Collins Cambridge IGCSE™ Combined Science Student's Book [2nd Edition]

Collins

Cambridge IGCSE™ Combined Science

• Reach your full potential with clearly differentiated extended material

This resource is endorsed by Cambridge Assessment International Education

✓ Supports the full syllabus for examination from 2025

✓ Has passed Cambridge International's rigorous quality-assurance process

✓ ✓ For Cambridge schools worldwide Developed by subject experts

978-0-00-8545918

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Registered Cambridge International Schools benefit from high-quality programmes, qualifications, assessments and a wide range of support so that teachers can effectively deliver in the classroom. Visit www.cambridgeinternational.org to find out more.

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• Refine your exam technique and improve your answers with practice questions, example answers and commentary

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• Understand how your learning applies in the wider world from the ‘science in context’ sections

STUDENT’S BOOK

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• Manage your own learning and progress with end-oftopic checklists and knowledge checks

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• Learn how to answer questions well with supportive worked examples

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• Develop your practical science skills with investigative tasks

Combined Science

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• Check your understanding and embed knowledge with questions throughout

Cambridge IGCSE™ Combined Science STUDENT’S BOOK

Collins Cambridge IGCSE™ Combined Science Student’s Book provides in-depth coverage of the latest Cambridge IGCSE Combined Science (0653) syllabus for examination from 2025 and is written by a team of experienced authors.

Cambridge IGCSE™

Find us at collins.co.uk/international facebook.com/collinsint @Collins_Int

Malcolm Bradley, Gurinder Chadha, Jackie Clegg, Susan Gardner, Sam Goodman, Sarah Jinks, Sue Kearsey, Gareth Price, Mike Smith and Chris Sunley

Contents Biology

C3 Stoichiometry.................................258

Getting the best from this book.............. 4

C4 Electrochemistry............................276

B1 Characteristics of living organisms........................................10

C5 Chemical energetics......................286

B2 Cells....................................................16

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B3 Movement into and out of cells....32 B4 Biological molecules.......................48 B5 Enzymes............................................56

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B6 Plant nutrition..................................68

C6 Chemical reactions........................300 C7 Acids, bases and salts...................320 C8 The Periodic Table..........................342 C9

Metals..............................................370

C10 Chemistry of the environment...388 C11 Organic chemistry.........................408

B7 Human nutrition..............................80

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C12 Experimental techniques and chemical analysis...........................438 B8 Transport in plants..........................94

Physics

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B9 Transport in animals.....................116 B10 Diseases and immunity................134

P1 Motion, forces and energy...........464

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B11 Gas exchange in humans.............146

B13 Drugs...............................................164

B16 Human influences on ecosystems.....................................204

Chemistry C1 States of matter.............................220

P5 Space physics.................................648 Developing experimental skills...........668 Preparing for assessment.....................685 Periodic Table..........................................688 Glossary ...................................................689 Answers....................................................707 Index.........................................................735

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C2 Atoms, elements and compounds.....................................230

P4 Electricity........................................598

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B15 Organisms and their environment..................................190

P3 Waves...............................................560

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B14 Reproduction.................................170

P2 Thermal physics.............................530

CONTENTS

B12 Respiration.....................................156

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Biology B11 Gas exchange in humans..............146

B2 Cells..........................................................16

B12 Respiration........................................156

B3 Movement into and out of cells........32

B13 Drugs..................................................164

B4 Biological molecules........................... 48

B14 Reproduction....................................170

B5 Enzymes................................................. 56

Practice questions for sections B9–14 ������������������������������������� 186

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B1 Characteristics of living organisms. 10

B6 Plant nutrition....................................... 68 B7 Human nutrition.................................. 80

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B8 Transport in plants................................94

B16 Human influences on ecosystems.....................................................204

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Practice questions for sections B1–8 ��������������������������������������� 106

B15 Organisms and their environment........................................190

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B9 Transport in animals..........................116

Practice questions for sections B15–16 ����������������������������������� 215

B10 Diseases and immunity..................134

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CONTENTS

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Around 1.9 million living species have been identified on Earth.

Around 50 000 of these species are classified as plants and around 1.37 million species are classified as animals. Over 66 000 of the animal species are vertebrates (animals with bony skeletons) and the rest are invertebrates (animals without backbones), of which the majority (around 1 million species) are insects.

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It is difficult to know how many species are still to be discovered, although it is thought that about 15 000 new species are discovered around the world every year. The smaller the organism, the greater the chance that there are species we don’t yet know about. So, although around 4000 species of bacteria have been identified, there could be many more species of bacteria than of all the other kinds of organism put together.

STARTING POINTS 1. What do we mean when we say that something is alive?

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2. How do the characteristics of life look different in different organisms?

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SYLLABUS SECTIONS COVERED B1.1 Characteristics of living organisms

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B1

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Characteristics of living organisms

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Characteristics of living organisms INTRODUCTION

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ΔΔFig. B1.1 Tiny tardigrades (about 1 mm long) are one of the toughest organisms known. They can survive temperatures below –200 °C, 10 days in the vacuum of space and over 10 years without water!

Deciding whether something is alive is one of the most important starting points of biology.

Scientists have decided on around seven characteristics that help decide whether something is alive. However, this is not always as straightforward as we might think, and trying to decide what the characteristics of life are can be difficult. Viruses are problematic to categorise as they share some of the characteristics of life, but not others. While scientists say that they are not alive, viruses are still studied within biology and they carry out some of the characteristics of life.

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LEARNING OBJECTIVES ✓✓Describe the characteristics of living organisms by defining: movement as an action by an organism or part of an organism causing a change of position or place. ✓✓Describe respiration as the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. ✓✓Describe sensitivity as the ability to detect and respond to changes in the internal or external environment. ✓✓Describe growth as a permanent increase in size and dry mass. ✓✓Describe reproduction as the processes that make more of the same kind of organism. ✓✓Describe excretion as the removal of the waste products of metabolism and substances in excess of requirements. ✓✓Describe nutrition as the taking in of materials for energy, growth, and development.

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Characteristics of living organisms

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KNOWLEDGE CHECK ✓✓Living organisms show a range of characteristics that distinguish them from dead or non-living material. ✓✓The life processes are supported by the cells, tissues, organs, and systems of the body.

CHARACTERISTICS OF LIVING ORGANISMS There are seven life processes that most living organisms will show at some time during their life.

• Movement: Organisms may move their entire body so that it changes position or place. Organisms may also move parts of their body. For example, plants may move their leaves in response to external stimuli such as light, while structures in the cytoplasm of all living cells move.

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• Respiration: This is a series of chemical reactions inside living cells that break down nutrient molecules and release energy. The energy released from respiration is used for all the chemical reactions that help to keep the body alive. Together, these reactions are known as metabolism.

• Sensitivity: Organisms are able to detect (or sense) and respond to changes in the environment around them. For example, we see, hear, and respond to touch.

Organisms can also detect and respond appropriately to changes inside their bodies (the internal environment).

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ΔΔFig. B1.2 Sunflowers respond to light by tracking the Sun across the sky during the day.

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• Growth: This is the permanent increase in size of an organism. Growth is also often defined as an increase in dry mass (mass without water content) of cells or the whole body of an organism. This is because total mass can vary, depending on how much the organism eats and drinks. Dry mass only measures the amount by which the body increases in size when nutrients are taken into the cells and used to increase their number and size.

• Reproduction: This includes the processes that result in making more individuals of that kind of organism, such as making gametes and the fertilisation of those gametes.

• Excretion: This is the removal from the body of substances that are toxic (poisonous) and may damage cells if they stay in the body. Living cells produce many products from the metabolic reactions that take place inside them. Some of these are waste products – materials that the body does not use; for example, animals cannot use the carbon dioxide produced during respiration. As these waste products may also be toxic they must be removed from the body by excretion.

Organisms also excrete substances that are in excess, where there is more in the body than is ΔΔFig. B1.3 Growth of a child can be needed. measured by recording their change in

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Characteristics of living organisms

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height over time.

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• Nutrition: This is the taking of nutrients into the body. Nutrients are the raw materials needed by the cells to release energy and to make more cells for growth, development and repair. Plant nutrition requires light, carbon dioxide, water, and mineral ions, such as nitrate and magnesium. Animal nutrition requires organic compounds such as carbohydrates and proteins, mineral ions such as iron and calcium, and usually water.

All these characteristics will be described in greater detail in later sections in this book.

QUESTIONS 1. For each of the seven characteristics, give one example for: a) a human

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b) an animal of your choice c) a plant.

2. For each of the seven characteristics, explain why they are essential to a living organism.

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An easy way to remember all seven processes is to take the first letter from each process. This spells Mrs Gren. Alternatively you may wish to make up a sentence in which each word begins with same letter as one of the processes, for example: My Revision System Gets Really Entertaining Now.

Characteristics of living organisms

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End of topic checklist Key terms cytoplasm, excess, excretion, growth, metabolism, movement, nutrition, plant, reproduction, respiration, sensitivity

During your study of this section you should have learned:

❍❍About the seven characteristics of life: movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition.

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End of topic questions

1. State and describe the seven processes of life.

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2. Give two life processes necessary for an organism to release energy.

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3. Explain why dry mass is often used to measure growth.

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4. When you place a crystal of copper(II) sulfate in a saturated solution of the same compound, the crystal will increase in size. Does this mean that the crystal is alive? Explain your answer.

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5. Plants cannot move about, as animals can. Does that mean animals are more alive than plants? Explain your answer.

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Characteristics of living organisms

6. During winter, an oak tree in the UK will lose its leaves and not grow. Is the tree still living during this time? Explain your answer using all the characteristics of life.

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This section will give you plenty of practice at writing word and symbol equations. You will also see the importance of balancing equations so that the number and type of atoms in the substances that are reacting is equal to the number and type of atoms in the products formed so that matter is neither created nor destroyed in the reaction.

Starting points 1. Do you know the chemical formulae of common substances such as water and carbon dioxide? Make a list if you think you know more.

2. Can you remember a word equation you have come across before?

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3. What do you think ‘state symbols’ might be?

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SYLLABUS SECTIONS COVERED C3.1 Formulas

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C3

Stoichiometry

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Formulas INTRODUCTION

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Stoichiometry is the branch of chemistry concerned with the relative quantities of reactants and products in a chemical reaction. A study of stoichiometry depends on balanced chemical equations which, in turn, depend on knowledge of the chemical symbols for the elements and the formulae of chemical compounds. This topic starts by considering how simple chemical formulae are written and then looks in detail at chemical equations.

ΔΔFig. C3.1 When this reaction is described as S(s) + O2(g) → SO2(g), it is understood by chemists all over the world.

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KNOWLEDGE CHECK ✓✓Elements are made up of atoms. ✓✓Compounds are formed when atoms combine together. ✓✓Molecules are formed in covalent bonding and ions are formed in ionic bonding.

FORMULAS When elements chemically combine, they form compounds. A compound can be represented by a chemical formula. The formula of a compound shows how many of each type of atom are present.

Where the compound contains only non-metals, and so is covalently bonded, the chemical formula is also known as a molecular formula.

All substances are made up from simple building blocks called elements. Each element has a unique chemical symbol, containing one or two letters. Elements discovered a long time ago often have symbols that don’t seem to match their name. For example,

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Stoichiometry

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LEARNING OBJECTIVES ✓✓State the formulas of the elements and compounds named in the subject content. ✓✓Define the molecular formula of a compound as the number and type of different atoms in one molecule. ✓✓Deduce the formula of a simple compound from the relative number of atoms present in a model or a diagrammatic representation. ✓✓Construct word equations and symbol equations to show how reactants form products, including state symbols. ✓✓Balance and interpret simple symbol equations, including state symbols. ✓✓SUPPLEMENT Deduce the formula of an ionic compound from the relative numbers of the ions present in a model or a diagrammatic representation or from the charges on the ions. ✓✓SUPPLEMENT Construct symbol equations with state symbols, including ionic equations.

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silver has the chemical symbol Ag. This is derived from argentum, the Latin name for silver.

‘Combining powers’ of elements There are a number of ways of working out chemical formulae. In this topic you will start with the idea of a ‘combining power’ for each element and then look at how the charges on ions can be used for ionic compounds.

There is a simple relationship between an element’s group number in the Periodic Table and its combining power. Groups are the vertical columns in the Periodic Table.

The combining power is linked to the number of electrons in the outer shell of atoms of the element.

Group number

I

II

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2

III

IV

V

VI

VII

VIII (or 0)

3

4

3

2

1

0

ΔΔTable C3.1 Combining powers of elements.

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Groups I–IV: combining power = group number Groups V–VII: combining power = 8 – (group number) If an element is not in one of the main groups, its combining power is included in the name of the compound containing it. For example, copper is a transition metal and is in the middle block of the Periodic Table. In copper(II) oxide, copper has a combining power of 2.

Sometimes an element does not have the combining power you would predict from its position in the Periodic magnesium oxide Table. The combining power of these elements is also included in the name of the compound containing it. For Mg O example, phosphorus is in Group V, so you would expect 2 2 it to have a combining power of 3, but in phosphorus(V) 1 1 oxide its combining power is 5.

O1 Mg1 MgO The only exception is hydrogen. Hydrogen is not included in a group, nor is its combining power given in the name Mg O Mg O of compounds containing hydrogen. It has a combining power of 1. calcium chloride

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Ca

Cl

2

1 (They don’t cancel) Cl2

CaCl2 Cl

Ca

Cl Ca

Cl

Cl

ΔΔFig. C3.2 Working out the chemical formulae for magnesium oxide and calcium chloride.

FORMULAS

Ca1

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Simple compounds Many compounds contain just two elements. For example, when magnesium burns in oxygen, a white ash of magnesium oxide is formed. To work out the chemical formula of magnesium oxide: 1. Write down the name of the compound.

2. Write down the chemical symbols for the elements in the compound.

3. Use the Periodic Table to find the ‘combining power’ of each element. Write the combining power of each element under its symbol.

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4. If the numbers can be cancelled down, do so.

5. Swap the combining powers. Write them after the symbol, slightly below the line (as a ‘subscript’).

6. If any of the numbers are 1, you do not need to write them.

Magnesium oxide has the chemical formula you would have probably guessed: MgO. H O C The chemical formula of a compound is not always immediately H C OH obvious, but if you follow these rules you will have no problems.

HO C H The formula of a simple compound can also be worked out from a H C OH simple model or diagram.

H C OH Fig C3.3 shows a diagram of the glucose molecule. If you count the number of atoms in the diagram you will find: 6 carbon (C) atoms, CH2OH 12 hydrogen (H) atoms and 6 oxygen (O) atoms. So the formula of ΔΔFig. C3.3 glucose can be written C6H12O6.

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Compounds containing more than two elements Some ionic compounds contain more than two elements. A small group of atoms bonded together can gain or lose electrons to give an overall charge to the group of atoms.

For example sodium sulfate, Na2SO4, contains sodium ions (Na+) and sulfate ions (SO42–).

Table C3.2 shows the names and formulas for some other ions that contain more than one element.

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Name of ion

Formula of ion SO42–

Nitrate

NO3–

Carbonate

CO32–

Hydroxide

OH–

ΔΔTable C3.2 Combining powers of common radicals.

When you learned about ionic bonds you saw that atoms of some elements can transfer one or more outer shell electrons to atoms of other elements. Two ions are formed. The charge on the ion depends on how many electrons are gained or lost.

The formula of an ionic compound can be worked out from the ions present. For example, sodium chloride is an ionic compound.

Sodium is in Group I and forms an ion with a charge of 1+, Na+. Chlorine is in Group VII and forms an ion with a charge of 1–, Cl–. When these ions combine, the charges must cancel each other out: NaCl (the 1+ and 1– charges cancel).

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Stoichiometry

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SUPPLEMENT

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Sulfate

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What is the formula of lead(II) bromide, which contains Pb2+ and Br– ions?

To cancel the 2+ charge, two 1– charges are needed, so the formula is PbBr2.

The formula of an ionic compound can also be worked out from the relative number of the ions present in a model or diagram.

Cl− Na+

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Fig C3.4 shows the arrangements of the ions in sodium chloride. This is only a small part of the whole structure but shows the repeating pattern in the structure. The structure illustrates that for every Na+ ion there is one Cl– ion. Therefore the formula of sodium chloride is NaCl.

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QUESTIONS

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1. How many atoms are there of each element in the following formulas: HCl; F2; CH4; H2O; LiOH; MgCl2; CaCO3?

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2. What is the chemical formulae of the following compounds: a) potassium bromide b) calcium oxide?

3. a) What is the formula of butane shown in the diagram?

Diagram of structure

H

State at room temperature and pressure

H

H

H

H

C

C

C

C

H

H

H

H

Gas H

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Butane

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Alkene

Diagram of structure

Propene

H H

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H

C

C

H

H

C H

State at room temperature and pressure Gas

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Alkene

FORMULAS

b) What is the formula of propene shown in the diagram?

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4. SUPPLemeNt What is the chemical formulae of the following compounds? a) a compound containing Zn2+ ions and Cl– ions b) a compound containing Cr3+ ions and O2– ions c) a compound containing Fe2+ ions and OH– ions 5. Suggest the formula of the ionic compound shown in the diagram.

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chloride ion

sodium ion

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WRITING CHEMICAL EQUATIONS In a chemical equation the starting chemicals are called the reactants and the finishing chemicals are called the products.

Follow these rules to write a chemical equation.

1. Write down the word equation.

2. Write down the symbols (for elements) and formulae (for compounds).

3. Balance the equation, to make sure there are the same number of each type of atom on each side of the equation.

4. Include the state symbols: solid (s); liquid (l); gas (g); solution in water (aq).

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State symbol (s) (l) (g) (aq)

Δ Table C3.3 States and their symbols.

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State Solid Liquid Gas Solution

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StoIChIometry

Remember that some elements are diatomic. They exist as molecules containing two atoms.

Element

Formula

Hydrogen

H2

Oxygen

O2

Nitrogen

N2

Chlorine

Cl2

Bromine

Br2

Iodine

I2

Δ Table C3.4 Some diatomic elements.

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WORKED EXAMPLES 1. When a lighted splint is put into a test-tube of hydrogen, the hydrogen burns with a ‘pop’. In fact the hydrogen reacts with oxygen in the air (the reactants) to form water (the product).

Write the chemical equation for this reaction. hydrogen + oxygen → water Word equation: Symbols and formulae: H2 + O2 → H 2O + O2 → 2H2O Balance the equation: 2H2 For every two molecules of hydrogen that react, one molecule of oxygen is needed and two molecules of water are formed.

Adding the state symbols: 2H2(g) + O2(g) → 2H2O(l) 2. What is the equation when sulfur reacts with oxygen in the air to form sulfur dioxide?

Word equation: sulfur + oxygen → sulfur dioxide → SO2 Symbols and formulae: S + O2 Balance the equation: S + O2 → SO2 Adding the state symbols: S(s) + O2(g) → SO2(g)

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Balancing equations Balancing equations can be quite tricky. It is essentially done by trial and error. However, the golden rule is that balancing numbers can only be put in front of the formulae.

For example, to balance the equation for the reaction between methane and oxygen: Start with the unbalanced equation

Products CO2 + H2O

1C ✓, 4H, 2O

1C ✓, 2H, 3O

CH4 + O2

CO2 + 2H2O

CH4 + 2O2

CO2 + 2H2O

1C ✓, 4H ✓, 4O ✓

1C ✓, 4H ✓, 4O ✓

ΔΔTable C3.5 Steps in balancing the equation for the reaction between methane and oxygen.

No atoms have been created or destroyed in the reaction. The equation is balanced.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

ΔΔFig. C3.5 The number of each type of atom is the same on the left and right sides of the equation.

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WRITING CHEMICAL EQUATIONS

1C ✓, 4H ✓, 4O

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1C ✓, 4H ✓, 2O

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Count the number of atoms on each side of the equation There is a need to increase the number of H atoms on the products side of the equation. Put a ‘2’ in front of the H2O Count the number of atoms on each side of the equation again There is a need to increase the number of O atoms on the reactant side of the equation. Put a ‘2’ in front of the O2 Count the number of atoms on each side of the equation again

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Reactants CH4 + O2

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QUESTIONS 1. Balance the following chemical equations: a) Ca(s) + O2(g) → CaO(s) b) H2S(g) + O2(g) → SO2(g) + H2O(l) 2. SUPPLEMENT What are balanced equations for the following word equations?

a) carbon + oxygen → carbon dioxide b) magnesium + oxygen → magnesium oxide c) copper(II) oxide + hydrogen → copper + water

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As mentioned earlier, the general method for balancing equations is by trial and error, but it helps if you are systematic – always start on the left-hand side with the reactants. Sometimes you can balance an equation using fractions. In more advanced study such balanced equations are perfectly acceptable. Getting rid of the fractions is not difficult though. Look at this example:

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WORKED EXAMPLE Ethane (C2H6) is a hydrocarbon fuel and burns in air to form carbon dioxide and water.

Unbalanced equation: C2H6(g) + O2(g) → CO2(g) + H2O(l) Balancing the carbon and hydrogen atoms gives: C2H6(g) + O2(g) → 2CO2(g) + 3H2O(l) The equation can then be balanced by putting 31/2 in front of the O2. By doubling every balancing number, the equation is then balanced using whole numbers.

2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l)

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During your course you will become familiar with balancing equations and become much quicker at doing it. Try balancing the equations below. The third is the chemical reaction often used for making chlorine gas in the laboratory.

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Stoichiometry

ΔΔFig. C3.6 Balancing the equation for burning ethane in air.

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QUESTIONS 1. Balance the following equations: a) C5H10(g) + O2(g) → CO2(g) + H2O(l) b) Fe2O3(s) + CO(g) → Fe(s) + CO2(g) c) KMnO4(s) + HCI(aq) → KCl(aq) + MnCl2(aq) + H2O(l) + Cl2(g)

SUPPLEMENT

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Ionic equations Ionic equations show reactions involving ions (atoms or groups of atoms that have lost or gained electrons). The size of the charge on an ion is the same as its combining power – whether it is positive or negative depends on which part of the Periodic Table the element is placed in.

In many ionic reactions some of the ions play no part in the reaction. These ions are called spectator ions. A simplified ionic equation can be written, using only the important, reacting ions. In these equations, state symbols are often used and appear in brackets.

The equation must balance in terms of chemical symbols and charges.

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WORKED EXAMPLES 1. When lead(II) nitrate solution is mixed with potassium iodide solution, the products are insoluble lead(II) iodide and soluble potassium nitrate. The lead iodide forms a coloured precipitate (Fig. C3.7).

The word equation is: Lead(II) nitrate + potassium iodide → lead(II) iodide + potassium nitrate In this reaction, the potassium ions and nitrate ions remain separate in the solution – they are spectators. The important ions are the ones that form the precipitate – the lead(II) ions (Pb2+) and the iodide ions (I–).

The simplified ionic equation is: Pb2+(aq) + 2I–(aq) → PbI2(s)

1 Pb ✓, 2I ✓ 0✓

WRITING CHEMICAL EQUATIONS

1 Pb ✓, 2I ✓ 2+ and 2– = 0

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Products PbI2(s)

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Symbols Charges

Reactants Pb2+(aq) + 2I–(aq)

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ΔΔFig. C3.7 This reaction occurs simply on mixing the solutions of lead(II) nitrate and potassium iodide. Lead(II) iodide is an insoluble yellow solid.

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The equation shows that any solution containing lead(II) ions will react with any solution containing iodide ions to form lead(II) iodide.

2. Any solution containing copper(II) ions and any solution containing hydroxide ions can be used to make copper(II) hydroxide, which appears as a solid: Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s)

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Reactants Cu2+(aq) + 2OH–(aq)

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Symbols Charges

Products Cu(OH)2(s)

1Cu ✓, 2O ✓, 2H ✓ 2+ and 2– = 0 ✓

1Cu ✓, 2O ✓, 2H ✓ 0✓

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Stoichiometry

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ΔΔFig. C3.8 Copper(II) hydroxide.

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End of topic checklist Key terms chemical formula, chemical symbol, diatomic, ionic equation, molecular formula, precipitate, product, reactant, state symbols

During your study of this topic you should have learned:

❍❍How to use the symbols of the elements to write the formulae of simple compounds.

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❍❍How to define the molecular formula of a compound. ❍❍How to deduce the formula of a simple compound from the numbers of atoms present.

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❍❍How to deduce the formula of a simple compound from a model or a diagram. ❍❍How to construct word equations and simple balanced symbol equations, including state symbols.

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❍❍Recall the formulas of some common elements and compounds, including

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hydrogen, water, carbon dioxide, ammonia, methane, sodium chloride and hydrochloric acid.

❍❍SUPPLEMENT How to determine the formula of an ionic compound from the

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relative number of ions present in a model or diagram or from the charges on the ions present.

❍❍SUPPLEMENT How to construct equations with state symbols, including ionic equations.

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Stoichiometry

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End of topic questions 1. Using the Periodic Table on page 688 determine the chemical formulae of the following compounds: a) sodium chloride b) magnesium fluoride c) aluminium nitride d) lithium oxide e) carbon dioxide.

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2. SUPPLEMENT Determine the chemical formulae of the following compounds: a) iron(III) oxide (contains Fe3+ and O2–)

b) chromium (III) bromide (contains Cr3+ and Br-)

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c) copper(II) sulfate (contains Cu2+ and SO42–).

3. Determine the chemical formulae of the following compounds:

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a) potassium carbonate b) lithium chloride

d) magnesium hydroxide

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c) sulfuric acid

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e) calcium bromide.

4. SUPPLEMENT Give symbol equations from the following word equations: a) carbon + oxygen → carbon dioxide

c) iron(III) oxide + carbon → iron + carbon dioxide (Fe3+ and O2– in iron(III) oxide)

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b) iron + oxygen → iron(III) oxide (Fe3+ and O2– in iron(III) oxide)

d) calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water. (CO32– is the carbonate ion) 5. SUPPLEMENT Determine ionic equations for the following reactions: b) iron(III) ions and hydroxide ions form iron(III) hydroxide c) silver(I) ions and bromide ions form silver(I) bromide.

270

Stoichiometry

a) calcium ions and carbonate ions form calcium carbonate

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Practice questions for Sections C1, C2 and C3 Note: practice questions, sample answers and comments have been written by the authors.

The marks awarded for these questions indicate the level of detail required in the answers. In ­ eferences to assessment and/or examinations, the way marks are awarded may be different. R assessment preparation are the publisher's interpretation of the syllabus requirements and may not fully reflect the approach of Cambridge Assessment International Education.

Example answer Question 1

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a) The diagrams show the arrangement of particles in the three states of matter.

Each circle represents a particle.

A

C

B

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pl ii) Correct – solidifying. iii) Incorrect – order should be ‘A, C’ because ethene is a gas, poly(ethene) a solid. iv) Correct – equation shows solid → gases.

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B

A

✓

1

C

✓

1

B

A

✗

C

A

✓

B

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i) Correct – evaporation process.

i) The formation of water vapour from a puddle of water on a hot day ii) The formation of solid iron from molten iron iii) The manufacture of poly(ethene) from ethene iv) The reaction whose equation is ammonium chloride(s) → ammonia(g) + hydrogen chloride(g)

1

(4)

PRACTICE QUESTIONS

A = gas, B = liquid, C = solid.

Starting Finishing state state

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a) It is important to identify the states of matter:

Change

271

COMMENTS

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Use the letters A, B, and C to give the starting and finishing states of matter for each of the changes in the table. For the mark, both the starting state and the finishing state need to be correct.

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b) Answer is ‘liquid’. In the Periodic Table at room temperature the majority of elements are solids, a few are gases but only two are liquids – mercury and bromine.

b) Which state of matter is the least common for the elements of the Periodic Table at room temperature? gases

✗

(1)

c) T he manufacture of sulfuric acid can be summarised by the equation:

c) i) Correct – sulfur.

2S(s) + 3O2(g) + 2H2O(l) → 2H2SO4(l)

ii) Incorrect – this is a ‘mixture’ of two elements.

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Tick one box in each line to show whether the formulae in the table represent a compound, an element or a mixture.

iii) Correct – a mixture of an element (O2) and a compound (H2O).

Compound Element Mixture

i) 2S(s)

✓

(Total 9 marks)

6 9

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Question 2

(4)

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The answers rely on using the state symbols for the equation and a thorough knowledge of the terms elements, mixtures and compounds.

✓ ✓1 ✓1

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iv) 2H2SO4(l)

✓

✓1 ✗

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iii) 3O2(g) + 2H2O(l)

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ii) 2S(s) +3O2(g)

✓

iv) Correct – sulfuric acid.

Identify which statement best describes the bonding in water.

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A A combination of hydrogen and oxygen molecules.

B Weak bonding within the molecule and strong bonding between molecules.

D The sharing of electrons between hydrogen and oxygen atoms.

(1) (Total 1 mark)

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PRACTICE QUESTIONS

C Ionic attraction between hydrogen and oxygen ions.

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Question 3 This question is about atoms. a) i) Choose words from the box to label the diagram of an atom.

Proton

Neutron

Electron

(3)

Ion

−

−

+ +

+

+

−

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−

+

+

−

m −

ii) Determine the proton number of this atom.

(1)

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iii) Determine the mass number of this atom.

(1)

b) i) An element has a proton number of 13. What is the electron configuration? (1)

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ii) An element has an electron configuration of 2, 8, 1. Which Group is the element in?

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Question 4

(1)

(Total 7 marks)

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a) Some elements combine together to form ionic compounds. Use words from the box to complete the sentences.

Each word may be used once, more than once or not at all. high metals

lost non-metals

low shared

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gained medium

Electrons are ……………………… by atoms of one element and ………………. by atoms of the other element.

(6)

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The ionic compound formed has a ……………….. melting point and a ………………. boiling point.

PRACTICE QUESTIONS

Ionic compounds are formed between …………………. and ………………………… .

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b) Two elements react to form an ionic compound with the formula MgCl2. (proton number of Mg = 12; proton number of Cl = 17) i) State the electronic configurations of the two elements in this compound before the reaction.

(2)

ii) Deduce the electronic configurations of the two elements in this compound after the reaction.

(2)

(Total 10 marks)

Question 5 Consider the structures of the substances shown here:

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a) Answer these questions using the letters A, B, C or D. ii) Identify which structure is a giant structure.

iii) Identify which two structures are hydrocarbons. iv) Identify which structure contains ions.

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i) Identify which structure is methane.

(1) (1) (1) (1)

b) Determine the simplest formula for substance B.

(1)

c) Is substance D an element or a compound? Explain your answer.

(3)

(Total 9 marks)

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PRACTICE QUESTIONS

v) Identify which structure has a very high melting point.

(1)

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Question 6 SUPPLEMENT Strontium and sulfur chlorides both have a formula of the type XCl2 but they have different properties.

Property Appearance Melting point /°C Particles present Electrical conductivity of solid Electrical conductivity of liquid

Strontium chloride White crystalline solid 873 Ions Poor Good

Sulfur chloride Red liquid –80 Molecules Poor Poor

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a) i) Use the Periodic Table to identify which Group strontium is in.

(1)

ii) Which ion does strontium form?

(1)

(3)

c) Use the kinetic particle theory to explain the differences between solid and liquid strontium chloride.

(2)

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b) Demonstrate the bonding in sulfur chloride using a dot-and-cross diagram.

Use x to represent an electron from a sulfur atom.

Use o to represent an electron from a chlorine atom.

pl

(Total 7 marks)

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PRACTICE QUESTIONS

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Pa 306098_IGCSE Combined_SB_Che.indb 275

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Humans are curious beings. We cannot look at the stars in the night sky, without asking questions. How did the stars get there? Is the Earth the only habitable planet in the Universe? The frontiers of science and technology are being pushed forward all the time. We can now send space probes to distant planets, and even land them on comets hurtling through space. We are learning more about the objects that are close to the Earth. We can use complex telescopes orbiting the Earth to study distant objects in space using not only visible light, but also X-rays. The Chandra X-ray Observatory was launched in 1999, and is still operational and orbiting the Earth. It continues to provide astronomers with stunning images – helping them to test their theories and advance our knowledge and understanding of space.

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STARTING POINTS 1. What is the name of the force that keeps the Moon in its orbit around the Earth?

2. How can you tell if a speck of light in the night sky is a planet and not a star?

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3. What is a galaxy?

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SYLLABUS SECTIONS COVERED P5.1 The Solar System

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P5.2 Stars and the Universe

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P5

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Space physics

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pl s

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Pa ΔΔThe Chandra X-ray Observatory continues to help astronomers learn about Space.

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The Solar System INTRODUCTION

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Our developing understanding of the Solar System, and the space beyond, has largely come from observations. In the past, these observations were Earth-based and simple instruments were used to measure and record the motion of the planets and our Moon across the night sky. We can learn more about our Moon, and the planets beyond, by using powerful telescopes onboard space probes, see Fig. P5.1.

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ΔΔFig. P5.1 Stunning image of the swirling atmosphere of Jupiter taken from the camera aboard NASA’s Juno spacecraft.

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KNOWLEDGE CHECK ✓✓Gravitational force acts on an object that has mass.

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LEARNING OBJECTIVES ✓✓Describe the Solar System as containing: one star, the Sun; the eight named planets and know their order from the Sun; minor planets that orbit the Sun, including dwarf planets such as Pluto and asteroids in the asteroid belt; moons, that orbit the planets.

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650

Space physics

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THE SOLAR SYSTEM The Solar System is the general name for the Sun and all the objects that orbit it. The Sun is a yellow star. It is a hot ball of glowing gases.

The Sun’s enormous gravitational pull is responsible for trapping all of the eight planets, minor planets, millions of asteroids, and comets in their orbits around it. Comets are small objects made mainly of ice and rock. As a frozen comet gets closer to the hot Sun, it heats up and its icy material turns into gas, creating a long visible tail that points away from the Sun (see Fig. P5.4).

Some of the planets have smaller objects orbiting them.

These are called moons or natural satellites. Our planet Earth has one moon (called the Moon). The two closest planets to the Sun, Mercury and Venus, have no moons.

The ringed planet Saturn may have as many as 82 moons.

Some of the stars we see in the night sky may also have their own system of orbiting planets.

Planets and asteroids Planets are so far away that they appear as tiny specks of light moving across the night sky. We can only see the planets because they reflect the light they get from the Sun.

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ΔΔFig. P5.2 Asteroid Psyche is a small object, just 230 m wide, and rich in iron and nickel.

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The planets in order of increasing distance from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, see Fig. P5.3.

Small rocky objects, the asteroids, occupy a region of space mainly between the orbits of Mars and Jupiter. This region is known as the asteroid belt. The largest asteroid, Vesta, is about 500 km in diameter and the smallest asteroids are only a few metres wide. Although spread ΔΔFig. P5.3 The Solar System. In order of increasing distance over a vast region of space, the total from the Sun we have Mercury, Venus, Earth, Mars, asteroid mass of all the asteroids is less than belt, Jupiter, Saturn, Uranus and Neptune. the mass of our Moon.

A minor planet is an object that orbits around the Sun that is neither a planet nor a comet. Dwarf planets are minor planets.

A dwarf planet is an object where its own gravity forms an ellipsoid (spherical or squashed sphere) object.

Currently there are five officially recognised dwarf planets.

The most famous of these is Pluto. It was demoted from a planet to a dwarf planet in 2006. The others are Ceres, Eris, Haumea, and Makemake. Makemake was discovered in 2015 in the frozen regions beyond the orbit of Neptune. There are many more dwarf planets waiting to be officially recognised by Fig. P5.4 Halley’s comet with astronomers. its long tail in the night sky.

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pl

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QUESTIONS

1. The diagram below is that of the Solar System.

9 Sun

2 4

6

5 7

8

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1 3

What are the numbers for the planets Venus and Uranus?

2. Name the most distant planet in the solar system.

3. Name the two planets with orbits closest to that of the Earth.

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4. Describe the location of most of the asteroids in the Solar System.

THE SOLAR SYSTEM

All the planets are numbered from 1 to 8. The object numbered 9 is Pluto – a dwarf planet.

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● SCIENCE IN CONTEXT

LIFE BEYOND OUR SOLAR SYSTEM

How do astronomers know that some stars may have their own system of orbiting planets? The planets beyond our own Solar System are called exoplanets.

Even the most powerful telescopes on the Earth, or in space, cannot physically see exoplanets. They are just too small and too far away. However, the brightness of a star, even a dim star, can be accurately measured using large telescopes. Its brightness will show a tiny dip every time an exoplanet crosses over the star, see Fig. P5.5.

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In 2017, observations collected by NASA’s Spitzer Space Telescope, orbiting high above the Earth’s atmosphere, led to the discovery of seven Earth-sized exoplanets around the red star called TRAPPIST-1. All of these exoplanets have the potential of having water on their surface, and hence the potential for life.

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Challenge Question: Why do you think it is harder to discover exoplanets using telescopes on the Earth’s surface?

exoplanet

orbit of exoplanet

pl exoplanet blocking light from star

time

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brightness

star

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ΔΔFig. P5.5 The variation in brightness of a star is used to discover an exoplanet.

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Space physics

ΔΔFig. P5.6 The TRAPPIST-1 system with its exoplanets.

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End of topic checklist Key terms asteroid, planet, Solar System

During your study of this topic you should have learned:

❍❍That the Solar System has one star (the Sun) and all the objects (planets, minor planets, asteroids, and dwarf planets) that orbit it.

❍❍That the eight planets orbiting the Sun in order of increasing distance are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

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❍❍That Pluto is an example of a dwarf planet. ❍❍That some planets have moons orbiting them.

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End of topic questions

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1. a) Which of the following objects does not orbit around the Sun? A Jupiter B Pluto C Moon D Asteroid

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b) Name a dwarf planet.

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c) A planet in our Solar System has two moons. Describe the motion of this planet and its two moons.

2. a) Name all the planets between the orbits of Mercury and Jupiter. b) Suggest why all planets appear as specks of light in the night sky.

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c) The Sun is a hot glowing ball of gas.

Suggest why the average temperature of the Earth is far greater than that of Jupiter.

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Space physics

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Stars and the Universe INTRODUCTION

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A galaxy is a collection of billions of stars held together in space by their own gravity. Our Sun belongs to a galaxy we call the Milky Way. Astronomers believe that there could be as many as a hundred billion galaxies in the space around us. Some are so far away that light ΔΔFig. P5.7 The bright patches of light are not stars, but from them has yet to reach us on thousands of distant galaxies imaged by the Hubble Space the Earth. Our nearest galaxy is Telescope.

Andromeda Galaxy. The gravitational force between our Milky Way and Andromeda Galaxy will one day make them collide into each other. We do not need to worry about this, because it will happen many billions of years from now.

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KNOWLEDGE CHECK ✓✓Microwaves, infrared, visible, and ultraviolet are regions of the electromagnetic spectrum. ✓✓Speed is distance travelled per unit time. ✓✓SUPPLEMENT Nuclear fusion is the joining of nuclei. ✓✓Like charges repel and unlike charges attract. ✓✓SUPPLEMENT Speed of light is 3.0 × 108 m/s in a vacuum.

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Space physics

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LEARNING OBJECTIVES ✓✓Know that: the Sun is the closest star to the Earth; astronomical distances can be measured in lightyears, where one light-year is the distance travelled in (the vacuum of) space by light in one year. ✓✓Calculate the time it takes light to travel a significant distance such as between objects in the Solar System. ✓✓Know that the Sun contains most of the mass of the Solar System and this explains why the planets orbit the Sun. ✓✓Know that the force that keeps an object in orbit around the Sun is due to the gravitational attraction of the Sun. ✓✓Know that the Sun is a star of medium size, consisting mostly of hydrogen and helium, and that it radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum. ✓✓SUPPLEMENT Define orbital speed from the equation v = 2πr/T, where r is the radius of the orbit and T is the orbital period; recall and use this equation. ✓✓SUPPLEMENT Know that the strength of the Sun’s gravitational field decreases and that the orbital speeds of the planets decrease as the distance from the Sun increases. ✓✓SUPPLEMENT Know that stars are powered by nuclear reactions that release energy and that in stable stars the nuclear reactions involve the fusion of hydrogen into helium. ✓✓Know that stable stars are formed as protostars from interstellar clouds of gas and dust due to gravitational attraction.

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✓✓Know that the next stages of the life cycle of a star depend on its mass, limited to: a small mass star (about the same mass as the Sun): red giant → white dwarf + planetary nebula; a large mass star: red supergiant → supernova → neutron star; a very large mass star: red supergiant → supernova → black hole. ✓✓SUPPLEMENT Know that the nebula from a supernova may form new stars with orbiting planets. ✓✓Know that: galaxies are made up of many billions of stars; the Sun is a star in the galaxy known as the Milky Way; other stars that make up the Milky Way are much further away from the Earth than the Sun is from the Earth. ✓✓Know that the Milky Way is one of many billions of galaxies making up the Universe and that the diameter of the Milky Way is approximately 100 000 light-years. ✓✓SUPPLEMENT Know that the Big Bang Theory explains many astronomical observations and states that: the Universe expanded from a single point of high density and temperature; the Universe is still expanding; the Universe is approximately 13.8 billion years old.

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THE SUN AS A STAR If you look up at the sky at night you will see stars with a range of colours and brightness. The colour of a star is linked to its surface temperature – blue stars are hotter than red stars. Most of the yellowcoloured stars are just like our Sun. Our Sun is a medium-size star consisting mostly of hydrogen and helium. The Sun is the closest star to ΔΔFig. P5.8 Our Sun is just a mediumthe Earth. It radiates its energy in the form of size star. electromagnetic radiation mostly in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum.

Some stars are astonishingly huge. VY Canis Majoris is a cooler red star that is about 1400 times bigger than the Sun. By contrast, some stars are tiny. The star awkwardly named EBLM J0555-57Ab, discovered in 2017, is only the size of Saturn.

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TIME TAKEN BY LIGHT TO TRAVEL ACROSS THE SOLAR SYSTEM The speed of light is immense at 300 million metres per second. Even at this speed, because of the vast distances involved, it can take many hours for the light to travel the length of the Solar System.

STARS AND THE UNIVERSE

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LIGHT-YEARS The light from planets and stars take time to reach us – light travels fast, but its speed is still finite. The light from the Sun takes about 8 minutes to reach us. This means that if there was a solar flare on the surface of the Sun, we would see it about 8 minutes later. Stars are even more distant – light from them can take much more time to arrive to the Earth. It can take thousands of years for light from the most distant stars to get to us.

Rather than measuring distances in metres, it is convenient to measure the distance of astronomical objects in light-years. One light-year is defined as the distance travelled by light in a vacuum in one year. This equates to about 9.5 × 1015 m.

Distances expressed in light-years help us to visualise the enormous distance in space. The bright star Sirius is 8.6 light-years away. The centre of our galaxy is about 25 000 light-years away.

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The Moon is our closest neighbour. The light from it takes about 1 second to reach us.

The Sun is further away, and it takes several minutes for the light to reach us. If a solar flare erupts now on the Sun’s surface, then we will see this event much later. The distant events we see have already happened – we are glimpsing into the past.

Using the speed of light in a vacuum (3.0 × 108 m/s) and the equation, we can calculate the time it takes light to travel a distance such as the distance to the Sun or other stars.

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Worked example The Sun is 150 million km away from the Earth.

Calculate the time, in minutes, it takes light to travel from the Sun to us.

Convert the distance into metres. d = 150 million km = 150 × 106 × 103 = 1.5 × 1011 m Substitute into the equation and rearrange. d v= t 11 1.5 × 10 = 3.0 × 108 × t 1.5 × 1011 = 500 s t= 3.0 × 108

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Convert the time into minutes; 1 minute = 60 s.

500 t= = 8.3 minutes 60 The light from the Sun takes about 8.3 minutes to reach the Earth.

REMEMBER

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Be careful with big numbers – make sure you are confident using your calculator.

Questions often jump back and forth between metres, kilometres, hours, days, and so on.

QUESTIONS

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1. The light from the Moon takes 1.28 s to reach the Earth. ­Calculate how far the Moon is from the Earth in metres.

2. The most distant planet Neptune is about 4.4 × 1012 m from the Earth. Calculate

THE ROLE OF GRAVITY The planets orbit the Sun because the Sun has most of the mass in the Solar System.

The Sun’s attractive gravitational force at the positions of the planets is big enough to make each planet move around the Sun. The Moon only orbits the Earth, and not the Sun, because the Earth’s gravitational force on the Moon is much larger than that from the Sun.

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Space physics

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the time, in hours, it takes light to travel from it to us (1 hour = 3600 s).

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QUESTION 1. Explain why the planets orbit the Sun. 2. Name the attractive force between the Sun and the planets.

3. The Moon orbits the Earth. Suggest why it goes around the Earth rather than the Sun. SUPPLEMENT

ORBITAL SPEED Through direct observations with telescopes and information collected from space probes, astronomers have managed to gather detailed data on the planets, see Table P5.1.

Mass / 1024 kg

Mean orbital distance from Sun / 106 km

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Planet

Orbital duration or period / Earth days

0.33

57.9

88.0

Venus

4.87

108.2

224.7

5.97

149.6

365.2

0.64

227.9

687.0

Earth Mars

m

Mercury

pl

Jupiter

1900

Saturn

570

Uranus

87

2872.5

30 589

Neptune

100

4495.1

59 800

778.6

4331

1433.5

10 747

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ΔΔTable P5.1 Some planetary data.

2πr 2π × 3.8 × 108 = = 920 m/s T 2.59 × 106 This is almost 1 km per second. Even at this speed the moon takes 30 days to complete one orbit around the Earth. v=

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Worked example The orbital period of the Moon around the Earth is about 30 days. The average radius of its orbit is 380 000 km. Calculate the orbital speed of the Moon in m/s.

First, convert the period into seconds and the radius into metres.

T = 30 days = 30 × 24 × 3600 = 2.59 × 106 s r = 380 000 × 103 = 3.8 × 108 m Now substitute these values into the equation and solve.

ORBITAL SPEED

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The average orbital speed v of any object in an orbit can be calculated using the equation: 2πr v= T Where r is the average radius of the orbit and T is the orbital period. This equation may be used for both circular and elliptical orbits. It is worth noting that for a circular orbit, the distance travelled in one period T is the circumference 2πr of the circle.

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QUESTIONS SUPPLEMENT Use Table P5.1 to answer the questions.

1. Suggest the relationship between orbital duration (period) of a planet and its distance from the Sun.

2. The Earth takes one year to orbit the Sun. Calculate Neptune’s orbital period in years.

3. The mass of the Sun is 2.0 × 1030 kg. Determine: a) how many times more massive is Jupiter than the Earth b) the total mass of all the planets in the Solar System c) total mass of planets and comment on your answer.

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mass of the Sun 4. Calculate the mean orbital speed in m/s of Mars.

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WHY ORBITAL SPEED VARIES The gravitational field of the Sun at its surface is about 290 N/kg. The field strength decreases as the distance from the Sun increases. The field strength at Mercury’s position is much greater than that at Neptune’s position. This is why the orbital speed of Mercury is much greater than that of Neptune. In summary, as the distance from the Sun increases: • the Sun’s gravitational field strength decreases • the orbital speed of a planet decreases.

Table P5.2 shows the orbital distance from the Sun and the orbital speed of all the planets.

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orbital speed / km / s 47.4 35.0 29.8 24.1 13.1 9.7 6.8 5.4

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orbital distance from Sun / 106 km 57.9 108.2 149.6 227.9 778.6 1433.5 2872.5 4495.1

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Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

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Planet

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Space physics

ΔΔTable P5.2 Orbital speed data for the planets.

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QUESTIONS SUPPLEMENT 1. Explain how gravitational field strength changes as the distance from an object increases.

2. Name the fastest moving planet.

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NUCLEAR FUSION How does the Sun produce its energy? The Sun produces energy by nuclear fusion reactions. In these reactions, enormous energy is released when hydrogen nuclei join, or fuse, together to form helium nuclei, see Fig. P5.9.

Fusion reactions take place deep within the core of the Sun where the pressures are immense and the temperature high at around 15 million °C. The positively charged hydrogen nuclei would normally stay away from each other because like charges repel.

However, at these high temperatures, the hydrogen nuclei are travelling fast enough to get close enough to fuse with each other, and produce helium and lots of energy.

P n

P

P Pn

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hydrogen nuclei

+

energy

helium nucleus

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ΔΔFig. P5.9 One of the fusion reactions taking place in the Sun involves fusing the nuclei of two different isotopes of hydrogen to produce a helium nucleus and lots of energy. A proton has the label p and a neutron has the label n.

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QUESTIONS SUPPLEMENT

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1. What is released when hydrogen nuclei fuse together?

2. Suggest why high temperatures help with fusion reactions.

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LIFE CYCLE OF A STAR A star is formed from interstellar clouds of gas and dust (a nebula). The ultimate mass of the star depends on the original mass of the interstellar gas and dust cloud.

Larger clouds will produce massive stars.

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Life cycle of a star

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Birth of a star The gas is mainly hydrogen, with tiny amounts of helium. The internal gravitational attraction of the gas and dust particles collapses the cloud and makes it spin, and also increases its temperature. The gas cloud eventually spins faster, heats up and becomes a protostar.

The temperature within the core of a protostar can be about 15 million °C and it releases enormous energy. The protostar glows brightly. It becomes a stable star when the inward force of gravitational attraction is balanced by the outward force due to the high temperature in the centre of the star. The star will keep shining for millions to billions of years. This is the stage of our Sun right now.

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Fate of the star The final fate of the star depends on its mass.

Small mass star

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When a star with a similar mass to our Sun starts to run out of hydrogen in the core, it can no longer generate energy. The core of the star becomes unstable and starts to contract. The outer layer of the star, which is mostly hydrogen, starts to expand. As it swells up, it cools and glows red in colour.

The star has now become a red giant. One of the brightest stars in the night sky is Arcturus – it has the same mass as the Sun but it is 25 times larger than our Sun. Eventually, the core of the red giant collapses again. The outer layers of the star are pushed away forming a planetary nebula (see Fig. P5.10) and the core collapses to become a white dwarf. In summary, a small mass star like our Sun has the following life cycle: small mass star → red giant → white dwarf ΔΔFig. P5.10 A planetary nebula in the constellation + planetary nebula Aquarius.

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Larger mass star

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A star can remain stable for billions of years. When a star with a much greater mass than our Sun reaches the end of its life, the core of the star becomes unstable and it gets hotter and hotter and expands. It becomes a red supergiant. The star Antares is an example of a red supergiant – it is about 700 times larger than our Sun. Eventually, the red supergiant explodes as a supernova. The explosion ejects into space a nebula containing hydrogen, and new heavier elements such as iron, gold and uranium are formed during this explosion. The remaining core of the star contracts and may form an extremely dense neutron star or, if the original star was extremely massive, a black hole. Black holes are even denser than neutron stars. Their gravitational pull is so huge that even light cannot escape from it.

In summary, stars with mass much larger than the Sun have the following life cycles: large mass star → red supergiant → supernova → neutron star very large mass star → red supergiant → supernova → black hole Fig. P5.11 shows that the life cycle of a star depends on its original mass.

The nebula from a supernova may, over billions of years, form new stars with orbiting planets, like our Solar System. The heavier elements found on the Earth were created during past supernovae. It is interesting that the iron in our blood was created in these ancient explosions.

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Space physics

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SUPPLEMENT

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Sa ΔΔFig. P5.11 Life cycle of stars. The fate of the star depends on its original mass.

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QUESTIONS

1. Name two objects created after the red giant stage for a low-mass star (like

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our Sun).

2. Name two objects created at the end of the life cycle of a star that is much more massive than our Sun.

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3. Describe how a protostar is formed.

Here are some amazing facts about white dwarfs like Sirius B.

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ΔΔFig. P5.12 Subrahmanyan Chandrasekhar was awarded the 1983 Nobel Prize in Physics for working out the maximum mass of a white dwarf.

Life cycle of a star

The surface of Sirius B has a temperature of about 25 000 °C. It is much hotter than our Sun. What makes it dim in the night sky is its physical size. It has a diameter the same as our Earth, yet its mass is almost that of our Sun. Sirius B, like many other white dwarfs, is extremely dense.

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To the naked eye, one of the brightest stars in the night sky is Sirius. Powerful telescopes however show that Sirius is not one but two stars orbiting around each other. The main star, Sirius A, is large and bright, and its smaller companion, Sirius B, is a dim white dwarf.

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WHITE DWARFS

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• The material of a white dwarf can be 200 000 times denser than water.

• The surface gravitational field strength can be 3500 000 N/kg. As a comparison, the surface field strength of the Sun is 290 N/kg and for the Earth it is only 10 N/kg.

• The mass of a white dwarf cannot exceed 1.44 times the mass of the Sun. This limit is known as Chandrasekhar limit, after Subrahmanyan Chandrasekhar, the Indian-born physicist.

• A white dwarf does not generate any energy from fusion reactions. It steadily cools down by radiating energy from its surface for a couple of billion years.

• A white dwarf can ‘steal’ material from a neighbouring star and eventually become a supernova, releasing about 1044 J of energy in a short period of time.

Challenge Question: The majority of the stars in our Milky Way will evolve into white dwarfs. Why is it that we do not see the night skies full of these mysterious white dwarfs?

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GALAXIES Our Sun, with its Solar System, is part of a galaxy known as the Milky Way. Far away from the bright city lights, the Milky Way can be seen in the night sky, see Fig. P5.13.

Our Milky Way has a diameter of around 100 000 light-years.

A galaxy consists of a large number of stars and dust held together by gravity. There could be as many as 100 billion stars in a galaxy. Our Milky Way is spiral in shape, very much like galaxy NGC 628 shown in Fig. P5.14. Our Sun is close to the edge of the Milky Way and is in one of the spiralling arms of the Milky Way, much like the spiralling arms of the NGC 628 galaxy.

All the stars we see in the night sky are in our own galaxy. They are all much further away from us than our Sun.

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ΔΔFig. P5.13 The Milky Way as seen in Namibia.

ΔΔFig. P5.14 The NGC 628 spiral galaxy is similar to our Milky Way. This galaxy is 30 000 000 light-years away from the Earth.

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THE UNIVERSE The Universe is everything we can see and detect around us. Our Sun is part of the Milky Way. The Milky Way is one of about 100 billion galaxies that make up the Universe. Some galaxies, like the Andromeda galaxy, are close to us, but some are so far away that light from them has yet to reach us. The space between galaxies is mostly vacuum. The Universe is huge – it could have a diameter of about 90 billion light-years.

QUESTIONS 1. What is the Milky Way? 2. Approximately how many stars are there in a galaxy?

3. A star is a distance of 7.2 light-years from us.

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How many years it would take for the light from this star to reach us?

4. The star VY Canis Majoris is 3.6 × 1019 m from the Sun. Calculate this distance in light-years. (1 light-year = 9.5 × 1015 m)

5. The centre of the Milky Way is about 25 000 light-years from us. Calculate this

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distance in metres. (1 light-year = 9.5 × 1015 m)

SUPPLEMENT

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Big Bang Theory The Big Bang Theory is a model used to explain how the Universe came into existence, and also its subsequent evolution. The Universe began from a single point of high density, then for some unknown reason it began to expand from a hot explosion. This event, known as the Big Bang, was the birth of the Universe. The Universe is about 13.8 billion years old. Before the birth of the Universe, there was no space, no matter, and no time.

Space has been expanding and stretching ever since the Big Bang. Stars and galaxies created soon after the Big Bang have been carried away by the stretching of space. The expansion of the Universe made it cooler. From the Earth, analysis of the spectrum of light from distant galaxies shows that all the galaxies are rushing away from us – this is the major evidence for the Big Bang and the expansion of the Universe.

Astronomers also detect microwaves coming from all directions in space. These microwaves can be thought of as the ‘left-over’ radiation from the Big Bang. The existence of these microwaves provides more evidence that the Universe had a beginning.

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QUESTIONS SUPPLEMENT 1. Describe what happened to the Universe after the Big Bang.

A Galaxies are moving apart from each other.

B Planets move around the Sun. C Stars are bright.

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D The Universe is getting hotter.

THE UNIVERSE

2. How do astronomers know that the Big Bang occurred?

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End of topic checklist Key terms galaxy, light-year, Milky Way, nuclear fusion SUPPLEMENT Big Bang Theory, black hole, nebula, neutron star, planetary nebula,

protostar, red giant, supergiant, supernova, white dwarf

During your study of this topic you should have learned:

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❍❍That the Sun is the closest star to the Earth. ❍❍That astronomical distances are measured in light-years. ❍❍That one light-year is the time taken for light to travel in vacuum for one year. ❍❍That all planets orbit the Sun because it contains most of the mass in the Solar System.

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❍❍That objects orbit the Sun because of the attractive gravitational force. ❍❍That the Sun is a small mass star containing mostly hydrogen and helium. ❍❍That the Sun emits electromagnetic waves – mostly infrared, visible and ultraviolet. ❍❍SUPPLEMENT That the orbital speed of an object can be calculated using v = 2πr/T. ❍❍SUPPLEMENT That the further a planet is from the Sun, the smaller is the

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gravitational field of the Sun and the orbital speed of the planet.

❍❍SUPPLEMENT That the Sun produces energy by fusing hydrogen into helium. ❍❍That a protostar is formed from the gravitational collapse of an interstellar cloud of gas and dust.

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❍❍That the life cycle of a star depends on its original mass. ❍❍That a small mass star → red giant → white dwarf + planetary nebula ❍❍That a large mass star → red supergiant → supernova → neutron star ❍❍That a very large mass star → red supergiant → supernova → black hole. ❍❍SUPPLEMENT That the nebula from a supernova can produce new stars with ❍❍That a galaxy is made up of billions of stars. ❍❍That the Sun is part of a galaxy called the Milky Way. ❍❍That the Universe has many billions of galaxies. ❍❍That the diameter of the Milky Way is about 100 000 light-years.

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orbiting planets.

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❍❍SUPPLEMENT That the Big Bang was when the Universe was born. ❍❍SUPPLEMENT That the Universe is 13.8 billion years old. ❍❍SUPPLEMENT That the Universe was hot and very dense at the time of the Big Bang. ❍❍SUPPLEMENT That the Universe has been expanding ever since the Big Bang.

End of topic questions 1. a) What is the name of the galaxy that contains the Earth and our Sun? A Andromeda

C Canis Major

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D Virgo

b) State the diameter of our galaxy in light-years. c) There are about 100 000 000 000 stars in a galaxy.

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The mass of a typical star is 2.0 × 1030 kg. Estimate the total mass of our galaxy.

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2. The star Tau Ceti is similar to our Sun. It is about 12 light-years from us. a) State three types of electromagnetic radiation emitted by this star. c) Define the light-year.

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b) Explain why Tau Ceti must be in our Milky Way.

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d) SUPPLEMENT Tau Ceti is a stable star that predominantly consists of hydrogen and helium. i) Name the type of reaction responsible for the energy production in this star.

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ii) Describe how energy is produced in this star.

3. a) Name the force that keeps all the planets orbiting the Sun.

Jupiter

Mars

Uranus

Mercury

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b) SUPPLEMENT List the following planets in the order of decreasing orbital speed.

d) SUPPLEMENT The orbital period of Mars around the Sun is 690 days. Mars has an orbital radius of 2.3 × 1011 m. i) Show that the orbital period of Mars is about 60 million seconds.

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ii) Calculate the orbital speed of Mars in m/s.

SPACE PHYSICS

c) SUPPLEMENT Describe the link between the orbital speed of a planet in the Solar System and the gravitational field strength of the Sun at the orbit of the planet.

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Practice questions Note: practice questions, sample answers and comments have been written by the authors.

The marks awarded for these questions indicate the level of detail required in the answers. In examinations, the way marks are awarded may be different. References to assessment and/or assessment preparation are the publisher’s interpretation of the syllabus requirements and may not fully reflect the approach of Cambridge Assessment International Education.

Example answer

COMMENTS

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a) The idea of gravitational attraction is correctly given.

Objects, such as planets, orbit the Sun because of its gravitational pull.

a) E xplain why the objects in the Solar System orbit around the Sun. Objects such as planets orbit the Sun because of its gravitational pull.

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To improve the answer, this needed to be expanded to include the idea that the Sun is far more massive than objects orbiting around it. The term ‘big’ is not equivalent to ‘having greater mass’.

The Sun being ‘hot’ has nothing to do with the question.

Question 1

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Planets orbit the Sun because it is big

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and hot.

b) The dwarf planet Pluto is 5.0 × 1012 m from the Earth.

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Calculate the time it takes for light from Pluto to reach the Earth.

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5.0 × 1012 = 3.0 × 108 time = 0.00006 s

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(3)

(Total 5 marks)

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5

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PRACTICE QUESTIONS

✓ × time ✓

= speed × time

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b) T his answer is the inverse of the correct answer – so the rearranging was incorrect. The correct answer is: 5.0 × 1012 = 16 700 s time = 3.0 × 108 (4.6 hours)

The speed of light in vacuum is 3.0 × 108 m/s.

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SUPPLEMENT

Question 2

a) Describe the Milky Way and state its approximate diameter.

(2)

b) Describe the life cycle of a star a small mass star.

(3)

c) Astronomers believe that there is a black hole at the centre of the Milky Way. What is that black hole?

A A hole in space B A very dense object C A very large giant star

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(Total 6 marks)

Question 3

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SUPPLEMENT

(1)

The speed of light in vacuum is 3.0 × 108 m/s.

(3)

b) A star is 100 light-years from the Earth. Calculate its distance in metres.

(2)

c) The Universe is about 14 billion years old. Suggest why astronomers cannot observe any objects in the Universe beyond a distance of 14 billion light-years.

(2)

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a) Define the light-year and show that it is about 9.5 × 1015 m.

A The universe has galaxies

C The universe is expanding D The universe is full of light.

(1)

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B The universe is empty space

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d) An astronomer observing a distant galaxy concludes that it is moving away from us. What is the implication of this observation?

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PRACTICE QUESTIONS

(Total 8 marks)

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