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Cambridge IGCSE0620

Water

Chemistry 0620 Chapter Notes

What this chapter covers

WaterFertilisersAir quality and climate
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In this chapter

  1. 1Chemical tests for water
  2. 2Testing the purity of water
  3. 3Distilled water in the laboratory
  4. 4What natural water may contain
  5. 5Harmful substances in water
  6. 6Treating the domestic water supply
  7. 7Ammonium salts and nitrates
  8. 8NPK: three elements
  9. 9Why fertilisers are used
  10. 10Mixing compounds to make an NPK fertiliser
  11. 11Fertilisers in river water
  12. 12Clean, dry air
  13. 13Carbon dioxide, carbon monoxide and particulates
  14. 14Methane, oxides of nitrogen and sulfur dioxide
  15. 15Reducing climate change and acid rain
  16. 16Photosynthesis
  17. 17How greenhouse gases cause global warming
  18. 18Oxides of nitrogen and catalytic converters
  19. 19Symbol equation for photosynthesis

1. Chemical tests for water

Two anhydrous salts test for water. Anhydrous means with no water. Each changes colour when water is added.

Anhydrous copper(II) sulfate turns from white to blue. Anhydrous cobalt(II) chloride turns from blue to pink.

Both tests show that water is present. Always name the salt and say anhydrous.

anhydrous copper(II) sulfate (white) + water → hydrated copper(II) sulfate (blue)

anhydrous cobalt(II) chloride (blue) + water → hydrated cobalt(II) chloride (pink)

Examiner insight

“The most common error was to state that the test paper became blue (that being the correct colour for anhydrous copper(II) sulfate being used as the test reagent).” “Very often, when the correct reagent was given, the essential words anhydrous, blue or white were missing.”

Worked example 12 marks

State the colour change when water is added to anhydrous copper(II) sulfate.

  1. 1

    from white [1]

  2. 2

    to blue [1]

2. Testing the purity of water

Pure water has a fixed melting point of 0 °C and a fixed boiling point of 100 °C. A sample with any other value is not pure.

The colour tests above only show that water is present. They do not show that the water is pure.

Examiner insight

“Many candidates did not read the question, which asked for a test for pure water and instead gave tests to show the presence of water.” “The “chemical test” was often correct, but a large minority gave a test for purity.”

Worked example 11 mark

Which test results show that a sample of water is pure?

1 Its boiling point is 100 °C. 2 Its freezing point is 0 °C. 3 It turns anhydrous cobalt(II) chloride from blue to pink. 4 It turns anhydrous copper(II) sulfate from white to blue.

A 1 and 2 · B 1 and 4 · C 2 and 3 · D 3 and 4

  1. 1

    A: 1 and 2. A boiling point of 100 °C and a freezing point of 0 °C show pure water. [1]

  2. 2

    Results 3 and 4 only show that water is present.

3. Distilled water in the laboratory

Distilled water is used in practical chemistry rather than tap water. It contains fewer chemical impurities.

Tap water is not pure. It contains dissolved solids, so it contains more dissolved ions than distilled water.

Examiner insight

“Candidates did not realise that tap water is not pure as it contains dissolved solids, and missed the term soluble in option 2.”

Worked example 11 mark

Why is distilled water used in practical chemistry rather than tap water?

A It has a higher boiling point than tap water. · B It has a lower melting point than tap water. · C It has fewer impurities than tap water. · D It contains more ions than tap water.

  1. 1

    C: distilled water has fewer impurities than tap water. [1]

4. What natural water may contain

Water from natural sources may contain many substances: dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, nitrates from fertilisers, and phosphates from fertilisers and detergents.

Some of these are beneficial. Dissolved oxygen is needed by aquatic life. Some metal compounds provide essential minerals for life.

Examiner insight

“Many candidates did not know ‘one source of phosphate ions’.” “Many candidates confused nitrates in water with oxides of nitrogen and suggested breathing difficulties.”

Worked example 11 mark

Water from natural sources may contain substances such as metal compounds, dissolved oxygen, phosphates and plastics. Which statement is correct?

A Some metal compounds provide essential minerals for life. · B Dissolved oxygen is always harmful to aquatic life. · C Phosphates increase the oxygenation of the water. · D Most plastics break down into harmless substances in water.

  1. 1

    A: some metal compounds provide essential minerals for life. [1]

5. Harmful substances in water

Some metal compounds are toxic. Some plastics harm aquatic life.

Sewage contains harmful microbes, which cause disease. Nitrates and phosphates lead to deoxygenation of water and damage to aquatic life. Details of the eutrophication process are not required.

Examiner insight

“There were many vague answers such as ‘toxic’, ‘water pollution’ or ‘breathing difficulties’ for both sewage and nitrates.” “Many candidates did not mention that the sewage caused disease.”

Worked example 12 marks

Water from natural sources can be polluted with harmful substances. State why sewage and phosphates in river water are harmful.

  1. 1

    sewage: contains microbes which cause disease [1]

  2. 2

    phosphates: deoxygenation of water, which harms aquatic life [1]

6. Treating the domestic water supply

Water is treated in stages to make it safe to drink.

Sedimentation and filtration remove solids. Carbon is added to remove tastes and odours. Chlorination kills microbes.

Distillation is not one of the stages.

Examiner insight

“Option A was the most common incorrect answer, suggesting candidates did not understand that sedimentation is used to separate insoluble solids which are often in suspension in water.”

Worked example 13 marks

State what happens at each stage in the treatment of the domestic water supply: sedimentation and filtration, addition of carbon, chlorination.

  1. 1

    sedimentation and filtration: to remove solids [1]

  2. 2

    addition of carbon: to remove tastes and odours [1]

  3. 3

    chlorination: to kill microbes [1]

Recap

  • Anhydrous copper(II) sulfate: white to blue with water.
  • Anhydrous cobalt(II) chloride: blue to pink with water.
  • These colour tests show water is present, not that it is pure.
  • Pure water: melting point 0 °C, boiling point 100 °C.
  • Distilled water has fewer chemical impurities than tap water.
  • Dissolved oxygen and some metal compounds are beneficial. Some metal compounds and plastics are harmful.
  • Sewage causes disease. Nitrates and phosphates cause deoxygenation of water.
  • Treatment: sedimentation and filtration remove solids, carbon removes tastes and odours, chlorination kills microbes.

7. Ammonium salts and nitrates

Ammonium salts and nitrates are used as fertilisers.

An ammonium salt contains the ammonium ion, NH₄⁺. A nitrate contains the nitrate ion, NO₃⁻. Ammonium nitrate contains both.

Ammonia reacts with nitric acid to make ammonium nitrate. Ammonium sulfate is another ammonium salt used as a fertiliser.

NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq)

Examiner insight

“Common errors were ammonia nitrate, ammonium (unqualified) or nitrate (unqualified).”

Worked example 11 mark

One compound that can be found in fertilisers contains the ions NH₄⁺ and NO₃⁻ only. State the name of this compound.

  1. 1

    ammonium nitrate [1]

8. NPK: three elements

NPK fertilisers provide the elements nitrogen, phosphorus and potassium. The letters are their chemical symbols: N, P and K.

Give the name of the element. Phosphate and nitrate are not elements.

Examiner insight

“Candidates should be reminded that N, P and K are chemical symbols.” “Many different elements were seen, including ‘hydrogen’ and ‘oxygen’ which were wrong and also species which are not elements like ‘phosphate’ and ‘nitrate’.”

Worked example 11 mark

Name the three elements present in an NPK fertiliser.

  1. 1

    nitrogen, phosphorus and potassium [1]

9. Why fertilisers are used

Fertilisers are used for improved plant growth.

Crops take nitrogen, phosphorus and potassium from the soil. A fertiliser replaces the nutrients taken from the soil by previous crops.

A fertiliser is not used to change the pH of the soil.

Examiner insight

“The best answers referred to ‘increased plant growth’ or ‘to replace nutrients taken from the soil by previous crops’.” “A significant number of candidates thought that fertilisers neutralised the soil or affected the soil pH in some way.”

Worked example 11 mark

Some fertilisers contain nitrates. State why fertilisers are used.

  1. 1

    improved plant growth [1]

10. Mixing compounds to make an NPK fertiliser

An NPK fertiliser can be made by mixing compounds. Together they must supply all three elements.

Nitrogen comes from an ammonium salt or a nitrate. Phosphorus comes from a phosphate. Potassium comes from a potassium compound.

Check each element in turn. A mixture with only two of the three is not an NPK fertiliser.

Examiner insight

“Candidates recognised two compounds that are fertilisers but did not realise that phosphorus was necessary.”

Worked example 11 mark

An NPK fertiliser is made by mixing two compounds. One of the compounds is Ca(H₂PO₄)₂. What is the other compound?

A potassium nitrate · B potassium sulfate · C sodium chloride · D ammonium nitrate

  1. 1

    A: Ca(H₂PO₄)₂ supplies phosphorus only, so the other compound must supply nitrogen and potassium. Potassium nitrate does both. [1]

11. Fertilisers in river water

If fertilisers wash into a river, they harm aquatic life. The harm is deoxygenation of the water: the level of oxygen falls.

Write about the oxygen. ‘It kills fish’ and ‘it is toxic’ are not enough.

Examiner insight

“The most common answers were to state that ‘it kills fish’ or that ‘it’s toxic’, which are insufficient. Reference should be made to the level of oxygen present to sustain life.”

Worked example 11 mark

Which statement explains why river water containing NPK fertilisers is harmful to aquatic life?

A NPK fertilisers cause disease in plant and animal life · B NPK fertilisers contain harmful microbes · C NPK fertilisers cause deoxygenation of the water · D NPK fertilisers contain plastic particles

  1. 1

    C: NPK fertilisers cause deoxygenation of the water. [1]

Recap

  • Ammonium salts and nitrates are used as fertilisers.
  • Ammonium ion NH₄⁺. Nitrate ion NO₃⁻. Ammonium nitrate is NH₄NO₃.
  • NPK fertilisers provide the elements nitrogen, phosphorus and potassium.
  • Fertilisers are used for improved plant growth. They are not used to change the pH of the soil.
  • A mixture is NPK only if it supplies all three elements.
  • Fertilisers in river water cause deoxygenation of the water.

12. Clean, dry air

Clean, dry air is approximately 78% nitrogen, N₂, and 21% oxygen, O₂. The remainder is a mixture of noble gases and carbon dioxide, CO₂.

Water vapour and pollutant gases are not part of clean, dry air.

N₂ ≈ 78%

O₂ ≈ 21%

noble gases and CO₂ = the remainder

Examiner insight

“Many candidates did not pick up that the question referred to clean, dry air so ‘water vapour’ or gaseous pollutants such as ‘methane’ or ‘nitrogen dioxide’ received no credit.” “Most candidates confused natural gas, methane, with the most common gases in clean, dry air and chose option B or option D.”

Worked example 11 mark

The table gives the percentage composition of gases on the planet Neptune.

Which statement about the atmospheres on Neptune and on the Earth is correct?

A There is more helium on Neptune than oxygen on the Earth. B There is less methane on Neptune than carbon dioxide on the Earth. C There is less hydrogen on the Earth than on Neptune. D There is more helium on the Earth than on Neptune.

gaspercentage composition / %
hydrogen80
helium18
methane1.5
other gases0.5
  1. 1

    Earth: 78% nitrogen and 21% oxygen, so the remainder, noble gases and carbon dioxide, is about 1%.

  2. 2

    A is wrong: 18% helium on Neptune is less than 21% oxygen on the Earth.

  3. 3

    B and D are wrong: carbon dioxide and helium on the Earth are each part of that 1%, less than 1.5% methane and 18% helium on Neptune.

  4. 4

    C: hydrogen is not one of the gases in clean, dry air, so there is less on the Earth than the 80% on Neptune. [1]

13. Carbon dioxide, carbon monoxide and particulates

Carbon dioxide comes from the complete combustion of carbon-containing fuels. Carbon monoxide and particulates come from the incomplete combustion of carbon-containing fuels.

Higher levels of carbon dioxide lead to increased global warming, which leads to climate change. Carbon monoxide is a toxic gas. Particulates increase the risk of respiratory problems and cancer.

Examiner insight

“Candidates should be aware that carbon monoxide is toxic or poisonous and this is the reason it should not be released into the atmosphere.” “Some answers which just said that carbon monoxide was “harmful” were too vague for credit.”

Worked example 11 mark

Which equation describes the formation of two atmospheric pollutants that are not present in clean air?

A 2C₆H₁₄ + 19O₂ → 12CO₂ + 14H₂O B 2C₇H₁₄ + 18O₂ → 8CO₂ + 6CO + 14H₂O C 2C₈H₁₈ + 19O₂ → 10CO₂ + 6C + 18H₂O D 2C₉H₁₈ + 16O₂ → 14CO + 4C + 18H₂O

  1. 1

    Carbon monoxide, CO, and particulates of carbon, C, are pollutants that are not in clean air. Carbon dioxide, CO₂, is in clean air.

  2. 2

    A makes neither. B makes only CO and C makes only C.

  3. 3

    D makes both CO and C. The answer is D. [1]

14. Methane, oxides of nitrogen and sulfur dioxide

Methane comes from the decomposition of vegetation and waste gases from digestion in animals. Higher levels of methane lead to increased global warming, which leads to climate change.

Oxides of nitrogen come from car engines. They cause acid rain, photochemical smog and respiratory problems.

Sulfur dioxide comes from the combustion of fossil fuels which contain sulfur compounds. It causes acid rain.

Examiner insight

“Candidates should note that ‘factories’ unqualified is not considered a source of pollutants.” “Students should realise that although carbon dioxide is an acidic oxide, it is not acidic enough to be classed as a contributor to acid rain.”

Worked example 11 mark

Three air pollutants, X, Y and Z, are described.

X is a toxic gas formed by the incomplete combustion of fossil fuels.

Y is formed by decomposing vegetation and increases global warming.

Z is a cause of breathing problems and acid rain.

What are X, Y and Z?

XYZ
Acarbon monoxidemethanenitrogen oxides
Bcarbon monoxideparticulatescarbon dioxide
Csulfur dioxidemethaneparticulates
Dsulfur dioxideparticulatescarbon dioxide
  1. 1

    X is carbon monoxide: toxic, from incomplete combustion.

  2. 2

    Y is methane: from decomposing vegetation, increases global warming.

  3. 3

    Z is nitrogen oxides: breathing problems and acid rain. The answer is A. [1]

15. Reducing climate change and acid rain

To reduce climate change: plant trees, reduce livestock farming, decrease the use of fossil fuels, and increase the use of hydrogen and renewable energy such as wind and solar.

To reduce acid rain: use catalytic converters in vehicles, and reduce emissions of sulfur dioxide by using low-sulfur fuels and flue gas desulfurisation with calcium oxide.

Explain a strategy by the pollutant it cuts. Trees take in carbon dioxide. Fewer livestock means less methane. Burning less fossil fuel makes less carbon dioxide. Catalytic converters remove oxides of nitrogen. Flue gas desulfurisation is a neutralisation reaction.

Examiner insight

“For example, many candidates who gave incorrect answers responded with ‘Burn less fuel’ rather than specifying ‘burn less fossil fuels’.” “Very few candidates could recall one method of reducing the emissions of sulfur dioxide.”

Worked example 11 mark

Which row describes a strategy for reducing global warming and a strategy for reducing acid rain?

strategy for reducing global warmingstrategy for reducing acid rain
Aburning more woodflue gas desulfurisation
Bdecrease use of petroleumincrease use of coal for electricity generation
Creduce livestock farminguse catalytic converters in vehicles
Duse low-sulfur fuelsreduce livestock farming
  1. 1

    A is wrong: burning more wood does not reduce global warming. B is wrong: using more coal does not reduce acid rain.

  2. 2

    D is wrong: its two strategies are the wrong way round.

  3. 3

    C: reduce livestock farming for global warming, and use catalytic converters in vehicles for acid rain. [1]

16. Photosynthesis

Photosynthesis is the reaction between carbon dioxide and water to produce glucose and oxygen in the presence of chlorophyll and using energy from light.

Plants take in carbon dioxide, so photosynthesis removes it from the air.

carbon dioxide + water xrightarrow[chlorophyll]light glucose + oxygen

Examiner insight

“Many candidates incorrectly thought that glucose is a reactant in photosynthesis.” “Many candidates identified oxygen as a product of photosynthesis. The commonest error was to suggest carbon dioxide.”

Worked example 11 mark

Which row identifies the reactants and products of photosynthesis?

reactantsproducts
Acarbon dioxide and wateroxygen only
Bcarbon dioxide and oxygenglucose and water
Cglucose and oxygencarbon dioxide and water
Dwater and carbon dioxideglucose and oxygen
  1. 1

    The reactants are carbon dioxide and water, so B and C are wrong.

  2. 2

    The products are glucose and oxygen, so A is wrong. The answer is D. [1]

17. How greenhouse gases cause global warming

Carbon dioxide and methane are greenhouse gases. They absorb thermal energy from the Earth and emit some of this energy towards the Earth. Some thermal energy is reflected back towards the Earth too.

This reduces thermal energy loss to space, so global warming increases.

Examiner insight

“Methane contributes to the enhanced greenhouse effect by causing some of the thermal energy emitted from the Earth to be reflected back towards the Earth rather than being lost to space. This trapping of heat contributes to global warming.” “Candidates knew that greenhouse gases absorb heat but did not think their answer through.”

Worked example 11 mark

Which statement describes how the C–H bonds in methane gas in the atmosphere contribute to global warming?

A They absorb thermal energy from the Sun and emit some of this energy into space. B They absorb thermal energy from the Sun and emit all of this energy towards the Earth. C They absorb thermal energy from the Earth and emit all of this energy towards the Earth. D They absorb thermal energy from the Earth and emit some of this energy towards the Earth.

  1. 1

    A and B are wrong: the thermal energy absorbed comes from the Earth, not the Sun.

  2. 2

    C is wrong: only some of the energy is emitted towards the Earth. The answer is D. [1]

18. Oxides of nitrogen and catalytic converters

In a car engine, nitrogen and oxygen from the air react at high temperatures to form oxides of nitrogen. The nitrogen does not come from the fuel. A catalytic converter removes oxides of nitrogen from the exhaust. It changes them to nitrogen, N₂, and changes carbon monoxide to carbon dioxide.

nitrogen + oxygen → nitrogen dioxide

2CO + 2NO → 2CO₂ + N₂

Examiner insight

“Candidates are reminded that petrol is a mixture of hydrocarbons and so it cannot be nitrogen from the petrol that is reacting and that the purpose of the catalytic converter is to remove oxides of nitrogen not to produce them.”

Worked example 11 mark

Nitrogen dioxide, NO₂, is an oxide of nitrogen and an air pollutant.

Write a word equation to show how nitrogen dioxide is produced from two gases present in air.

  1. 1

    The two gases are nitrogen and oxygen: nitrogen + oxygen → nitrogen dioxide [1]

19. Symbol equation for photosynthesis

The symbol equation has the same reactants and products as the word equation. Glucose is C₆H₁₂O₆. Six molecules of carbon dioxide react with six of water to give one of glucose and six of oxygen, O₂.

Check that it is balanced by counting atoms on each side: 6 C, 12 H and 18 O.

6CO₂ + 6H₂O →(light)(chlorophyll) C₆H₁₂O₆ + 6O₂

Worked example 13 marks

Write the balanced symbol equation for photosynthesis. Show that it is balanced by counting atoms.

  1. 1

    Formulae: reactants CO₂ and H₂O, products C₆H₁₂O₆ and O₂. [1]

  2. 2

    6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ [1]

  3. 3

    Left: 6 C, 12 H and 12 + 6 = 18 O. Right: 6 C, 12 H and 6 + 12 = 18 O. [1]

Recap

  • Clean, dry air is about 78% nitrogen, 21% oxygen, and the remainder noble gases and carbon dioxide.
  • Carbon dioxide: complete combustion. Carbon monoxide and particulates: incomplete combustion.
  • Methane: decomposing vegetation and animal digestion. Oxides of nitrogen: car engines. Sulfur dioxide: burning fossil fuels with sulfur compounds.
  • Effects: carbon dioxide and methane give global warming. Carbon monoxide is toxic. Oxides of nitrogen and sulfur dioxide give acid rain.
  • Photosynthesis: carbon dioxide + water gives glucose + oxygen, with chlorophyll and light.
  • Extended: greenhouse gases absorb thermal energy from the Earth, emit some towards the Earth, and reduce thermal energy loss to space.
  • Extended: oxides of nitrogen form from air in a hot engine. Catalytic converters remove them: 2CO + 2NO → 2CO₂ + N₂.
  • Extended: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State the colour change when water is added to anhydrous cobalt(II) chloride.2 marks
  2. A white solid turns blue when a few drops of a colourless liquid are added to it. Name the solid and say what the result shows.2 marks
  3. A sample of water melts at −2 °C. State the melting point of pure water and what the result shows about the sample.2 marks
  4. A student adds anhydrous copper(II) sulfate to a colourless liquid. It turns blue. The student says that the liquid is pure water. Explain why this is not justified and describe a test that would show whether the water is pure.3 marks
  5. A student rinses a burette with distilled water, not tap water, before a titration. Explain why.2 marks
  6. A student says that all metal compounds in river water are harmful. Explain why this is wrong.2 marks
  7. Name one source of nitrates in natural water and two sources of phosphates.3 marks
  8. A river is polluted with sewage and with phosphates. State why each is harmful.4 marks
  9. A student tests three colourless liquids, X, Y and Z. The table shows the results. (a) State which liquids contain water. Give the evidence. (b) State which liquid is pure water. Explain your answer, referring to both liquids that contain water. (c) The student predicts that Z will turn anhydrous copper(II) sulfate from blue to white. Correct the prediction. | liquid | anhydrous cobalt(II) chloride added | boiling point / °C | | --- | --- | --- | | X | stays blue | 78 | | Y | blue to pink | 100 | | Z | blue to pink | 103 |6 marks
  10. A town takes its water from a lake. Fertilisers from farms and sewage from a village drain into the lake. Fish in the lake are dying. (a) A resident says that nitrates from the fertilisers poison the fish. Give the correct reason why nitrates damage aquatic life. Name the beneficial substance in the water that the fish need. (b) At the treatment works the lake water goes through sedimentation and filtration. Carbon is then added. The water is now clear and has no smell. An engineer says that it is safe to drink. Explain why the engineer is wrong and name the stage that is missing. (c) After full treatment the water is safe to drink. Explain why distilled water, not this water, is still used in practical chemistry.8 marks
  11. Ammonium salts are used as fertilisers. Name the other type of compound used as a fertiliser.1 mark
  12. Give the formula of the ammonium ion and the formula of the nitrate ion.2 marks
  13. Name the compound with the formula (NH₄)₂SO₄.1 mark
  14. Name the three elements in an NPK fertiliser and give the symbol for each.3 marks
  15. A student says that NPK fertiliser is added to a field to neutralise the soil. State why the student is wrong and give the correct reason for using the fertiliser.2 marks
  16. A farmer grows the same crop in a field each year. Explain why the farmer adds NPK fertiliser to the field.2 marks
  17. Which mixture of compounds can be used as an NPK fertiliser? A ammonium sulfate + sodium phosphate · B potassium chloride + ammonium nitrate · C ammonium nitrate + potassium phosphate · D sodium nitrate + calcium phosphate1 mark
  18. Potassium nitrate is mixed with ammonium phosphate. State which of nitrogen, phosphorus and potassium each compound supplies. Say whether the mixture is an NPK fertiliser.3 marks
  19. A bag of fertiliser is labelled ‘NPK’. The label lists two compounds only: (NH₄)₂SO₄ and KNO₃. (a) Name the two compounds. (b) A student says the label is correct because both compounds are fertilisers. Explain why the student is wrong. (c) One more compound is added so that the label is correct. Choose the compound from this list: potassium chloride, calcium phosphate, ammonium nitrate, sodium sulfate.5 marks
  20. A factory makes a fertiliser in two stages. Stage 1: ammonia reacts with nitric acid to make salt R. Stage 2: salt R is mixed with one other compound to make an NPK fertiliser. (a) Name salt R and give the formulas of the two ions it contains. (b) The compound for stage 2 is chosen from K₂SO₄, Ca₃(PO₄)₂ and K₃PO₄. State which compound must be used. Explain why each of the other two is not suitable. (c) Rain washes some of the fertiliser from a field into a lake. State why this is harmful to aquatic life.6 marks
  21. State the approximate percentages of nitrogen and oxygen in clean, dry air. Name the two kinds of substance that make up the remainder.3 marks
  22. State one source of each pollutant: (a) carbon dioxide, (b) carbon monoxide, (c) oxides of nitrogen.3 marks
  23. State the adverse effect of (a) carbon monoxide and (b) sulfur dioxide.2 marks
  24. State two ways to reduce climate change and two ways to reduce acid rain.4 marks
  25. Write the word equation for photosynthesis. State the two other things the reaction needs.4 marks
  26. Near a town there is a coal-fired power station and a farm with a large herd of cattle. The coal contains sulfur compounds. (a) Name the pollutant from the power station that causes acid rain. (b) Name the pollutant the cattle release and state its adverse effect. (c) State one way the power station could cut its emissions of sulfur dioxide.4 marks
  27. Describe how methane in the atmosphere causes global warming.3 marks
  28. (a) Explain how oxides of nitrogen form in a car engine. (b) State the purpose of a catalytic converter. (c) Balance the equation for a reaction in a catalytic converter: CO + NO → CO₂ + N₂4 marks
  29. A test car has an engine that burns hydrogen, H₂, in air in place of petrol. Petrol is a mixture of hydrocarbons. The exhaust of the hydrogen engine contains an oxide of nitrogen, NO, but no carbon monoxide. (a) Explain why the exhaust contains no carbon monoxide. (b) Explain how NO forms in this engine. (c) A student writes: ‘The NO causes global warming, just as carbon dioxide does.’ State what is wrong with this and give two adverse effects of oxides of nitrogen. (d) In a petrol car a catalytic converter removes NO by the reaction 2CO + 2NO → 2CO₂ + N₂. Explain why this reaction cannot remove the NO from the hydrogen engine.6 marks
  30. A power station pipes its waste gas into a sealed greenhouse full of fast-growing plants. The plants take in carbon dioxide from the waste gas by photosynthesis. (a) Write the symbol equation for photosynthesis. (b) The plants take in 4800 molecules of carbon dioxide. Calculate the number of molecules of glucose and the number of molecules of oxygen that are made. (c) Use your answer to (b) to show that every carbon atom taken in ends up in glucose. (d) Explain how taking carbon dioxide out of the waste gas reduces global warming. Use ideas about thermal energy.8 marks

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