles in a solid are moving backwards and forwards about a fixed position. The particles in a liquid move more quickly and can move past each other. The particles in a gas can move freely in all directions at high speeds. When a substance is heated, the particles receive more energy and move faster.

Modelling increasing temperature You will need: a tray with 2 cm high sides and thirty marbles.

Process 1 Place the marbles in the tray so that they lie next to each other in rows. They represent a solid. 2 Model what happens when the temperature of the solid is increased by gently shaking the tray and watching the marbles jiggle about. 3 Model increasing the temperature by shaking the tray more strongly. 4 Make a model of particles in a gas and show what happens to the particles as you increase the temperature of the gas.

LET’S TALK

What kinds of energy are affecting your body at this moment? For each one you identify, describe what it is doing.

CHALLENGE YOURSELF In the Modelling increasing temperature activity, steps 1 and 2 help you to model the effect that heat has on particles in a solid. What are the strengths and limitations of this model? In step 3, a model of a melting solid should have been made. What are the strengths and limitations of this model? In step 4 a model of a hot gas is made. What are the strength and limitations of this model?

Science extra: Electromagnetic energy There is a form of energy that can travel through space at the speed of light. This kind of energy travels in waves that have some properties of electricity and some properties of magnetism. They are called electromagnetic waves. As these waves make up rays of light and heat, this form of energy is sometimes called radiation energy. There is a huge range of possible wave sizes, or wavelengths. A wavelength is shown in Figure 12.6.

Figure 12.6 A wave showing wavelength.

Electromagnetic waves are split into seven groups according to their wavelength. The different groups have different properties and

different uses. The two most familiar groups are light and radio waves.

Light energy Light is the electromagnetic energy that we can detect with our eyes. One way in which light energy is different from sound energy, is that light energy can travel through space.

Science in context Using heat from the Sun You probably have seen light from the sun reflected off a smooth surface such as a mirror. Light is a form of electromagnetic energy, and the infra red radiation we feel when we are in the sunshine is also a form of electromagnetic energy. It can be reflected like light and if the reflecting surfaces are in the form of a shiny curve they can bring all the infra red radiation to one point, which becomes extremely hot. Scientists have used this discovery to make a solar furnace. The heat that collects in the furnace is so great that it can be used to melt steel, to test materials used on space craft and to make tiny materials called nanomaterials. Scientists and engineers have made much smaller versions of the solar furnace – solar cookers. They can cook food without using fuel or generating carbon dioxide, and are portable, so can be used on camping expeditions or even as an alternative to other cookers as long as the Sun is shining.

Energy transfers Seeing, feeling, hearing and measuring energy

We use energy in many ways – for example, to cook food, light our homes and move cars and buses. When energy is used it always affects the energy that we can see, feel or measure, and some is always moved in the heating process. For example, when you switch on a light, electrical/ energy is transferred to stores of energy that we can see or feel as light and heat. When you play a guitar, chemical energy in your body is transferred into stores of energy that we can see and hear as movement and sound.

Figure 12.7 Energy is transferred when a guitar is played.

6 What main energy transfers take place in the following examples? a a clockwork toy b a child kicking a football c a boiling kettle on a gas ring d a person walking upstairs

Which makes the loudest sound? You will need: a metal tray, a floor covered in a hard surface such as floor tiles, and a noise meter app.

If you hold up a metal tray above the floor, you give it gravitational potential energy. If the tray is released, it then has kinetic energy and when it reaches the floor some of the energy is transferred to sound energy.

Hypothesis As a tray is held higher and higher, it gains more gravitational energy, so when it falls, more energy is transferred to a store of sound energy, which is transferred to the surroundings. Is this hypothesis testable? If you cannot decide, look back at the Did you know? on page xiv.

Prediction Which will make a louder sound: a tray held high or a tray held low? Make a prediction.

Plan and investigation Make a plan of an experiment to test your prediction and show it to your teacher. If the plan is approved, try it, then analyse and evaluate your results and draw a conclusion. You are living at this moment because of energy transfers taking place in your body. As the energy is transferred, it is shifted to other stores of energy. However, it is important to remember that it is not energy that is changing but how we are able to see, feel, hear or measure it. Stored chemical energy in your food is transferred to a store of kinetic (movement) energy when you raise your hand to turn the pages of this book. Some of the chemical energy is also transferred, which heats your body, keeping you warm. Energy that is found in one store shifts to another store. 7 If you are wearing a watch, what energy transfers are taking place in it right now? All the changes you can detect around you are due to energy transfers. If you are reading this book by an electric light, stored electrical energy is being transferred into a store of light energy so that you can see the words. If you can hear someone shuffling about in their seat next to you, some of

the stored kinetic energy of their body is being transferred into a store of sound energy, which is then transferred to the surroundings and reaches your ears. Also, at each energy transfer, some energy always heats the surrounding air.

Energy transfer diagrams Energy transfers can be shown by energy transfer diagrams – models that represent a scientific idea. In these diagrams you need to think about the energy that you can see, feel, hear or measure. The energy itself does not change. There are three parts to an energy transfer diagram: 1 an arrow showing the energy input 2 a box showing where the energy is converted from one form to another (something which converts energy in this way is also called an energy transducer) 3 arrows showing energy output.

The following are some examples of energy transfer diagrams.

Figure 12.8 Releasing a catapult.

A Releasing a catapult strain energy →

→ kinetic energy

B Burning gas in a Bunsen burner chemical energy →

→ light energy and heat energy

C Blowing up a balloon kinetic energy →

→ strain energy

D A plant making food light energy →

→ chemical energy

Figure 12.9 Blowing up a balloon.

8 Draw energy transfer diagrams for: a winding up a clock-work car b letting a clock-work car run c letting a battery-powered car run. 9 What materials does the plant from point D above use to make its stored chemical energy? (Look back at page 6 if you cannot remember). 10 What investigation could you make with a balloon to find out if different amounts of stored strain energy are transferred into different amounts of kinetic or sound energy?

Energy and ourselves Have you ever cooked meat on a barbecue? If you have, you might have seen fat dropping through the grill and bursting into flames as it hit the hot charcoal below. If you did not take enough care when cooking the meat, you may even have seen it catch fire too. Chemical energy (in the meat and fat) is transferred when the food burns, heating the surrounding air, just like the burning charcoal fuel that is heating them.

11 Draw an energy transfer diagram for respiration. 12 What do you think might happen to a seedling shoot if it was growing from a seed that had been planted too deeply? If you look at packets of food, you will find that many have an information box like the one shown in Figure 12.10. It tells you about the ingredients used and the nutrients present in the food. It also tells you about the amount of chemical energy stored in a 100 g mass of that food. The units used to measure energy in food are the joule (J) and kilojoule (kJ). They are the same units as those used to measure energy and work in scientific investigations.

Figure 12.10 Food packet label.

The chemical energy in food is released in a process called respiration. Oxygen is needed for this process and your body takes it in from the air you breathe in through your lungs. When the energy is released, carbon dioxide and water are produced. The carbon dioxide is released into the air when you breathe out. The water is used in your body and released in sweat and urine. Most of the energy that is released in your body is used for movement and for keeping the body warm.

Figure 12.11 This runner is releasing a lot of energy.

13 Draw an energy transfer diagram for the human body. The energy input is the stored chemical energy in food.

Science in context A change in the idea about energy Scientists working on experiments over two hundred years ago had a clear idea about energy. They thought that, just like when you are holding a hot drink, if you picked something up, you could feel heat energy flowing into your hand. That was what they thought energy was – a liquid that flowed. They thought that everything had this liquid inside it. They even gave it a name. They called it caloric. They believed that when something gave out heat like the cup holding a hot drink, caloric flowed out through tiny holes in the cup into your hand. In 1789, an American scientist called Benjamin Thomson was watching a cannon being made. The cannon was being made by drilling a cylinder in a large piece of metal. As he watched he saw that the metal became hotter and hotter.

Scientists at the time believed that the metal was just letting caloric flow from it, but Benjamin thought that the metal was getting too hot for that explanation. He thought that if caloric was in the metal, it would eventually be used up. He tried an experiment in which he used a very blunt drill to make as much heat as possible as it turned, and he surrounded the metal with water. After two and half hours of drilling, the water boiled. Benjamin believed that if the metal held caloric, it would melt, and so since it stayed a solid, he reasoned that the heat was being made by the moving drill, and that energy in the moving drill was being transferred to the water as heat energy. More experiments had to be done before other scientists realised that caloric did not exist and that energy is actually transferred from one form to another. Today we use this idea when we make energy transfer diagrams.

Wasted energy or energy we can’t use We have seen that when an energy transfer takes place, more than one form of energy may be produced. This spreading out of energy into different forms is called the dissipation of energy. Usually only one of the forms is useful to us at a particular time and the rest of the dissipated energy is wasted. This wasted energy can also be called dissipated energy. When we turn on a lamp, for example, it is because the light is useful to us. Energy that is transferred through the heating of the surrounding air can be thought of as wasted energy. The heat in this example is dissipated energy. Sometimes that wasted energy can cause problems. For example, some machines make so much noise (wasted sound energy) that people using them have to wear ear protection (Figure 12.12).

Figure 12.12 This worker’s ears are protected from noise energy.

14 What is the wasted energy in each of the energy transfers in Question 8 on page 136?

When there is not enough energy Have you ever switched on your computer or mobile phone and found the screen blank? The battery does not have enough chemicals left to produce electrical energy to light it up or to let you make a call. Every action such as lighting the phone screen and allowing you to make a call needs a certain amount of energy. If that amount is not available, nothing happens. If you close this book and put it on a table then gently push it, nothing happens. This is because you are not using enough energy to move it. Push harder, use more energy and the book moves. 15 In a film, the star is running away from danger. She jumps in a car, turns the key and the engine makes a ‘rrr rrr rrr’ sound but

will not start. What could be the cause?

Summary • Gravity is the force of attraction between any two objects; the size of the gravitational force is related to the mass of the object. • Transfers of energy are a result of an action or a process. • Energy can dissipate and, in doing so, it becomes less useful.

End of chapter questions 1 Name three forms of stored energy. 2 When you raise your hand to turn the page of this book, what kind of energy does your hand have? 3 What form of energy are radio waves? 4 What happens to energy during the dissipation of energy? 5 Why is dissipated energy also called wasted energy?

6 A group of students was investigating the potential energy in a nail 15 cm long. They suspended it above a block of soft clay, measured the distance to its tip, and then let it go. The students measured the depth to which the nail sank in the clay. Table 12.1 shows their results for four experiments. Table 12.1

Height of nail above clay/cm

Depth of indent/cm

25

0.9

50

1.6

75

2.3

100

3.0

a How do you think they measured the depth of the indent in the clay?

b Plot a graph of their results. c How could you use the graph to predict the indent made by the nail from a height greater than 1 m?

7 A second group of students investigated the potential energy of a brass sphere, diameter 2.5 cm, which was dropped from different heights into soft clay. They measured the diameter of the indent made by the sphere. Table 12.2 shows their results for four experiments. Table 12.2

Height of sphere above clay/cm

Diameter of indent/cm

5

1.0

20

1.7

50

2.2

70

2.5

a How do you think the students measured the diameter of the indent? b Plot a graph of their results. c How do these results compare with the results of the experiment described in Question 6? d Suggest a reason for any differences you describe. e Can the graph be used to predict indentations produced by falls from any height greater than 70 cm? Explain your answer.

CHALLENGE YOURSELF On your way home from school, make a list of examples of energy transfers. When you get home, draw an energy transfer diagram for

each example. Now you have completed Chapter 12, you may like to try the Chapter 12 online knowledge test if you are using the Boost eBook.

13 Sound In this chapter you will learn: • about the vibration of particles in a sound wave • why sound cannot travel in a vacuum • about echoes in the reflection of sound waves.

Do you remember? • Make a sound with a soft volume. Give two examples of sounds with a soft volume. • Make a sound with a loud volume. Give two examples of sounds with a loud volume. • Make a sound with a low pitch. Give two examples of sounds with a low pitch. • Make a sound with a high pitch. Give two examples of sounds with a high pitch. Figure 13.1

• Using the photo, describe how the sounds are made by the different instruments.

Can you demonstrate volume and pitch using a guitar? You will need: a guitar.

Process 1 Demonstrate how you can make a sound with a soft volume and then change it to a sound with a loud volume. 2 Demonstrate how you can change the pitch of the sound by using different strings. 3 Demonstrate how you can change the pitch of the sound made by one of the strings. You have probably performed some experiments on sound without knowing it. At some time, most people have made a ruler vibrate by holding one end over the edge of a desk and ‘twanging’ it. The end of the ruler moves up and down rapidly and a low whirring sound is heard which becomes higher as you pull the ruler in from the edge of the desk. Figure 13.2 Making a ruler vibrate.

From vibration to sound wave Any object can make a sound wave when it vibrates. In practical work on sound, you might use an elastic band, a guitar string or a tuning fork, because they all vibrate easily. A vibration is a movement about a fixed point. This movement may be described as a to-and-fro movement or a backwards-and-forwards movement (see Figure 13.3). Figure 13.3 Vibration is a to-and-fro movement.

Figure 13.4 Producing sound by vibration.

Sound waves can travel in a gas, a liquid or a solid, because they all contain particles. When an object vibrates, it makes the particles next to it in the gas, liquid or solid vibrate too. For example, when an object vibrates in air, it pushes on the air particles around it. As the vibrating object moves towards the air particles it squashes them together. The particles themselves are not compressed, they just come

closer together. As the object moves away from the air particles next to it, it gives the particles more space, so they spread out. This movement of air particles from a vibrating object can be modelled by using a long spring, as shown in Figure 13.5. One end is held firmly by the hand on the right and the other end is pushed and pulled by the hand on the left. Figure 13.5 A spring shows how sound waves move.

1 When using the spring, which coils represent the air particles a being close together b spreading out?

Sound and the particle model of matter The movement of sound through the air can be explained using the particle model of matter. Figure 13.6 shows how the side of a vibrating object pushes on the air around it. Figure 13.6

Figure 13.7 shows that the particles move apart when the side of the vibrating object moves away from them, but you can also see that air particles have pushed together a little further away. Figure 13.7

If we could see the air particles at a greater distance from the vibrating object, we should see the particles squashing together and spreading out and making a wave of sound as shown in Figure 13.8.

Figure 13.8

2 Compare the particle model of sound shown in Figure 13.8 with the spring shown in Figure 13.5. What are the strengths and limitations of using the spring as a model to show how air particles pass a sound wave through the air?

Figure 13.9 These whales communicate by sound-waves.

Sound waves are generated and travel in liquids and solids in the same way as they do in gases. The particles in liquids and solids are held close together by forces of attraction. In a liquid, however, the particles are further apart than in a solid and can move around one another. Sound travels very well through a liquid. It moves faster and further than it does in a gas. The humpback whale emits a series of sounds, called songs, which travel thousands of kilometres through the ocean. It uses its songs to communicate with other whales. When sound travels through a solid, it moves even faster than through a liquid because of the close interaction of the particles. However, the sound does not travel as far. A snake detects vibrations in the ground with its lower jaw-bone. The bone transmits the vibrations to the snake’s ears and allows the snake to detect the footsteps of its prey. Figure 13.10 This snake is listening for vibrations in the ground.

LET’S TALK If you carefully and gently hold the front of your neck you will find an air pipe going down to your lungs. In this pipe there is a structure called the voice box which you may feel as a lump. It contains two vocal cords which you can think of like the ends of rulers hanging off a desk. If you hold your voice box and talk or sing you should be able to feel the voice box vibration. How can you use this information to explain what we hear when you talk?

Figure 13.11 Sound cannot be heard through a vacuum.

3 Why is it that a bell in a sealed bell-jar: a can be heard when the jar is full of air? b cannot be heard when a vacuum is created in the jar? Sound waves cannot pass through a vacuum, because a vacuum does not contain any particles. Figure 13.11 shows an experiment that demonstrates this. As air is drawn out of the bell-jar with a pump, the sound of the bell becomes quieter. When a vacuum is established in the bell-jar, the bell cannot be heard, although the hammer can still be seen striking it.

Reflection of sound When light strikes a shiny surface, it is reflected. You may even see an image of yourself reflected in a surface. You can find out what happens when sound strikes a surface through the following simple experiment.

Are sound waves reflected? You will need: this textbook and a ruler.

Hypothesis Light waves can be reflected, so sound waves should be reflected too. Is this hypothesis testable? Explain your answer.

Prediction If a sound is reflected from a surface, you should be able to hear it.

Investigation 1 Close your book and hold it out about 20 cm in front of you, below the level of your mouth.

2 Keep the book below your head and say ‘ahhh’ for as long as you can. 3 Listen to the sound, then raise the book so that it is about 20 cm in front of your mouth.

Examining the results Compare the sounds you hear when the book is below your mouth and in front of your mouth.

Conclusion Compare your evaluation with your prediction and draw a conclusion. Scientists like to check their discoveries by performing different investigations. The simple investigation using the ears to detect the reflection of sound can be developed into a second investigation using a sound detection meter which measures the energy in a sound wave. A cell phone can be converted into a sound meter by downloading a decibel meter app. A decibel is a measure of sound energy and its symbol is dB.

CHALLENGE YOURSELF Plan a modified investigation with the book, voice and ears to include a mobile phone with a decibel meter app. If your teacher approves your plan, try it. How will it confirm or contradict the evidence of the first investigation?

Modelling sound reflection You will need: a long spring held firmly at one end by a friend. A second friend with a cell phone camera (optional).

Process 1 Stretch out the spring between you and your friend.

2 Ask your friend to hold their end firmly, then push and pull the end you are holding so that you set up waves as shown in Figure 13.5. 3 Look for waves reaching the end held firmly and setting up waves back towards you. 4 If you find evidence that the model shows the reflection of sound, ask another friend to film it.

CHALLENGE YOURSELF What are the strengths of the spring model and what are its limitations? Sometimes scientists make a model before they try an experiment. Here is an example.

Modelling surfaces and sound reflection The movement of sound energy can be modelled by using a tennis ball (to represent an air particle). The surfaces can be provided by using a metal tray and a cushion. You will need: a tennis ball, a metal tray and a cushion.

Process 1 Ask a friend to stand about two metres away and hold up the tray. 2 Throw the ball at the tray and observe what happens to it. 3 Repeat steps 1 and 2 with the cushion instead of the tray.

Conclusion What conclusion do you draw from using this model?

CHALLENGE YOURSELF What are the strengths of the surfaces and sound reflection model and what are its limitations? The conclusion drawn from the model can now be used in an experiment.

How does the surface affect sound reflection? The movement of sound energy can be modelled by using a tennis ball (to represent an air particle). The surfaces can be provided by using a metal tray and a cushion. You will need: a metal tray, a cushion, and a mobile phone with a decibel meter app.

Hypothesis The sound reflected from different types of surface differs in some way. Is this hypothesis testable? Explain your answer.

Prediction Make a prediction based on the hypothesis about what will happen when the sound from a hard and a soft surface are compared.

Plan and investigation 1 Look back at the first experiment in this chapter and the modelling activities and think about how you can use what you discovered to plan an experiment. 2 Make a plan to test the question in the title of this experiment. In your plan think about: – what you will use to make the sound – your voice, a buzzer or something else.

– how far your sound source will be from the surface. – how far the mobile phone will be from the surface. – how many times you will test each surface. – how you will record your data. 3 Show your teacher your plan and if it is approved, try it.

Examining the results Compare the sets of data you have collected and describe any differences.

Conclusion 1 Compare your evaluation with your prediction and draw a conclusion. 2 What are the limitations of your conclusion? 3 How could the investigation be improved? If you want to catch someone’s attention in the distance, you may shout at them. If they do not hear, you might cup your hands and shout again. This time they may hear you. If someone whispers to you in class, you might cup your hand around your ear to hear better. Figure 13.12

Figure 13.13

4 Why is your shout louder when you cup your hands around your mouth? 5 Why do you cup your hand to your ear to hear a whisper?

CHALLENGE YOURSELF Plan experiments to test the observations in Figures 13.12 and 13.13. If your teacher approves, try them and report your findings.

Recording bird-song Scientists use the reflection of sound to help them record bird-song. To do this, they use a parabolic reflector. Sound from the singing bird strikes the inside of the reflector and, because of its curved shape, the reflected soundwaves are directed to a point in front of the reflector where the sound appears to be amplified. A microphone is placed at that point to record the bird-song. Figure 13.14 Using a parabolic reflector to record bird-song.

CHALLENGE YOURSELF Could you make a parabolic reflector to use with your mobile phone to record bird-song? What equipment would you use? (For example, a large bowl, an umbrella, a soft surface material like cloth, or a hard surface material such as metal kitchen foil.) What other ideas do people have? Work out a plan, show it to your teacher and, if it is approved, try it and assess the equipment you have made. Suggest any improvements it might need.

Science extra: Reverberations A living room in a home usually has soft furnishings in it, things such as chairs, a sofa and curtains. The surfaces of these items absorb sound and make the room quiet. If the room is being decorated, the furniture is taken out and the curtains are taken down, leaving the hard surfaces of the walls, ceiling and windows behind. When a sound is made in such a room, it sounds louder and seems to have a slight echo. This is called a reverberation and is made by many reflected sounds reaching the ear very close together.

Echoes

The human ear can only hear two sounds separately if they reach the ear more than one-tenth of a second apart. If they arrive in a shorter time than this you may hear a reverberation but, in certain circumstances, if they arrive more than one-tenth of a second apart you may hear an echo. Sound travels at about 340 metres per second, or 34 metres in one-tenth of a second. To make a sound which produces an echo, you need it to travel at least 34 metres from you and back again. 6 What is an echo?

Figure 13.15

A crash of thunder, called a thunderclap, is made when a streak of lighting flashes through the air. The heat of the lightning makes the air expand so fast that it makes the noise. During a thunderstorm, you sometimes hear a few claps of thunder close together. This is called a roll of thunder and is caused by the sound of one thunderclap being reflected off the clouds, making a multiple echo.

CHALLENGE YOURSELF Make an echo by standing just over 17 metres from a high wall and clapping your hands repeatedly.

Science in context Ultrasounds We do not make use of the echoes we can hear in our lives, apart from having fun, but we do use the echoes of sounds beyond our hearing, called ultrasounds. A familiar use of collecting the echoes of ultrasound is in checking that a fetus (an unborn baby) is developing healthily in the womb. The word ‘sonar’ is used to describe equipment that helps ships navigate in shallow water and submarines navigate underwater. The name stands for ‘sound navigation ranging’ and produces ultrasounds which pass through the water, strike the sea bed and

are reflected back to the vessel to help to steer it safely on its journey. Figure 13.16 An ultrasound scan of a fetus.

Figure 13.17 Operating sonar.

Bats and dolphins also use echolocation to find their prey. Both types of animals send out ultrasounds which reflect off their prey and return to the animals’ ears. In a bat, the brain compares the ultrasounds released with those that return (the echo), and uses them to swoop down on an insect such as a moth. A dolphin compares the ultrasounds released and echoed back to find shoals of fish. Figure 13.18 A bat catching a moth.

Figure 13.19 A dolphin feeding on fish.

LET’S TALK Can you have thunder without a flash of lightning? Explain your answer. Why don’t you usually hear echoes of your sounds as you walk around your surroundings?

Science in context Reducing sound reflections If you went into a recording studio to record a song with your band, you would find that the walls of the room are covered in acoustic panels. The surfaces of the panels are made of soft materials and may be flat, wedge- or pyramid-shaped. Their purpose is to absorb

the reflected sound made by your band, and to prevent echoes and reverberations so that your music can be recorded clearly. Figure 13.20 A recording studio showing panels to absorb reflected sound.

Acoustic panels are also used in rooms called anechoic chambers. These rooms are used by scientists who study sounds made by a variety of things, from headphones, to cars and even aeroplanes. The panels are so well made that they absorb all the sound that is produced in the room. Anechoic chambers are the most silent places on Earth. Figure 13.21 Acoustic panels in an anechoic chamber.

7 If you were checking the sounds produced by various parts of a car, what would they be?

Science extra: Early experiments on the speed of sound In the past, many scientists have performed experiments to find the speed of sound. Isaac Newton (1642–1727) investigated the speed of sound in air by measuring the time between a sound being made and its echo from a wall being heard. Other scientists measured the time taken between seeing a distant cannon fire and hearing its sound. The speed of sound in water was investigated using the apparatus shown in Figure 13.23. The experiment was performed at night. When the lever was pulled down, both the arm carrying the bell hammer and the device carrying the match moved. Figure 13.22 Measuring the speed of sound in air.

Figure 13.23 Measuring the speed of sound in water

8 Why do you think the experiment to find the speed of sound in water was done at night?

9 Using your knowledge from your work on echoes, what sort of surface do you think Newton may have used in his investigation?

Summary • Sounds are made by vibrating objects that cause a sound wave. • Sound cannot travel in a vacuum because a vacuum does not contain any particles. • Echoes occur when there is a time interval between the transmitted and received sounds of over one-tenth of a second.

End of chapter questions 1 Write down a word that completes the following sentence:

2 3 4 5

6

When the end of a ruler goes up and down it is said to __________________. Why can sound travel in gases, liquids and solids? Does sound travel through a vacuum? Explain your answer. What happens when a sound wave is reflected? When you clap your hands 18 metres from a wall a what happens when the sound waves reach the wall? b what do you hear shortly after? Describe how a sound wave moves through the air. You may use some diagrams in your answer.

Now you have completed Chapter 13, you may like to try the Chapter 13 online knowledge test if you are using the Boost eBook.

14 Electricity In this chapter you will learn: • to model electricity as a flow of electrons around a circuit • that electrical conductors are substances that allow electron flow • that electrical insulators are substances that inhibit electron flow • to measure the current in series circuits • how adding components into a series circuit can affect the current to use diagrams and conventional symbols to represent, make and compare circuits.

Do you remember? • What do you need to make a circuit with a lamp in it that you can turn on and off? • Where is the electricity produced in the circuit? • What do you need to put in a circuit so that when electricity flows though it a sound is made? • What advice would you give to someone who was setting up a circuit?

Introducing the electron The idea of the atom was first raised by the Greek philosopher Democritus in around 400BC. Since then, many discoveries have been made about atoms. Here is one which will help in our study of electricity. Figure 14.1 The structure of an atom.

An atom (see Figure 14.1) has two parts. In the centre of the atom is a part called the nucleus. Around the nucleus are particles which are much smaller

than the nucleus – these are called electrons. Some of the electrons around the atom are free to move around.

Electrons on the move Scientists’ first investigations were on electricity that, once generated, did not move. This electricity is called static electricity. You may have already met static electricity with a party balloon trick. If you rub a balloon on a woollen sleeve and then press it on a wall, it sticks there.

LET’S TALK Imagine you are a group of atoms in the wool on the sleeve of your sweater. A balloon is rubbed on the wool. Explain what happens to you when this happens. Why is the rubbing of a balloon on a woollen sleeve an example of static electricity?

Figure 14.2 Balloons stick to the wall because of static electricity.

Figure 14.3 Electrons are transferred from the wool to the balloon.

This static electricity was generated by the action of rubbing the balloon, which made some of the electrons move onto the balloon and they stayed there. Once pressed to the wall, the electrons generated an electric charge in the wall that held the balloon in place.

Conductors and insulators It did not take scientists long before they discovered how to make electrons move through some materials, which they called electrical conductors, and until they identified materials which did not let electrons pass, which they called electrical insulators. The device that was invented to drive the electrons is called a cell, and when a group of cells are linked together, they form a battery. The moving electrons form a current of electricity.

Science in context Early investigations with electricity Luigi Galvani (1737–1798) believed that he had discovered ‘animal electricity’. Galvani studied how human and animal bodies were constructed, but he also had a machine which generated static electricity in his laboratory. He discovered that when the machine was working, the muscles in dissected frogs’ legs twitched. He also discovered that the muscles twitched when they touched two different metals, such as copper and iron. From his observations, he concluded that the muscles contained electricity. Figure 14.4 Luigi Galvani.

Alessandro Volta (1745–1827) believed that muscles were not important in the generation of electricity and replaced them with a salt solution. He arranged a row of bowls of salt solution with strips of copper and zinc dipping into each one. The bowls with their metal strips were joined together and were called a battery. This was the first device made that could produce a steady flow of electric current. Figure 14.5 Copper and zinc strips in sodium chloride solution.

Later Volta re-designed his battery into a pile of copper and zinc discs. The copper and zinc discs were separated by cardboard that had been soaked in salt solution. This device became known as the voltaic pile (see Figure 14.6). Volta wrote to the Royal Society in London about his invention so that other scientists could learn how to construct it. When William Nicholson (1753–1815) and Anthony Carlisle (1768–1840) constructed a voltaic pile, they connected it to two pieces of metal that were dipping into water. Bubbles of gas were produced on the metal pieces. The bubbles on one piece of metal contained hydrogen, and the bubbles on the other piece of metal contained oxygen.

Figure 14.6 Volta demonstrates his battery to Napoleon.

Humphry Davy (1778–1829) was an English chemist. He realised that Nicholson and Carlisle’s discovery that electricity could split up a substance into its elements could be more widely used. At the time, substances such as potash and lime were thought to be compounds of elements, but reactions with other chemicals could not split them up. Davy built a very large battery, heated each substance until it melted, then let the current flow through it. When Davy tested potash he discovered tiny pieces of shiny metal that caught fire when they were placed in water. He had discovered potassium. Davy made other investigations using current, which led to the discovery of the elements sodium, barium, calcium, strontium and magnesium. 1 What other conclusion could Galvani have drawn about his work on metals and frogs’ legs? 2 What did Volta take from Galvani’s experiment to make his first device? Which material that Galvani used did Volta change in his first device? 3 What idea did Davy have after learning about the experiment of Nicholson and Carlisle? 4 How did the study of electricity help the study of chemistry?

Science extra: Chemical reactions in a cell In the chemistry section of this book, we learnt that substances can be divided up into chemical elements and that substances made from two or more elements, such as salt, are called chemical compounds. In the cells made by Volta, a chemical reaction between zinc, copper and salt took place when a wire was connected to the two metals. This chemical reaction moved electrons along the wire and created an electric current.

Figure 14.7 A lemon battery.

A cell can also be made by using a lemon. In Chapter 10 on acids and alkalis we found that lemons contain the chemical citric acid. If this is combined with copper (the coins in the lemons shown above are made of copper) and zinc (the nails in the lemons shown above are coated in zinc), as Figure 14.7 shows, it also makes a cell. There are more components that can be placed in a simple circuit besides the cells, wires, lamps and switches you may have used in an earlier science course. Another component that releases light energy like the lamp is the LED. These letters stand for ‘light emitting diode’. You can find out more about LEDs on page 166.

How many lemons are needed to light an LED? You will need: eight lemons, eight galvanized steel nails (they are coated in zinc), eight copper coins or eight short copper wires, nine wires (preferably with crocodile clips at each end), a three-volt LED, and a knife to cut a slice in the lemons.

Hypothesis The juice of a lemon can be used to generate a current of electricity when a lemon is made into a cell. Is this hypothesis testable? Explain your answer.

Prediction How many lemons do you think will be needed for a battery of lemon cells to produce enough electric current to light an LED? Make a prediction.

Investigation

1 Set up each lemon as shown in Figure 14.7. 2 Connect one wire to the nail and one to the coin. Connect the LED between them and see if it lights up. 3 If the LED does not light up, connect a wire between the copper coin of one lemon and the nail of the other. Connect wires to the free metal objects of the lemons to the LED and see if it lights up. 4 If the LED does not light up, keep adding lemons in a row until your battery of lemons lights the LED.

Examining the results Compare your results with your hypothesis and prediction.

Conclusion How many lemons were needed to light the LED? Make a conclusion based on your results. Was your prediction accurate?

Modelling moving electrons Electrons are sometimes shown in a circuit as balls moving along in single file in a wire when the circuit is switched on. Try and make a 3-D model of this to show electrons moving along a wire. You will need: cardboard tubes from kitchen rolls, modelling clay or marbles. (These are some suggested materials but you can use others.)

Process 1 Make a model to show electrons moving along a wire. 2 Use your model to explain how electrons behave when the circuit with the wire in it is switched on and then switched off. You may think that a strength of the last model activity is that it gives you an the idea of moving electrons, but it is limited because it does not show the path of electrons around a circuit. Try the following modelling activity

and see if it helps you to understand the path of electrons around a circuit. This model features a cell which has the power to move the electrons.

How do electrons move around a circuit? You will need: yourself (as the cell), a friend (as a component, such as a buzzer – see page 166) and a rope or thick string 2 metres long, tied into a loop.

Process 1 Sit at a table and grab hold of the rope with both hands. 2 Ask your friend to sit opposite you and grab the rope with both hands. 3 Imagine that the circuit you have made is switched on. This makes the cell push the electrons around the circuit. You can model the pushing of the cell by pulling some rope in with one hand and pushing some rope away with your other hand. The moving rope represents the moving electrons. 4 Your friend will take up the slack rope that you have made between you and push it through their hands back to you, so that the rope (the electrons) moves completely round the circuit back to you – the cell. 5 As your friend passes the rope through their hands, they could imagine that they are a buzzer and make a buzzing sound, which represents a change in energy from electrical energy to sound energy.

Examining the results What are the stengths and limitations of this model?

Simple circuits

Simple circuits are used in science to investigate the properties of electricity and the effects of bringing different electrical components together. The components in simple circuits are cells, wires, switches, lamps, buzzers and ammeters. On the following pages you can find out more about how electricity moves through the components in a simple circuit.

A cell in a simple circuit If you set up this equipment and close the switch, the lamp comes on. Figure 14.8 A simple circuit.

The wires of the circuit are composed of atoms that are held tightly together, but around them are many electrons that are free to move. The metal filament in the lamp and the metal parts of the switch also have free electrons. When the switch is closed, the wires on either side of the switch are linked by metal contacts and a path is made, along which the electrons can flow. When you open the switch, the lamp goes out. The path is broken and the electrons cannot flow. The energy to move the electrons comes from the cell. The chemical reactions that take place in the cell make the electrons leave the cell at the negative terminal when the circuit is completed. They push their way into the wire and move the other electrons along, creating a flow or current of electricity. At the positive terminal, electrons are drawn back inside the cell. The wire in the lamp filament is more resistant to the flow of electrons than the other wires in the circuit. This means that it acts to stop the electrons moving freely. It slows them down and makes them rub and push against the material in the wire. As the current moves through the filament, some of its electrical energy is transferred to stores of heat energy and light energy. 5 Describe the path of an electron around the circuit in Figure 14.8 when the switch is pressed down. 6 a How does the wire in the filament behave differently to other wires in the circuit when the current flows? b What property of the wire accounts for this difference?

CHALLENGE YOURSELF Simulate the action of electrons moving through a wire of high resistance by pressing your hands together and then rubbing them on each other. What do you feel? In time, the chemicals that take part in the reaction inside the cell are used up. They can no longer release energy to make the electrons move and the current stops. The number of electrons in the circuit does not change – it is the chemical energy released by the cell that changes.

Science extra: Conventional current Earlier in the chapter we saw the terms ‘positive’ and ‘negative’ used to describe the electrical charge on a material. This idea about the terms arose during the early investigations on electricity. One of the scientists at the time was called Benjamin Franklin, and he believed that electricity was a type of liquid which flowed from a positively charged substance to a negatively charged substance. His idea was taken up by other scientists until it was discovered that it was the flow of negatively charged electrons that produced a current. Franklin’s idea is still used today, however – it is known as the conventional current direction.

Circuit diagrams When circuits are drawn, symbols are used for the parts or components. The use of symbols instead of drawings makes diagrams of circuits quicker to make, and the connections between the components are easier to see. The symbols have been standardised, like the SI units, and are recognised by scientists throughout the world. The circuit in Figure 14.8 is shown as a circuit diagram using symbols in Figure 14.9. The components for the circuit are the wires, cell, lamp and switch. Figure 14.9 A circuit diagram and the symbols used.

7 In Figure 14.10, the bases of the cells (on the right) are the negative terminals and the caps (on the left) are the positive terminals. How can you distinguish between the positive and negative terminals in a cell in the circuit diagram in Figure 14.9? In everyday life, cells are almost always called batteries but this is scientifically incorrect. In science, a battery is made of two or more cells joined together. The symbols for batteries of two cells, three cells and more cells are shown in Figure 14.10. Figure 14.10 The circuit symbols for two cells, three cells and any number of cells.

Other circuit components are also represented by unique symbols, and later in the chapter you will see the symbols for the ammeter and buzzer. The lining up of cells next to each other in a row, end to end, as shown in Figure 14.10, is described as arranging them in series. Other electrical components can also be arranged in series, as you will see later.

Amperes The rate at which electrons flow through a wire is measured in units called amperes. This word is usually shortened to amps and the symbol for it is A. One amp is equal to the flow of 6 million, million, million electrons passing any given point in the wire in a second! 8 To think about current and electron flow, try these simple calculations. How many electrons are flowing per second past a point in a circuit in which there is a current of: a 0.5 A b 5A c 30 A?

Measuring current

The current flow in a circuit is measured using an instrument called an ammeter. This is a device that has a coil of wire set between the north and south poles of a magnet. The coil has a pointer attached to it and it turns when a current passes through it. The amount by which the coil turns depends on the size of the current and is shown by the movement of the pointer across the scale. When an ammeter is used, it is connected into a circuit with its positive (red) terminal connected to a wire that leads towards the positive terminal of the cell, battery or power pack. It is always connected in series with the component through which the current flow is to be measured (Figure 14.11). Ammeters usually have a very low resistance so that the current passes through them without affecting the rest of the circuit. Figure 14.11 An ammeter connected in a circuit (left) and the circuit diagram showing its symbol (right).

9 Towards which terminal of the power supply should the negative (black) terminal of an ammeter be connected?

What is the size of the current in a circuit? You will need: a cell, a switch, four wires, a lamp, an ammeter.

Plan 1 Set up the components of the circuit as shown in Figure 14.11. 2 Close the switch and read the ammeter to find the size of the current. 3 Record the current for the next enquiry. The following enquiry can be used for you to show your science communication skills as well as making an enquiry.

How does changing the number of cells in a circuit affect the size of the current? You will need: three cells, six wires, a switch, four wires, a lamp, an ammeter, a camera.

Process 1 Make a circuit diagram for a circuit with two cells in series with an ammeter, switch and lamp. 2 Make a circuit using your diagram and photograph it. 3 Close the switch and read the ammeter to find the size of the current. 4 Record the size of the current. 5 Make a prediction about the size of the current in a circuit when three cells are present in it. 6 Make a circuit diagram for a circuit with three cells in series with an ammeter, switch and lamp. 7 Make a circuit using your diagram and photograph it. 8 Close the switch and read the ammeter to find the size of the current. 9 Record the size of the current.

Examining the results Compare your results with your prediction. Is there a trend or pattern? Explain your answer.

Conclusion Draw a conclusion from the examination of your results and explain its limitations. How could the investigation be improved?

CHALLENGE YOURSELF Use your diagrams, photographs and results to make a presentation about your answer to the enquiry question. You may like to make a video to show to the class, your friends and your family. How well did they think you explained what you had done and found out? What improvements would you consider when making your next presentation?

Lamps and current size The wires connecting the components in a circuit have a low resistance, while the wires in the filaments of lamps have a high resistance. When lamps are connected in series, their resistances combine – they add up. They therefore offer a greater resistance to the current than each lamp would separately. The size of the current flowing through a circuit can be estimated by looking at the brightness of the lamps in the circuit. A lamp shines with normal brightness when it is connected to one cell as shown in Figure 14.12a. The lamp shines more brightly than normal when it is in a circuit with two cells (Figure 14.12b) and shines less brightly when it is in a circuit with one cell and another lamp as shown in Figure 14.12c. Figure 14.12 Three arrangements of cells and lamps in series.

10 A wire carrying a current of electricity can be described as being similar to a stream carrying a current of water. In what ways are the wire and the stream similar? 11 Predict the brightness of the lamps in the circuits in Figure 14.13 compared with that of a single lamp in a circuit with one cell. Use one of the following descriptions in each case: very dim, dimmer, the same, brighter, very bright.

(All the lamps are identical and all the cells have the same voltage.) 12 Compare the circuit in Figure 14.14 with the one in Figure 14.13b. Do you think the lamps will glow with the same brightness? Explain your answer. Figure 14.13

Figure 14.14

LET’S TALK Why is it better to use an ammeter than a lamp to compare the current in two different batteries?

Are your answers scientifically correct? Process 1 Look at the circuit diagrams in Figures 14.13 and 14.14. 2 Gather electrical circuit components so that you can make all of the circuits in turn. 3 Assemble the circuit to match circuit diagram a and use your own observation to check your answer to Question 12. 4 Repeat step 3 with all the other circuits. 5 What do you conclude?

Other circuit components There are many components that can be added to circuits. If you made the lemon battery, you used one to test it – the LED or light-emitting diode. It performs the same task as a lamp and is found in many complicated

electrical circuits. LEDs are also used in electrical devices where it is the lamp that shows that the device, such as a TV or a kettle, is switched on. Figure 14.15 Four light-emitting diodes (LEDs) and their symbol.

A buzzer is an electrical device in which one part vibrates strongly when an electric current passes through it. The vibrations produce the sound. Figure 14.16 A buzzer and its symbol.

What happens when you change the number of cells in a circuit? Process 1 2 3 4 5

Draw a circuit diagram with one cell, a buzzer and a switch. Gather the components you have put in your diagram. Assemble the circuit to match your diagram. Open and close the switch and describe what happens. What will happen to the buzzer when another cell is added to the circuit? Explain your answer. 6 Add another cell and test your answer to step 5. What do you conclude? Explain your answer.

CHALLENGE YOURSELF In your primary science course, you may have worked with a buzzer before. Perhaps you used it to make a burglar alarm with a switch made from two small sheets of aluminium foil. Try to make a device which will buzz when you close this book. You may find a cell, three wires with crocodile clips, a buzzer and two small sheets of aluminium foil helpful, but you may use other materials if you

choose. How good would this device be in making sure someone has got their book open in the science lesson?

Summary • • • •

Electricity is a flow of electrons around a circuit. Electrical conductors are substances that allow electron flow. Electrical insulators are substances that inhibit electron flow. An ammeter measures the rate of flow of electrons (or current) in series circuits. • Adding components (such as a buzzer) into a series circuit can affect the current. • We use models, diagrams and conventional symbols to represent, make and compare circuits.

End of chapter questions 1 Where would you find electrons in an atom? 2 What is the name of materials that let electrons flow through them? 3 The results from an early electrical experiment suggested that muscles contained electricity. What did Alessandro Volta replace the muscles with in his experiment? 4 a What component is used to test to see if a lemon battery is producing a current of electricity in a circuit? b How does this component show electricity is flowing? 5 What is the circuit component which is used to start and stop a flow of electricity? 6 When you set up a circuit, why must you make sure that all the components are securely fastened together? 7 What is the shorter word for ampere and what is its symbol? 8 How would you measure current flow in a simple circuit with an ammeter?

9 How is the symbol for a buzzer different from the symbol for a lamp used in the circuit diagrams in this chapter?

CHALLENGE YOURSELF Electrical fruits The lemon can be used to make a battery of cells to light an LED. What other fruit do you think could be made to do the same? 1 Select your fruit. 2 Work out a plan to investigate it. If your teacher approves, try it. 3 Describe your investigation and set out your conclusions. Now you have completed Chapter 14, you may like to try the Chapter 14 online knowledge test if you are using the Boost eBook.

15 The Earth in space In this chapter you will learn: • how planets are formed from dust and gas, pulled together by gravity • that gravity is the force that holds components of the solar system in orbit around the Sun • that tidal forces on earth are a result of the gravitational attraction between the Earth, Moon and Sun • how solar and lunar eclipses happen.

Do you remember? • Name the planets of the solar system. • What else is in the solar system besides the planets? • Describe how the Earth moves through the solar system.

Gravity and planet formation There is a force of attraction between any two objects in the universe, called gravity. The objects may be small, such as an ant and a pebble, or they may be very large, such as a star and a planet. When a star forms, a cloud of gas and dust also forms around it. The objects in the gas and dust clouds are very, very small, but the force of gravity exists between them and, in time, pulls them together. Dust particles are pulled together by gravity and, when they touch, they stick together to form larger rocky particles. These rocky particles are pulled together by gravity and form larger pieces of rock. These large pieces are pulled together by gravity to make even larger pieces.

Figure 15.1 The early stages of the formation of the solar system.

This process of rocky pieces coming together and sticking by the force of gravity continues, and, in time, a planet is formed. The force of gravity between the planet and the gases around it bring the gases closer to the planet, and they form an atmosphere over its surface.

Modelling planet formation You will need: a small lump of modelling clay for each person in the class (this small lump represents a piece of dust in the cloud around a star) and a video camera (optional).

Plan 1 Look at Figure 15.1 and read again the text about planet formation. 2 Think about how the people in the class could be a model of the cloud of gas and dust, and how the pieces of dust could be drawn together, to represent how dust in a gas cloud is drawn together to make a planet. 3 Discuss your idea with your class and your teacher, and work out a way you might model the formation of a planet. 4 When you have a plan, arrange for someone to video how your class makes a model planet.

Examining the results What are the strengths and limitations of your model?

The formation of the solar system Scientists believe that about 4.6 billion years ago, the Sun and the solar system formed from a huge cloud of gas and dust. They think an exploding star nearby made the cloud begin to rotate. As the cloud turned, it formed a

disc. The force of gravity between hydrogen and helium atoms pulled them together, and they collected at the centre of the disc and formed a star – our Sun. The force of gravity between the dust particles in the disc brought them together and, when they touched, they stuck together and formed rocky particles. In time, the force of gravity between the dust and rocky particles in the cloud produced the first four planets that move around the Sun. Other dust and rocky particles produced the centres of the next four planets, known as the gas giants. Gravity acts between any two objects in the universe and is related to the masses of the two objects. This means that the smaller planets orbit the much more massive Sun and this is why smaller objects, such as the moon, orbit the Earth. Therefore, it is this gravity that keeps the components of the solar system in orbit around the Sun. However, this fact was not understood for a long time. Figure 15.2 Map of our solar system.

DID YOU KNOW? The term ‘Common Era’ is used to describe the time starting just over two thousand years ago, which is when Christians believe that Jesus was born. It is often shortened to ‘CE’. The time before the common era is called ‘Before Common Era’ and is shortened to BCE. The terms are often used in newer books, but in older books you will find ‘AD’ which also relates to the birth of Jesus, and ‘BC’ which relates to time before the birth of Jesus. This means that CE = AD and BCE = BC when looking at historical dates.

Science in context Early studies of the solar system People have looked to the heavens (the sky and everything in it) since the earliest times and they had many ideas about what it might be. Around 12 000 BCE, it was believed in China that the heavens were a giant cover over the Earth. As the centuries went by, this idea was changed to the heavens being like a giant egg,

with the Earth at the centre, but the movement of the stars and planets across the sky were closely observed, noted and used for measuring time from about 8000 BCE. In India from about 5000 BCE, observations and measurements on the sky were also used for measuring time, and in the production of calendars. Timekeeping helped people plan when to sow their seeds to raise crops. In Ancient Greece, observations and ideas about the heavens developed over time to produce a model of the Sun and planets as shown in Figure 15.3. It was believed that each object in the heavens – the planets and Sun – was set in a separate crystal sphere which moved around the Earth. Figure 15.3 The Ancient Greeks’ view of the solar system.

As time went on, this arrangement of crystal spheres did not fully fit with the observations then being made. For example, Mercury and Venus did not move so far from the Sun as the crystal sphere model suggested, nor did the model explain the backward motion of Mars, Jupiter and Saturn that sometimes occurred. A new model was constructed by Ptolemy, an Egyptian astronomer who lived about 1800 years ago, with looped paths of the planets which seemed to explain the observations (such as the occasional backwards movements of the planets). Nearly 800 years ago, the Persian Islamic scholar Muhammad ibn Muhammad ibn al-Hasa alTūsī, sometimes known as Tusi, developed a new model to explain how the planets might move. This model is today called the Tusi couple, which some science historians think might have helped Nicholas Copernicus with his model of the solar system with the Sun at the centre instead of the Earth (see Figure 15.4 on the next page). Figure 15.4 A Copernicus model of the solar system.

At first there was great resistance to the idea that the planets went round the Sun instead of the Earth, because this meant that the

Earth was not the centre of the known universe. This resistance began to weaken due to the scientific work of Galileo in Italy, Tycho Brahe in Denmark, Kepler in Germany and Newton in England in the sixteenth to eighteenth centuries. Galileo Galilei, using his telescope, showed that Jupiter had four moons that moved around the planet. This proved that some things in the sky did not move around the Earth but moved around other objects instead. Tycho Brahe made a huge number of measurements of the objects in the night sky, including a comet whose movement showed that the crystal spheres did not exist. Johannes Kepler used Brahe’s data to refine the Copernicus model and show that the planets moved in ellipses. This new model of the solar system is the one we use today, and was used by Isaac Newton to show how the planets were drawn around the Sun by the force of the Sun’s gravity. 1 List the models that have been used to explain the heavens and the movement of the objects in it. 2 What were the earliest observations on the stars and planets used for? 3 Was our knowledge of the solar system built up from observations and ideas in one place on the Earth? Explain your answer. 4 What planets are missing from the Copernicus model of the solar system? Use the internet to find out who discovered them and when.

Exploring the solar system The first method that was used to explore the solar system was to look up at the night sky and note the positions of the planets as they moved in their orbits. Later, with the invention of the telescope, more discoveries were made. Today, we send out space probes (like the one shown in Figure 15.5) to collect more data and use it to revise the information that we have already collected.

Figure 15.5 A space probe.

You can explore the solar system and make an up-to-date spreadsheet of information by finding out facts about the planets. Remember that as you discover their movements around the Sun, it is the force of gravity that is pulling them round.

Science in context Interstellar objects An interstellar object is an object which moves across space between the stars. They are believed to have moved into a position where the pull of gravity of their star is no longer strong enough to keep them in orbit, so they move away across the universe. The first interstellar object was discovered in 2017 by an astronomer at the Haleakala Observatory in Hawaii. It was named ‘Oumaumau’ which is an Hawaiian word for ‘messenger sent from the distant past to reach out to us’ or, alternatively, simply ‘scout’. Oumaumau was travelling very fast. This was due to the lack of any force, like air resistance, to slow it down in the vacuum of space. It was travelling so fast that it was not pulled into orbit by the Sun’s gravity and continued across the universe. Figure 15.6 An artist’s impression of the appearance of Oumaumau.

A second interstellar object was discovered in 2019 by a Russian engineer who built telescopes, and explored the sky as an astronomer in his spare time. When he reported his discovery, other astronomers checked his work and, within weeks, the New Hubble Space Telescope had found it and photographed it. The object is believed to be a comet and is named comet 2I/Borisov after its discoverer, Gennadiy Borisov. Figure 15.7 The New Hubble Space Telescope’s photo of the interstellar comet 2I/Borisov.

All reports of discoveries and enquiries in any science are peerreviewed. This means that other scientists check the observations, measurements and methods that a scientist or group of scientists has used to draw their conclusions. If the scientists who take part in the peer review draw the same conclusions, then the results of the discovery or enquiry are considered to be reliable. 5 Use the internet to search for the latest reports of interstellar objects. Check more than one report about the same object. Look for different ideas about the object. Some reports may be biased towards aliens and claim that the object is an alien spacecraft! Other reports may use our knowledge and understanding of the universe to explain the object. Make a report of what you find.

The Moon, the Sun and the tides We have seen that the force of gravity exists between any two objects in the universe, and have used this knowledge to explain the movement of the planets in a planetary system, such as our solar system. The force of gravity between the Moon, the Sun and the Earth also have an effect on the water in the seas and oceans of our planet. The movement of the Moon affects the tides at the edges of our oceans and seas. The water in the oceans is not only pulled down by the Earth’s gravity but it is also affected by the gravitational pull of the Moon, and to a smaller extent by the gravitational pull of the Sun. Differences in the force of attraction of the Moon on different parts of the Earth cause the oceans beneath the Moon and on the opposite side of the Earth to rise up slightly, causing a high tide. The level of water in the other parts of the oceans falls and produces a low tide in those places. As the Earth rotates, different regions of the seas and oceans move under the Moon and experience the full force of its gravitational pull. This brings a period of high tide to that region, which is followed by a period of low tide as the Earth continues its rotation.

Figure 15.8 The attraction of the Moon produces high and low tides.

6 The Sun is much larger than the Moon, yet its gravitational pull on the oceans is less than that of the Moon. Why? 7 Describe the tidal forces on Earth as a consequence of the gravitational attraction between the Earth, Moon and Sun. Twice a month, the Sun and the Moon are in line as the Moon orbits the Earth (Figure 15.9). At these times, the difference in the sea-level at high and low tide (see Figure 15.11) is greatest as the small gravitational pull of the Sun reinforces the effect of the stronger pull of the Moon. The tides at these times are called spring tides. When the Moon is furthest out of line with the Sun and the Earth, the difference in the high and low tides is at its least since the gravitational pull of the Sun counteracts that of the Moon. The tides at these times are called neap tides.

DID YOU KNOW? As the Moon moves around the Earth, it also spins on its axis. The speed at which it rotates makes the Moon always keep the same part of its surface facing the Earth. This is why the surface of the Moon always appears the same to us.

Figure 15.9 How the positions of the Sun and Moon cause a change in the tides.

You can use this knowledge, along with your knowledge of the phases of the Moon, to predict the state of the tides around the world. Here is some information about the phases of the Moon to help you make your prediction.

The phases of the Moon The Moon moves around the Earth in about 28 days. Only the side of the Moon’s surface that is facing the Sun reflects light so, as its orbit progresses, the illuminated part that we can see from Earth changes shape. The different shapes are known as phases of the Moon (Figure 15.10).

Figure 15.10 Phases of the Moon.

Figure 15.11 High tide (left) and low tide (right).

LET’S TALK If you saw the full Moon one night, what changes in shape would you expect to see in the following days? Explain your answer.

How can we predict the tides? You will need: a clear night with the Moon in view, and internet access.

Process 1 Look at the phase of the Moon using Figure 15.10 to help you. 2 Predict how the Moon will change in the next seven days. 3 Predict the tide at the coast nearest to you over the next seven days. For example, is it going to change from neap to spring or spring to neap? 4 Use the internet to find the tide tables for the coast nearest you.

Examining the results Compare the pattern shown in the tide tables with your prediction. How do they compare?

Eclipses of the Sun and Moon If you look back at Figure 15.2, the map of our solar system (page 170), you will see that the orbits of all the planets line up as if they were on an imaginary surface of a flat disc. Scientists believe that this is due to the way the planets formed from the disc of gas and dust around the Sun.

The imaginary surface in which an orbit lies is called the plane of the orbit. Figure 15.12 shows that the plane of the orbit of the Moon is different from the plane of the orbit of the Earth around the Sun. This means that even when the Moon passes between the Sun and the Earth in its orbit, there is not always an eclipse of the Sun. In fact, eclipses of the Sun are very rare. Figure 15.12 Planes of the orbits of the Moon and the Earth.

When the Sun, Moon and Earth do line up exactly, a total eclipse of the Sun occurs for viewers on a certain part of the Earth’s surface (Figure 15.13). The Moon blocks out the light of the Sun. Figure 15.13 An eclipse of the Sun.

Figure 15.14 An eclipse of the Moon.

Sometimes the Sun, Earth and Moon line up as in Figure 15.14, and an eclipse of the Moon takes place. The Earth blocks out light from the Sun that would normally fall on the Moon at the full Moon phase. Figure 15.15 The Earth’s shadow on the Moon during a partial eclipse of the Moon.

8 Explain what solar and lunar eclipses are and the conditions required for them to happen. 9 How are an eclipse of the Sun and an eclipse of the Moon a similar b different?

LET’S TALK How would you explain that an eclipse of the Sun is different from an eclipse of the Moon?

Modelling eclipses You will need: a large, medium and small ball, and a torch or flashlight.

Process 1 Use the information in Figures 15.13 and 15.14 to create models of an eclipse of the Sun and the Moon using the three balls. 2 Use the information in Figures 15.13 and 15.14 to make more complex models of the eclipses using a torch instead of the large ball. 3 Demonstrate your models and test your audience’s understanding.

CHALLENGE YOURSELF What are the strengths of your eclipse of the Sun model and what are its limitations? What are the strengths of your eclipse of the Moon model and what are its limitations?

Summary • Gravitational force acted on atoms of gas and particles of dust to form the Sun and the planets in the solar system. • Gravitational force holds the components of the solar system in orbit around the Sun. • Tidal forces on Earth are a result of the gravitational attraction between the Earth, Moon and Sun. • When the Earth, Sun and Moon are in certain positions, eclipses occur.

End of chapter questions 1 a What sort of particles do planets form from?

b What pulls these particles together? 2 3 4 5

What was in the cloud the solar system formed from? What do scientists think started causing this cloud to rotate? Name two gas giants in the solar system. a What did scientists originally think was at the centre of the solar system? b What did the model of Copernicus have at the centre of the solar system?

6 What is an interstellar object? 7 How do the tides move on a shore? 8 What kind of tide occurs when the Moon, Earth and Sun are all in line? 9 What kind of eclipse occurs when the Moon is between the Sun and the Earth? 10 What kind of eclipse occurs when the Earth is between the Sun and the Moon? 11 Describe how a planet forms. 12 There is a unit of measurement called the astronomical unit, which is the average distance of the Earth from the Sun. Here are the average distances from the Sun of the first six planets in the solar system in astronomical units: 1.0 0.4 9.6 5.2 0.7 1.52 a Produce a table of these data, using Figure 15.2 (page 170) to name the planets. Scientists look for patterns in their data. In the eighteenth century, astronomers looked for a pattern in the orbital distances in the following way. Start with the number sequence 0, 3, 6, 12 … Then continue to double the last number. Add 4 to each number, and then divide the total by 10. For example:

0 + 4 = 4 4 ÷ 10 = 0.4 3 + 4 = 7 7 ÷ 10 = 0.7 The resulting numbers are called Bode numbers after the German astronomer Johann Bode (1747–1826). b Calculate the first seven Bode numbers in order and write down how they compare with the orbital distances of the planets. What conclusions can you draw from these values? c When Uranus was discovered it was found to have a distance from the Sun of 19.2 astronomical units. How did this fit in with the relationship known as Bode’s law? d When Neptune was discovered it was found to have an average distance from the Sun of 39.0 astronomical units. How did this fit in with Bode’s law? e Does the presence of the asteroids between the orbits of Mars and Jupiter support Bode’s law? f Pluto was believed to be a main planet of the solar system for about 70 years, and when it was discovered in 1929 it was found to have an average distance from the Sun of 40.4 astronomical units. How did this fit in with Bode’s law? g Why do you think Bode’s law is no longer used? Now you have completed Chapter 15, you may like to try the Chapter 15 online knowledge test if you are using the Boost eBook.

16 A closer look at the Earth In this chapter you will learn: • about the model of plate tectonics, in which a solid outer layer moves because of flow lower in the mantle • how earthquakes, volcanoes and fold mountains occur near the boundaries of tectonic plates • about the composition of gases contained in clean, dry air • how the composition of clean, dry air can change because of pollution and natural emissions • about the water cycle • how science can have a global environmental impact.

Do you remember? • • • •

What are the three types of rock? What is a fossil and how does it form? Name three types of soil. In the water cycle, a where does evaporation take place? b where does condensation take place? • What is precipitation?

The structure of the Earth In the last chapter, we saw how the planets were made from collisions between dust and rocks in orbit around the Sun. As the collisions took place and the rocks stuck together, energy released as heat kept the rocky planets molten. Inside the molten planets, gases built up which exploded onto the surface through volcanoes, to produce the first atmospheres. Based on the

data they have collected, scientists believe that the Earth formed about 4.6 billion years ago. In time, the planet cooled and developed the structure it has today, shown in Figure 16.1. Figure 16.1 The major parts of the Earth’s structure.

The Earth beneath your feet is divided into three regions – the crust, the mantle and the core.

The core The core is divided into two parts – the inner core and the outer core. The inner core is a ball of iron and nickel, which is 2740 km in diameter. It is very hot at the centre of the Earth, but the metals in the inner core stay solid due to all the materials in the upper layers of the Earth squeezing them, as a result of the force of gravity.

DID YOU KNOW? There are radioactive elements in the inner core of the Earth that give out heat and keep the core at a temperature of about 5000 °C! The outer core is 2000 km thick and is composed of more iron and nickel. The metals here are not being squeezed as hard as the metals in the inner core, and so are in liquid form. 1 Why is the centre of the Earth hot?

The mantle The mantle is made of rocky material and is 2900 km thick. It is composed mainly of the elements iron, silicon, oxygen and magnesium. The atoms of these elements are joined together to make substances called compounds. The main compounds in the mantle are called silicates. They are made from silicon and oxygen atoms, which combine with atoms of other elements.

The mantle is very hot – for example, it is 1500 °C at a depth of 2000 km below the Earth’s surface. This is above the normal melting point of the rock, but the pressure of the materials above it keep the rock solid. The upper mantle, near the crust, is cooler and is under less pressure. This allows the rocky material to behave like a very thick liquid and it flows, a little like toothpaste does when you squeeze the tube gently.

The crust The Earth’s crust is made from much cooler rocks than the mantle. Although the rocks at the surface can feel cool or cold, miners and cavers can feel an increase in temperature as they go down into the Earth’s crust. Figure 16.2 This cave is in Sichuan Province, China. As the caver goes deeper the cave will become warmer and warmer.

2 How is the mantle different from the core? 3 How is the crust different from the mantle?

LET’S TALK If you could get in some sort of capsule and travel down to the centre of the Earth, how do you think the conditions around you would change as you made your journey?

Modelling Earth with a hard boiled egg You will need: a hard boiled egg and a knife.

Hypothesis The structure of the Earth is similar to the structure of a boiled egg. Is this hypothesis testable? If you cannot decide, look back at the

Did you know? on page xiv.

Investigation Investigate this statement by removing some of the shell, then cutting the egg in half and looking at the inside. Which part could be the core, the mantle and the crust?

Conclusion Compare the model with Figure 16.1. How accurate is the model? Explain your answer.

Science extra: Types of crust There are two types of crust – oceanic crust and continental crust. Oceanic crust is much thinner than the continental crust. It is only about 7 km thick, while continental crust can be up to 80 km thick. Oceanic crust is also much younger. The oldest oceanic crust is less than 200 million years old, while the average age of continental crust is about 2 billion years.

The tectonic plates It has been found that the Earth’s crust is divided up into huge pieces of rock called tectonic plates, as figure 16.3 shows. The scientific theory of plate tectonics describes how the hot, thick liquid mantel moves around under the plates, causing them to move too. 4 On which tectonic plate do you live? 5 If you were to take another boiled egg and use it as a model of the Earth, how would you use a spoon to make it more realistic after learning about tectonic plates?

Figure 16.3 Map of the major tectonic plates.

Science extra: A cross-section through a tectonic plate If you could cut off a piece of a tectonic plate and look at its edge you would see a top layer called the crust. This would be made of either oceanic or continental crust. Beneath it you would see a rigid layer of the mantle. The crust and this layer together form the tectonic plate. Beneath them is a layer which is made of softer mantle materials which can move slowly.

DID YOU KNOW? Tectonic plates move at a similar speed to the speed your fingernails grow.

CHALLENGE YOURSELF Make a diagram of a cross-section of a tectonic plate using the information in the paragraph above. Compare your diagram with ones drawn by other students in your class. Do they all show the same information?

Movement of the tectonic plates You may think that the ground you walk about on is not moving. The ground is the surface of a tectonic plate, and tectonic plates are always moving very slowly. Under the plates is the mantle material and this moves about due to the heat inside the planet. The moving mantle makes the tectonic plates on the surface of the planet move too – but very, very slowly. Scientists call the edges of a tectonic plate its boundaries. As tectonic plates move, one of three things may happen at their boundaries. Figure 16.4 The three types of tectonic boundary.

1 Their boundaries may slide by, next to each other, as in Figure 16.4a.

As the boundaries of the plates slide past each other, they vibrate and cause earthquakes. Many examples of this type of sliding occur under the oceans, but some are found on land. The San Andreas fault-line in North America is one example. 2 Their boundaries may move apart, as in Figure 16.4b. When plates move apart in this way, the hot runny rock in the mantle, called magma, pushes its way up through the crust and spreads out on each side of the boundary. If this movement occurs in the oceans, when the hot molten rock meets the much colder ocean water, it freezes on either side of the boundary. 3 Their boundaries may crash into each other, as in Figure 16.4c. As this happens, one boundary goes underneath the other. When the boundaries of the two plates collide, they can cause earthquakes.

Fold mountains At some places where the boundaries crash together, the rocks in the upper part of the crust buckle and bend. They form mountains called fold mountains, as Figure 16.5 shows. Fold mountains occur at many places on Earth, such as Mount Everest in the Himalayas. Figure 16.5 How fold mountains are created.

Modelling fold mountains You will need: two small, cloth hand towels and a cell phone camera (optional).

Process 1 Place the two towels side by side on a smooth tabletop. 2 Push them together and observe what happens as they are squashed to half their width.

3 Make a drawing or take a photograph of your fold mountains. 4 Compare your fold mountains with Figure 16.5.

Examining the results What are the strengths and limitations of your model? If one of the plates is an oceanic plate and the other is a continental plate, the oceanic plate is forced downwards, back into the mantle, where its rocks melt to form magma. This pushes its way back up through the crust, breaks through the surface in an eruption and forms a volcano. As the two plates continue to press together, the layers of rock on the continental plate are squeezed together, which buckle and rise to make fold mountains.

Volcanoes The first rocks to be identified as igneous rocks were those that formed from volcanic eruptions. These eruptions have been taking place since the Earth first formed, and they continue today.

Volcanoes and boundaries Figure 16.6 (below) shows the distribution of volcanoes worldwide. Compare this map with Figure 16.3 on page 186 showing the major tectonic plates. Figure 16.6 Worldwide volcano distribution.

6 What conclusion can you draw from comparing Figures 16.6 and 16.3? How good is your evidence for your conclusion? Explain your answer.

Earthquakes We have seen how the edges of tectonic plates can meet and that, when they do, they can cause stresses in the rocks, which make them crack. When this

happens, energy is released as waves which travel though the planet and cause vibrations on the surface far away from where the crack occurred.

LET’S TALK When you stand completely still on a rock in the countryside are you really not moving? Explain your answer.

Science extra: Epicentre and hypocentre The place where the crack happens and the energy is released is called the hypocentre, or focus, of an earthquake. The point on the planet’s surface directly above the hypocentre is called the epicentre (see Figure 16.7). Figure 16.7 Focus and epicentre of an earthquake.

Measuring the intensity of earthquakes The waves of energy produced by an earthquake cause the environment to shake or vibrate. The Italian Earth scientist, Guiseppe Mercalli, observed the whole range of vibrations produced by earthquakes. He made them into a scale in 1902 which was later revised to make the Modified Mercalli intensity scale (MM or MMI).

LET’S TALK If you have experienced an earthquake, describe what happened. Use the Modified Mercalli intensity scale to find the level you experienced. Earthquakes can have tragic consequences, so if you have experienced a tragedy, consult your teacher before taking part in this activity.

How can we use the Modified Mercalli intensity scale? You will need: access to the internet.

Investigation 1 Look up the Modified Mercalli intensity scale in your search engine and compare the effects produced by the different levels. 2 Visit several sources and, if there is a difference in what the sources are saying, consider the authority of the website – for example, see if it is well known, such as NASA, or less well known, such as someone’s blog – and then make a choice about which source to rely on most heavily for information. 3 Which levels are concerned with the feelings of people and which are concerned with damage to buildings?

Science in context The seismometer The waves which are generated in an earthquake and pass though the planet are called seismic waves, and the piece of equipment used to detect and measure their strength is called a seismometer. Figure 16.8 The replica of Zhang Heng’s seismometer.

In 132 CE, Chinese scientist, Zhang Heng, invented the first seismometer. Although the object itself has been lost, there was enough information about it left behind for a replica to be made (see Figure 16.8). Scientists are not sure how the inside of the device worked, but when an earthquake occurred, a ball left the mouth of one dragon, which indicated the direction of the earthquake. Since then, scientists have continued to work on equipment to detect

earthquakes. In 1259 CE, the Maragheh Observatory was set up in Persia (now present-day Iran), and Islamic scientists used seismometers to detect earthquakes there. In the late-nineteenth century, a team of scientists working in Japan made a seismometer, which has been further developed into the style of device that is used worldwide today (see Figure 16.9). Figure 16.9 A seismometer.

This seismometer has two parts – one which moves when a vibration occurs and another which stays in place. In this seismometer, the rotating drum moves horizontally (to the left and right) when it detects seismic waves and the pen remains still. This movement makes the pen draw a line on the paper as the drum turns. As the drum moves to and fro, the pen makes a wavy line on the paper. The size of the wave on the paper gives an indication of the energy in the seismic wave. In addition to measuring the strength of earthquakes, scientists around the world have set up seismometers to investigate the structure of the Earth. They have done this by collecting the data from all the seismometers to detect how the waves pass through the planet.

The Richter magnitude scale This scale was created by the American twentieth-century scientist Charles Richter for use with the data collected by the seismometer. It is used to measure the energy in an earthquake and relate it to the effects the earthquake produces.

How can we use the Richter magnitude scale? You will need:

access to the internet.

Investigation 1 Look up the Richter magnitude scale in your search engine and compare the effects produced by the different levels. 2 Visit several sources and, if there is a difference in what the sources are saying, consider the authority of the website – for example, see if it is well known, such as NASA, or less well known, such as someone’s blog – and then make a choice about which source to rely on most heavily for information. 3 Which levels are concerned with the feelings of people and which are concerned with damage to buildings?

What can you learn from vibrations? You will need: a small tin with a lid and a range of small objects made from different materials collected by your teacher.

Hypothesis As the knowledge of the structure and materials inside the Earth has been deduced by observing the vibrations of seismic waves, it should be possible to deduce the structure and type of materials inside a container by making the container vibrate. Is this hypothesis testable? Explain your answer.

Prediction When a container containing an object is shaken, the structure and material of the object inside can be deduced.

Investigation 1 Ask your helper to put one of the items they have collected in the tin and secure the lid.

2 Take the tin and shake it as gently, forcefully, slowly or quickly as you like, as many times you like and then make a deduction about the item inside. 3 Take out the item and compare it with your deduction. 4 Repeat steps 1–3 with all the items.

Examining the results How many items were correctly deduced? What percentage of items was deduced correctly?

Conclusion Compare your results to the hypothesis and prediction and draw a conclusion. How could your experiment be improved?

Science in context Predicting earthquakes and eruptions In 2009, a colony of toads being studied by scientists suddenly moved away and, a few days later, an earthquake occurred that caused widespread devastation. Less scientific observations on animals have revealed that some vertebrates and insects show a change in behaviour before an earthquake occurs. Many hypotheses have been put forward for this, including being extra sensitive to seismic waves arriving in their habitat, changes in ground-water, gases in the air, or charged particles in the atmosphere but, at the time of writing, no definite conclusions have yet been found. Scientists are trying to improve their accurate prediction rate, which uses data from the past in a particular area to calculate the probability of another earthquake occurring soon. Providing accurate predictions allows people living in an earthquake area to make preparations. An app for smartphones has been developed which connects to seismographs set up in an area and sets off an alarm when large seismic waves are building up. This gives the people living there a

ten-second warning to prepare for an earthquake. In time, this technology may be developed for worldwide use. Predicting volcanic eruptions like the one shown in Figure 16.10 is a little more accurate than earthquakes due to identifying volcanoes as active (they frequently erupt), dormant (they have not erupted for some time) and extinct (they have not erupted for a very long time). Before a volcano erupts it may generate earthquakes, change the composition of the gases it is releasing at the vent and increase its slope as it fills up with magma to release. All of these features can be monitored by seismometers, gas sensors and tiltmeters respectively. Any changes detected by these devices can be used to make a prediction about the eruption. Figure 16.10 An erupting volcano.

7 Area A has had two weak earthquakes in the last 200 years and area B has had four strong earthquakes during the same time. Which area will probably have an earthquake first? Could you make a guess at about the time each area will probably experience its next earthquake? 8 Someone put a post on the internet saying that because of the way the Earth is made, anyone anywhere on the planet could be affected by an earthquake and a volcanic eruption at any time. How would you explain that this is not true?

The Earth’s atmosphere So far, we have looked at the structure of the solid part of the Earth. The solid part is surrounded by layers of gases known as the atmosphere. The very first atmosphere was probably made from hydrogen and helium that was present in large quantities as the solar system developed. In time, most of the small atoms of these elements drifted off into space, although some are still present, especially in the uppermost layer of the atmosphere

today. There were a huge number of volcanoes on the surface of the early Earth and they released gases into the atmosphere as they erupted, or between eruptions. The composition of the early atmosphere is not known for certain, but it probably contained large amounts of carbon dioxide and small amounts of other gases such as methane and ammonia. Figure 16.11 The composition of the atmosphere.

When the first plants developed 3 billion years ago, they produced oxygen as a waste product and the oxygen reacted with ammonia to produce nitrogen. Clean, dry air of the modern atmosphere contains 78% nitrogen and 21% oxygen, and these percentages remain constant. There are also small amounts of other gases such as carbon dioxide, ozone and nitrous oxides. The exact amount of these gases is not fixed and can be affected by pollution and natural emissions, such as respiration.

The Earth’s changing atmosphere Figure 16.12 An artist’s impression of the atmosphere of early Earth.

DID YOU KNOW? Evidence for the composition of the atmosphere a long time in the past comes from a variety of sources, and one example is the analysis of bubbles of atmosphere that were trapped when ice formed over thousands of years. So far we have considered the natural emissions that occur on Earth, but there are also pollutants due to the activities of the human population. Most scientists agree that the burning of fossil fuels to release energy is increasing the amount of carbon dioxide in the air, and that is contributing to climate change. This energy is released to generate electrical power, move trucks, cars and work machines in industry. Other substances are produced in this release of energy and are most noticeable in towns and cities. They are nitrogen dioxide and sulfur dioxide, which can damage the

respiratory system, and carbon monoxide, which can affect the nervous system and cause headaches and sickness (in large amounts it is fatal). In addition to the gases in the air, there are tiny particles which occur naturally, such as pollen grains, dust, mould spores, soil and sand. When fossil fuels are burnt, they produce tiny particles too. These are composed of many chemicals, such as ozone, and can combine with water droplets in the air to make smog. When the Sun shines on this mixture, chemical reactions take place that can turn the smog above cities into a brown haze, while down in the streets it causes irritation to the eyes, nose and throat, and deeper in the body it can cause damage to the lungs and heart. Figure 16.13 People walking through city smog.

How can you assess air pollution? You will need: a white piece of wood, a hand lens and access to a space outside where the wood can be safely left for two days.

Hypothesis Air particles have mass and are pulled down by gravity, so will settle on surfaces. The number of particles seen will give an assessment of air pollution. Is this hypothesis testable? Explain your answer.

Prediction What do you predict to find if you leave a white piece of wood outside for two days?

Process 1 Clean the surface of the wood and examine it with a hand lens to check for particles.

2 Set up the wood in a safe place outside where it is exposed directly to the sky and leave it for two days. 3 Examine the surface of the wood with the hand lens and record how many particles have fallen on it.

Examining the results Compare the surface of the wood before and after exposure to the air.

Conclusion Compare your evaluation to your hypothesis and prediction and make a conclusion. What are the limitations of your conclusion? How could the investigation be improved?

Water in the atmosphere and the water cycle A major gas escaping from the early volcanoes was water vapour which, in time as the Earth cooled, condensed to form the water of the oceans. The modern atmosphere also contains water vapour and the amount varies between different places and can change throughout the year. The water moves between the atmosphere and the planet’s surface in the water cycle, as Figure 16.14 shows. Figure 16.14 A simple water cycle.

DID YOU KNOW? Most of the water you give a plant passes out through its leaves in a process called transpiration. In this process, water evaporates into spaces inside the leaf and then the water vapour passes through holes in the leaf out into the air. Transpiration is an important feature of the water cycle.

Evaporation Evaporation occurs at the surface of water anywhere, from a drink in a glass to an ocean. In this processs, water changes into a gas called water vapour, which mixes with other gases in the air and can rise up into the atmosphere.

Condensation The amount of water vapour that can be held in the air depends on temperature. Warm air can hold more water vapour than cold air. When the air containing a large amount of water vapour cools, the water vapour can no longer stay as a gas and it condenses on dust particles to produce water droplets.

Clouds If you look up into the sky in most parts of the world, at some point you will see a cloud. These are made of water droplets that scatter sunlight in all directions and make them appear white. 9 What do you think makes clouds dark and predicts the coming of rain?

Science extra: Precipitation Meteorology is the scientific study of the weather and, in this branch of science, precipitation means the falling of liquid or solid water from the sky, or the condensation of water on the ground. Precipitation can be classified as follows: • Rain: water droplets 0.5–8 mm across. • Drizzle: water droplets below 0.5 mm across. • Snow: ice crystals which have fallen through clouds of very cold water droplets, collided with them and formed snowflakes. • Sleet: a term used in many parts of the world for a mixture of rain and snow. In North America it is used to describe pellets of ice which form when rain falls through very cold air.

• Hail: formed from ice crystals in a thunder-cloud. There are rising air currents in a thunder-cloud that take the ice crystals upwards, and as they rise, more cloud water freezes onto them until they are too heavy for the air currents and fall out of the cloud. • Dew: formed from water vapour in the air which touches the cold ground and condenses to form liquid water. • Frost: formed from water vapour that has formed a liquid on the ground and then frozen to form ice crystals.

LET’S TALK What forms of precipitation occur in your country over the year? Are there times when one form of precipitation occurs very frequently? Explain your answer. In what way does this precipitation affect people’s lives?

Run-off When the amount of rain is great and continues for a long time, water runoff occurs. This is caused by the ground being so soaked that it cannot take in any more water, or if the surfaces on which the rain falls are impervious to water. These impervious surfaces are the tiles on roofs and the tarmac on roads. Run-off can also be caused when snow covering the ground melts. Water running over the surface of the ground can damage it or even destroy it and carry it away, as Figure 16.15 on the next page shows. Figure 16.15 Surface run-off.

In many places, such as farms and cities, drainage systems have been set up to channel the run-off water into rivers without damaging the environment. However, rising river levels further away may cause flooding. If the rain is particularly heavy over a short time, even a drainage system may not be able to carry all the water away, and a flash-flood occurs (see Figure 16.16). Figure 16.16 Flash flooding in India.

Open water sources Examples of open water sources are rivers, natural lakes and artificial lakes called reservoirs. These surfaces are open to the air. The water from open water sources can be used directly to supply crop plants by setting up an irrigation system, which distributes water through a system of channels (see Figure 16.17). Some irrigation systems use pipes and sprinklers. Figure 16.17 Irrigation channels.

Ground-water Water soaks into the ground by a process called infiltration to form groundwater. Ground-water is simply water held in the ground between soil particles or in cracks in the rocks. Water entering the soil sinks down through the soil, gravel and porous rock beneath it, until it reaches a layer of rock which is impervious to it. The water then collects in this rock and can be reached by sinking a well. This whole structure, from porous soil down to impervious rock, is called an aquifer. One of the most striking effects of ground-water is the formation of an oasis in a desert. An oasis is a spring in the desert around which plants grow. It forms because, under the desert surface, there is a layer of porous rock over the top of a layer of non-porous rock. The two layers stretch back from the desert under the surrounding hills and mountains. When it rains in these places, the water drains into the porous rock but is prevented from sinking further by the non-porous rock below. The water then moves out along the layer of porous rock under the desert. In a place where the overlying surface is thin, the water may burst through, forming a spring and a pool. The damp soil around the pool provides a habitat for plants, which in turn provides a habitat for animals. Figure 16.18 How water reaches the desert from the mountains and forms an oasis.

10 Why can an oasis be a lifesaver to travellers lost in the desert? 11 Which property does the non-porous rock have that lets it stop water passing through it? 12 You are given two samples of rock from an oasis. Plan an investigation to find out which one prevents water sinking into it and which one allows water to pass through it.

Figure 16.19 An oasis in the desert of Ica, Peru.

CHALLENGE YOURSELF How can you use spongy material to make a model oasis? What other materials and items do you need? Make a drawing of your model and show it to your teacher. If approved, make your model and demonstrate it on film. What are the strengths and limitations of your model?

Summary • The major features of the Earth’s structure are the crust, mantle and core. • The Earth’s crust is made up of several tectonic plates; a solid outer layer moves because of flow lower in the mantle. • Earthquakes, volcanoes and fold mountains occur near the boundaries of tectonic plates. • The composition of gases contained in clean, dry air contains 78% nitrogen and 21% oxygen, and these percentages remain constant; there are also small amounts of other gases such as carbon dioxide, ozone and nitrous oxides. • The composition of clean, dry air can change because of pollution and natural emissions. • Earth’s modern atmosphere contains water vapour; the water moves between the atmosphere and the planet’s surface in the water cycle.

End of chapter questions 1 What is the part at the centre of the Earth called? 2 What is the part that we walk around on called? 3 How does the temperature change as you go from the Earth’s surface to its centre? 4 What are tectonic plates? 5 What causes earthquakes? 6 What is the difference between the Modified Mercalli intensity scale and the Richter magnitude scale? 7 What two gases do scientists think formed the first atmosphere on the Earth? 8 Which gas makes up most of the atmosphere today? 9 Name three gases produced by the human civilisation that is harmful to human health. 10 What is smog made from? 11 a What process produces the water vapour in the air today? b Where does this process take place? c What happens to water vapour when it cools down? 12 Using the information in this chapter, write a short essay with the title ‘The changing Earth’. Now you have completed Chapter 16, you may like to try the Chapter 16 online knowledge test if you are using the Boost eBook.

Glossary A absorbent – The property of a substance that allows it to take in and hold another substance. For example, a sponge is absorbent because it can take in water. acid – A substance with a pH less than 7 that reacts with metals to produce hydrogen. adaptation – The way a living thing is suited to its habitat so that it can survive there. Adaptation can also mean the process by which living things become more suited to their habitat. alkali – A substance that is soluble in water and makes an alkaline solution – that is, a solution with a pH greater than 7. alloy – A mixture of two or more metals, or of a metal such as iron with a non-metal such as carbon. antenna (antennae) – A flexible limb, usually long, attached to the head of an insect, which is sensitive to touch. Some are also sensitive to taste. atom – A particle from which all substances are made. For example, diamond is made up of carbon atoms. axis (axes) – An imaginary line or rod running through the Earth from the North Pole to the South Pole, around which the Earth turns. Also the horizontal or vertical line on a graph, labelled with a scale and units showing what has been measured. When you draw a graph, you plot the points using the scales on the axes.

B

boiling – The process in which a liquid turns to a gas at the liquid’s boiling point. brittle – The property of a material that means it is hard and breaks easily. bud – A growth on a plant that develops into a leaf, flower or shoot. In yeast, an outgrowth that separates to form a new yeast cell.

C cell – The basic unit of life. The cell contains a nucleus, cytoplasm, mitochondria and a membrane around the outside. The bodies of most living things are made from large numbers of cells. Also the name of a device containing chemicals that react and produce a current of electricity in a closed conducting circuit. cell membrane – A very thin sheet of material that surrounds the cytoplasm in a cell. cell wall – A layer made from cellulose that encloses the cell membrane in plant cells. cellulose – A tough material found in plant cell walls which gives support to the cell. It is made up of long chain particles containing hydrogen, carbon and oxygen. chloroplast – A component of some plant cells. It is green and absorbs some of the energy from sunlight for use in photosynthesis. cilia – Short hair-like projections on the surface of some cells. They can beat backwards and forwards to help with the movement of fluids in animal systems. circulatory system – The heart and blood vessels, through which blood passes as it is pumped round the body. class – A group in the biological classification system. A class is smaller than a kingdom, and larger than an order. classification – Putting things, such as living organisms, into groups so that they can be studied more easily. comet – A huge lump of ice and dust, sometimes called a ‘dirty snowball’, which travels from a great distance beyond the planets to move in orbit

around the Sun. As it approaches the Sun, part of it melts and makes a tail of gas and dust. The tail can often be seen from Earth. compound – A substance made from the atoms of two or more elements that have joined together by taking part in a chemical reaction. condensation – The process in which a gas cools and changes into a liquid. conductor – A material that allows electricity or heat to pass easily through it by conduction. core – The central part of the Earth’s structure, made up of iron and nickel at a high temperature and pressure. It is in two parts – the solid inner core and the liquid outer core. crust – The solid outer shell of the Earth’s structure, made up of rocky material. cytoplasm – A fluid-like substance in the cell in which processes take place to keep the cell alive.

D decay – The breakdown of dead organic material by decomposing organisms (decomposers). Also the breakdown of an atom to form an atom of a different element, releasing a form of energy known as radiation. dichotomous key – A series of choices set out in pairs to help identify a range of living organisms quickly. digestive system – The organs of the alimentary canal, where the process of digestion happens, breaking down large food particles into small ones so that they can be absorbed by the body. dissolve – The process in which a solute mixes with a solvent to form a solution. DNA (deoxyribonucleic acid) – A substance in the nuclei of cells that contains information, in the form of a code, about how an organism should develop and function.

E

ecosystem – An ecological system in which the different species in a community react with each other and with the non-living environment. Ecosystems are found in all habitats, such as lakes and woods. elastic materials – Materials that can return to their original size and shape after being stretched or squashed. electricity – A type of energy due to charged particles staying in one place (static electricity) or flowing from one place to another (electrical current). electrons – Tiny, negatively charged particles which move around the nucleus of an atom. Electrons can produce an electric current in certain circumstances. element – A substance made of one type of atom. It cannot be split up by chemical reactions into simpler substances. energy – Something that exists in different forms – for example, electrical energy, light, sound and heat – and that allows matter to move. environment – The surroundings or conditions in which an organism lives. epidermis – A layer of cells forming a protective surface in an organism – for example, on the surface of a leaf. epithelial cells – Cells which form a tissue lining the surfaces of organs such as the stomach. evaporation – The process in which a liquid turns into a gas without boiling. excretion – Getting rid of wastes made by the body – for example, as urine. Solid waste formed from undigested food, called faeces, leaves the body in a process called egestion.

F family – A group in the biological classification system. A family is smaller than an order, and larger than a genus. flexible – Property of a material that means it can be bent or squashed, but when the pushing or pulling force is removed it springs back to its original shape.

flower – The part of a plant that produces seeds, consisting of the reproductive organs, often surrounded by coloured petals. food chain – A series of organisms linked together by the passage of food between them. When a food chain is drawn as a diagram, arrows show the direction of the energy flow through the system. force – A push or pull that changes the way an object is moving, or makes it change shape. formula (formulae) – A number of words or symbols set out to show a relationship between items that are being studied. fossil fuel – A fuel produced from the fossilised remains of plants and animals that lived long ago. fossil – The remains or impression of a plant or animal that lived in the distant past, preserved in rock. freezing – The changing of a liquid into a solid at the material’s freezing point. It is the reverse of melting. (The freezing point of a substance is the same temperature as its melting point.) fuel – A material such as coal, oil, gas or wood that is burned to release its store of chemical energy, transferring it to heat and light stores.

G gas – The state of matter in which the volume of a certain mass can change, so it takes up the shape of the container holding it. A gas flows easily and it is easy to compress. gravity – A pulling force that exists between any two masses in the universe. The larger the masses, the larger the force.

H habitat – The home of a plant or animal – the place where it lives. hypothesis – An idea suggested to explain something. In science a hypothesis must be able to be tested by making a scientific enquiry.

I

igneous rocks – Rocks formed from magma, which is molten rock beneath the Earth’s crust. They contain crystals of minerals. insulator – A material that does not let heat or electricity pass through it, because its particles do not pass the energy from one to the next. Good insulators are also known as bad conductors. invertebrate – A group of the animal kingdom. Invertebrates do not have an internal skeleton of cartilage or bone. The group includes 95% of all animal species.

K kinetic energy – Energy that an object has because it is moving.

L leaf – A green outgrowth of a plant in which food is made by photosynthesis, using light energy from the Sun. liquid – The state of matter in which a substance flows easily and takes up the shape of the bottom of the container holding it. The volume of a certain mass of liquid cannot change, and it cannot be easily compressed.

M magnification – A measure of the amount by which an image – in a microscope or photograph, for example – is bigger than the original object (magnification = image size ÷ real size). malleable – The property of a material that means it can be shaped by hammering or pressing without the material cracking. It stays in shape after the shaping process has ended. mantle – The part of the Earth’s structure between the solid crust and the core, about 2900 km thick. It is made of rocky material, which is molten in the upper mantle (magma) and flows like a very thick liquid. mass – The amount of matter in a substance. Two units used to measure mass are grams (g) and kilograms (kg). matter – Any substance that takes up space in the universe and has mass.

melting – The changing of a solid into a liquid at the material’s melting point. It is the reverse of freezing. (The melting point of a substance is the same temperature as its freezing point.) membrane – A very thin, flexible sheet of material. microorganism – An organism with a body made from only one cell. Most can only be seen using a microscope. mineral – A substance formed from an element or group of elements. It may have a crystal structure. In plants, mineral substances are taken up from soil water by the roots and used for growth and development. Minerals are also essential nutrients in animal diets. Examples include nitrogen, potassium, phosphorus and iron. mitochondrion (mitochondria) – A tiny structure in a cell which releases energy from food for use in the cell, to keep the cell alive. model – Something which represents something else in the real world. A model helps us to understand something that is happening (in the case of a chemical equation) or the structure of something (in the case of a diagram). monitor – A way of checking on something by comparing one set of data with another taken at a later time.

N neurone – A nerve cell or brain cell. neutral – Neither acidic nor alkaline (pH 7). neutralisation – A process that occurs when an acid reacts with an alkali, in which a salt and water are formed. The pH of the solution after the reaction is 7. nucleus (nuclei) – The control centre of a living cell, containing the genetic material (DNA). Also, the positively charged centre of an atom, which may split, in radioactive elements, releasing a great deal of energy.

O opaque – The property of a material that means it does not let light pass through it.

orbit – The path taken by one object in space around another object, such as the path of a planet around the Sun, or a moon around a planet. order – A group in the biological classification system. An order is smaller than a class, and larger than a family. ore – Rock that possesses a large quantity of a metal compound. An ore can be mined and then processed, usually using heat, to release the metal from its compound in the rock. organ – A part of the body, made up of groups of specialised tissues, that performs a special task to help the organism live. organ system – A group of organs that work together to carry out a task to keep a living organism alive. organism – A living thing, whose organ systems work together to carry out the seven life processes. (In single-celled organisms, the life processes are carried out by structures inside the cell.)

P palisade cells – Plant cells with a tall, narrow shape that allows them to pack closely together in the upper part of a leaf, near the light. They have large numbers of chloroplasts in them to trap as much light energy (from the Sun) as possible for photosynthesis. palisade mesophyll – Plant leaf tissue made up of palisade cells. periodic table – The arrangement of the elements set out in order of their atomic number. pH scale – A scale running from 0 to 14 that is used to compare the strengths of acids and alkalis. The strongest acid is pH 0 and the strongest alkali is pH 14. A solution with a pH of 7 is neutral. phenomenon (phenomena) – A piece of information, such as the presence of light, or an event, such as the formation of a rainbow, that we detect with our senses but that may need further study before it is understood. plate tectonics – A scientific theory which describes how the huge slabs of rock which are present in the Earth’s crust move across the surface of the planet.

potential energy – Stored energy. For example, gravitational potential energy is the energy stored in an object that is above the Earth’s surface, which gives it the potential to move if it is released. precipitate – A solid substance which forms in a solution due to a chemical reaction taking place there. predator – An animal that feeds on other animals. prey – An animal that is fed on by another animal. producer – The plant in a food chain, which makes food by photosynthesis using energy from sunlight. products – The substances (solids, liquids and gases) produced as a result of a chemical reaction.

R radiation – The energy given out by hot or light-emitting objects, such as the Sun. Also, the form of energy released when an atom of a radioactive element decays. Some kinds of radiation can be harmful to living cells. radioactive element – An element whose atoms break down to form smaller atoms of other elements, with the release of large amounts of energy. reactants – The substances (solids, liquids and gases) that take part in a chemical reaction. reproduction – Making more of an organism. Reproduction keeps a plant or animal species in existence in nature, because although individuals are constantly dying, new individuals are constantly produced that replace them. respiration – The process occurring in all living organisms in which energy is released from food inside cells. Glucose reacts with oxygen to release energy for life processes, and carbon dioxide and water are produced. (Not be confused with breathing, which is the process of moving air in and out of the body.) respiratory system – The organs involved in bringing oxygen into the body and getting rid of waste carbon dioxide – the windpipe, bronchi,

lungs, ribs, intercostal muscles and diaphragm. rigid – The property of a material that means it cannot be bent or squashed. root – The underground part of a plant, which anchors it in the soil and takes up water and dissolved minerals. root hair cell – Cell growing a short distance behind the root tip, adapted for taking up water. Root hair cells have long, thin extensions, giving them a large surface area.

S secrete – To produce a liquid. sediment – A collection of solid particles that settle out from a mixture of a solid and a liquid – for example, the rocky fragments of soil settling out after being mixed with water. sensitivity – Being sensitive to the surroundings. sensory system – The sense organs (eyes, ears, nose, tongue and skin), which together detect changes in the surroundings of an animal and send messages as electrical signals along nerves to the brain. solid – The state of matter in which a substance has a definite shape and its volume does not change. It does not flow and it is hard to compress (squash) it. solute – A substance that is dissolved in a liquid (solvent), forming a solution. solution – A liquid (solvent) that has one or more substances (solutes) dissolved in it. solvent – A liquid in which a substance (solute) will dissolve, forming a solution. species – The smallest group in the biological classification system. A species is smaller than a genus, and all the organisms it contains are of the same type. Individuals of a species have a large number of similarities and are able to reproduce with each other to form offspring, but are not able to breed with different species.

specimen – An object or part of an object to be examined in a scientific investigation. spongy mesophyll – Plant leaf tissue made up of roughly round cells with air gaps between them. Spongy mesophyll tissue is important for allowing air to get to leaf cells for photosynthesis, and in drawing water up into the leaf from the xylem tissue. spore – A tiny reproductive structure, a bit like a seed in some ways, forming part of the life cycle of some plants (such as ferns), fungi, bacteria, algae and protozoa. It has a protective case that helps it survive in very hot, cold or dry conditions. stem – The part of a plant that supports the leaves and the flowers and also transports water and food. stoma (stomata) – An opening in the epidermis of a leaf through which water vapour can escape from the spongy mesophyll, and through which air can enter the leaf. The opening and closing of each stoma is controlled by a pair of cells around the opening. There are usually more stomata on the underside of a leaf than on the upper surface.

T theory – An explanation of how something occurs based on the results of many investigations. tissue – A group of the same kind of cells, which together perform a special task in the life of an organism. Different tissues join together to make a larger group of cells called an organ. translucent – The property of a material that lets light pass through it but the light rays are scattered, so you cannot see objects clearly. transparent – The property of a material that lets light pass through it without the light rays being scattered, so you can see objects clearly through transparent materials.

U unit – A standard quantity used for measurement and comparison. Also, a basic structure from which more complex structures are built up.

urine – A mixture of urea, water and other dissolved substances, which is produced in the kidneys and stored in the bladder before it is released from the body.

V vacuole – A large, permanent space in the cytoplasm of a plant cell that has a membrane around it and is filled with a liquid called cell sap, containing dissolved sugars and salts. variable – A factor that can change in value. In a scientific enquiry, you investigate the effect of one variable on another by changing one (the independent variable) and measuring the other (the dependent variable). All other factors must be kept the same to make sure it is a fair test. vertebrate – A group of the animal kingdom, which includes mammals, birds and fish. Vertebrates have an internal skeleton of cartilage or bone. volume – The space occupied by a certain amount of matter.

W waterproof – The property of a material that does not let water pass through it. weight – The downward force on an object due to gravity.

X xylem tissue – Tubes in plant roots, stems and leaf veins, which carry water from the roots up to the leaves.

Acknowledgements The author would like to thank Katie Mackenzie-Stuart, Helen Cunningham, James Maroney and Kate Crossland-Page and her team for their advice and support in the preparation of this book. The Publishers would like to thank the following for permission to reproduce copyright material: Photo credits p.vi t, l © Digital Vision/Getty Images; t, c © Monkey Business/Stock.adobe.com; t, r © Photodisc/Getty Images; b © Richard Carey – Fotolia; p.vii t © Shotsstudio/Stock.adobe.com; c © James KingHolmes/Alamy Stock Photo; b © Kay Nietfeld/EPA/Shutterstock; p.viii t, l © Andrew Lambert Photography/Science Photo Library; t, c © Andrew Lambert Photography/Science Photo Library; t, r © Andrew Lambert Photography/Science Photo Library; c, l © Andrew Lambert Photography/Science Photo Library; c, r © Andrew Lambert Photography/Science Photo Library; b, l © Andrew Lambert Photography/Science Photo Library; b, r © Andrew Lambert Photography/Science Photo Library; p.xiii t © YEINISM/Shutterstock.com; b © Agencja Fotograficzna Caro/Alamy Stock Photo; p.xiv l © Rita Petcu/Stock.adobe.com; c © JohanSwanepoel Fotolia.com; r © Jason Stitt/Stock.adobe.com; p.xvii © Familieeisenlohr.de/Stock.adobe.com; p.1 © Peter Chadwick/Science Photo Library; p.2 © Claude Nuridsany & Marie Perennou / Science Photo Library; p.3 © 2707195204/Stock.adobe.com; p.4 © Andrea Izzotti/Stock.adobe.com; p.5 © Anatol – Fotolia; p.6 © Robert Hardholt – Fotolia; p.9 © NASA/Science Photo Library; p.13 © George Bernard/Science Photo Library; p.16 t, l © James White/Danita Delimont/Stock.adobe.com; t, r © Maria Harvey/Shutterstock.com; b © Willyam/Stock.adobe.com; p.17 t © The Art Gallery Collection/Alamy Stock Photo; b © Alessandro Mancini/Alamy Stock Photo; p.21 t © Copyright © 2019 Sonali Garg et al; b © Dong Lei/Nature Picture Library/Alamy Stock Photo; p.24 t © The Art Gallery Collection/Alamy

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