IGCSE Chemistry Paper 4 (Theory / Extended)

Topic 10: Chemistry of the Environment -- Mock Exam 2

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0620

Instructions

Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.

Question Navigation

Question 1 -- Substances in Natural Water

Total: 12 marks

A conservation group surveys a lake in the Lake District. The lake receives run-off from farmland, an outflow from a sewage works and litter from a nearby car park.

(a) [4]

(i) Name the dissolved gas in the lake water that is essential to aquatic life, and state why it is needed. [2]

(ii) Explain how nitrates and phosphates washed off the farmland can eventually cause the death of fish in the lake. [2]

Model Answer -- 1(a)

(i) Dissolved oxygen [1]

(i) Fish and other aquatic organisms need it for respiration [1]

(ii) The nitrates and phosphates act as nutrients, causing rapid growth of algae and water plants which later die [1]

(ii) Bacteria decomposing the dead algae use up the dissolved oxygen, so the fish can no longer respire and they die [1]

⚠ If you missed marks here: The fertiliser effect is indirect, and the examiner is looking for the chain: nutrients feed algae, the algae die, decomposer bacteria consume the oxygen, then the fish suffocate. Answers that say the nitrates "poison the fish" miss the entire mechanism and score nothing for (ii).

1 mark -- dissolved oxygen named 1 mark -- needed for respiration of aquatic organisms 1 mark -- nutrients cause rapid algal growth then algae die 1 mark -- bacteria decomposing algae use up oxygen so fish die

(b) [4]

Plastic litter and untreated sewage also reach the lake.

(i) Explain why plastic waste remains in the lake for many years. [2]

(ii) State one disease that can be caught from water contaminated with sewage, and name the type of substance in the sewage that causes it. [2]

Model Answer -- 1(b)

(i) Plastics are chemically unreactive and insoluble in water, so they are not broken down by water or oxygen [1]

(i) They are not biodegradable, so microbes cannot decompose them; animals may swallow the fragments and be harmed [1]

(ii) A named water-borne disease such as cholera or typhoid [1]

(ii) It is caused by harmful microbes (bacteria) present in the sewage [1]

⚠ If you missed marks here: For plastics, unreactivity alone gains one mark; the second needs either non-biodegradability or the harm caused when animals swallow the fragments. In (ii) the cause must be identified as microbes or bacteria — writing "dirty water" or "chemicals in the sewage" does not identify the living organisms responsible.

1 mark -- plastics unreactive and insoluble not broken down 1 mark -- not biodegradable and harm animals that swallow them 1 mark -- named water-borne disease such as cholera or typhoid 1 mark -- caused by harmful microbes or bacteria in sewage

A student boils some lake water and finds that it boils at 100.6 °C at standard atmospheric pressure. He then distils the lake water and tests the distillate.

(i) Explain what the boiling point of 100.6 °C tells him about the lake water. [2]

(ii) Predict the result of testing the distillate with anhydrous copper(II) sulfate, and predict its boiling point. [2]

Model Answer -- 1(c)

(i) The lake water is not pure, because pure water boils at exactly 100 °C at standard atmospheric pressure [1]

(i) The higher boiling point is caused by dissolved solids, which raise the boiling point of the water [1]

(ii) The anhydrous copper(II) sulfate would turn from white to blue, showing water is present [1]

(ii) The distillate would boil at exactly 100 °C, because distillation leaves the dissolved solids behind [1]

⚠ If you missed marks here: Two separate marks are on offer in (i): the conclusion that the water is impure, and the reason that dissolved solids raise the boiling point. In (ii) the prediction of exactly 100 °C must be justified by the fact that the solids stay behind in the flask; an answer of "about 100 °C" is too vague, since the whole point of the purity test is that the value is sharp.

1 mark -- water impure since pure water boils exactly 100 1 mark -- dissolved solids raise the boiling point 1 mark -- distillate turns anhydrous copper sulfate white to blue 1 mark -- distillate boils at exactly 100 solids left behind

Question 2 -- Making and Testing a Fertiliser

Total: 12 marks

A class in Nairobi prepares a sample of ammonium nitrate in the laboratory by titrating dilute nitric acid with aqueous ammonia, and then tests the salt they have made.

(a) [4]

(i) Write the balanced symbol equation for the reaction between aqueous ammonia and dilute nitric acid. [1]

(ii) Describe how the class can obtain pure dry crystals of ammonium nitrate from the neutral solution they have made. [3]

Model Answer -- 2(a)

(i) NH₃ + HNO₃ → NH₄NO₃ [1]

(ii) Warm the neutral solution to evaporate some of the water and produce a saturated solution [1]

(ii) Leave the concentrated solution to cool so that crystals form [1]

(ii) Filter off the crystals and dry them between sheets of filter paper (or in a warm oven) [1]

⚠ If you missed marks here: Crystallisation is a three-step answer and each step carries a mark: evaporate to concentrate, cool to crystallise, filter and dry. Boiling the solution completely to dryness is penalised, because it leaves an impure powder rather than crystals and can decompose the salt.

1 mark -- equation NH3 plus HNO3 gives NH4NO3 1 mark -- evaporate some water to make saturated solution 1 mark -- cool the solution so crystals form 1 mark -- filter off crystals and dry them

(b) [4]

The class checks that their product contains ammonium ions.

(i) Describe the test for the ammonium ion, including the reagent, the conditions and the result. [3]

(ii) Name the gas that is produced in this test. [1]

Model Answer -- 2(b)

(i) Add aqueous sodium hydroxide (or another alkali) to the solid or its solution [1]

(i) Warm the mixture gently [1]

(i) Hold damp red litmus paper in the gas given off; it turns blue [1]

(ii) The gas is ammonia [1]

⚠ If you missed marks here: All three details are separately credited: the alkali, the warming, and the damp red litmus turning blue. Leaving out "warm" is the most common single-mark loss, because at room temperature very little ammonia escapes. The litmus must be damp — dry paper shows no colour change at all.

1 mark -- add aqueous sodium hydroxide or other alkali 1 mark -- warm the mixture 1 mark -- damp red litmus paper turns blue 1 mark -- gas identified as ammonia

(i) Calculate the percentage by mass of nitrogen in ammonium nitrate, NH₄NO₃. (Aᵣ: H = 1, N = 14, O = 16) [3]

(ii) Calculate the mass of nitrogen supplied by a 25 kg bag of ammonium nitrate. [1]

Model Answer -- 2(c)

(i) Mᵣ of NH₄NO₃ = 14 + 4 + 14 + 48 = 80 [1]

(i) Mass of nitrogen per formula unit = 2 × 14 = 28 [1]

(i) Percentage of nitrogen = 28 ÷ 80 × 100 = 35% [1]

(ii) Mass of nitrogen = 35/100 × 25 = 8.75 kg [1]

⚠ If you missed marks here: Ammonium nitrate contains two nitrogen atoms in different parts of the formula, one in NH₄ and one in NO₃; counting only one halves the percentage. In (ii) apply the percentage to the mass of the bag — the answer is a mass in kilograms, not a percentage, and it must carry its unit.

1 mark -- relative formula mass 80 1 mark -- nitrogen mass 28 from two nitrogen atoms 1 mark -- percentage 35 1 mark -- 8.75 kg of nitrogen in the bag

Question 3 -- Vehicle Exhaust Pollution

Total: 12 marks

A city council in Manchester monitors the exhaust gases from vehicles in the city centre and is considering a low-emission zone that would require all vehicles to have working catalytic converters.

(a) [4]

Complete the table by naming the pollutant described in each row and stating one harmful effect of each.

(i) A gas formed by incomplete combustion of the fuel. [2]

(ii) A gas formed from two gases in the air at high temperature inside the engine. [2]

Model Answer -- 3(a)

(i) Carbon monoxide [1]

(i) It is toxic: it binds to haemoglobin so the blood cannot carry enough oxygen [1]

(ii) Nitrogen monoxide / oxides of nitrogen [1]

(ii) They cause acid rain, photochemical smog and respiratory problems [1]

⚠ If you missed marks here: Read the clue in each description: "incomplete combustion of the fuel" points to a carbon compound, while "two gases in the air" means nitrogen and oxygen. Each effect must belong to its own pollutant — giving acid rain as the effect of carbon monoxide is a classic mismatch that loses the mark.

1 mark -- carbon monoxide named 1 mark -- toxic binds haemoglobin reducing oxygen transport 1 mark -- oxides of nitrogen named 1 mark -- cause acid rain smog or respiratory problems

(b) [4]

Inside a catalytic converter, carbon monoxide reacts with nitrogen monoxide.

(i) Write the balanced symbol equation for this reaction. [2]

(ii) Explain why this single reaction is useful in reducing air pollution. [2]

Model Answer -- 3(b)

(i) 2CO + 2NO → 2CO₂ + N₂ — correct formulae [1]

(i) Equation correctly balanced [1]

(ii) Two harmful pollutants, carbon monoxide and nitrogen monoxide, are removed together in one reaction [1]

(ii) The products, carbon dioxide and nitrogen, are both normal components of clean air and are far less harmful [1]

⚠ If you missed marks here: Balancing needs the 2 in front of both CO and NO, giving 2CO₂ and a single N₂. In (ii) both halves are credited: two pollutants are destroyed at once, and the products are ordinary constituents of clean air. Do not claim the reaction removes carbon dioxide — it actually produces it, which is why converters do not help with climate change.

1 mark -- correct formulae CO NO CO2 N2 1 mark -- equation correctly balanced 2CO plus 2NO 1 mark -- two pollutants removed in a single reaction 1 mark -- products are harmless components of clean air

Diesel vehicles in the city also release particulates and small amounts of sulfur dioxide.

(i) Explain what particulates are and how they are formed. [2]

(ii) Explain how the sulfur dioxide emissions from these vehicles could be reduced, and why this method works. [2]

Model Answer -- 3(c)

(i) Particulates are very small solid particles, mainly of carbon (soot) [1]

(i) They are formed by the incomplete combustion of the fuel when the oxygen supply is limited [1]

(ii) Use low-sulfur diesel, in which most of the sulfur compounds have been removed at the refinery [1]

(ii) All the sulfur dioxide comes from sulfur in the fuel, so less sulfur in the fuel means less sulfur dioxide is produced when it burns [1]

⚠ If you missed marks here: Particulates must be described as solid carbon particles — calling them "a gas" contradicts the whole idea. In (ii) naming low-sulfur fuel gets one mark and the reason gets the second: the sulfur dioxide originates in the fuel itself, so removing the sulfur removes the emission at source. Flue gas desulfurisation is not an option for a moving vehicle.

1 mark -- particulates are tiny solid carbon particles 1 mark -- formed by incomplete combustion in limited oxygen 1 mark -- use low-sulfur diesel fuel 1 mark -- sulfur dioxide comes from sulfur in the fuel

Question 4 -- Water Supply for a Village

Total: 12 marks

An aid agency is building a small water treatment plant for a village in Bihar. The only water source is a muddy river that is used by cattle and receives waste from the village.

(a) [4]

The plant will use four stages: sedimentation, filtration, treatment with carbon and chlorination.

State what each stage removes from the water or does to it.

Model Answer -- 4(a)

Sedimentation: the larger, heavier insoluble particles settle out under gravity [1]

Filtration: beds of sand trap the remaining fine insoluble particles [1]

Carbon: removes dissolved substances that give the water unpleasant tastes and odours [1]

Chlorination: chlorine is added, which kills harmful microbes [1]

⚠ If you missed marks here: Each stage must be given its own distinct job. The commonest error is to say that filtration or sedimentation removes bacteria — only chlorination does that. Equally, carbon does not filter out solids; it deals with dissolved substances responsible for taste and smell.

1 mark -- sedimentation larger insoluble particles settle out 1 mark -- filtration sand traps fine insoluble particles 1 mark -- carbon removes dissolved tastes and odours 1 mark -- chlorination kills harmful microbes

(b) [4]

(i) Explain why the sedimentation tanks are built before the sand filters rather than after them. [2]

(ii) Explain why chlorination is the final stage of the treatment. [2]

Model Answer -- 4(b)

(i) Settling removes the bulk of the mud cheaply, using only gravity [1]

(i) If the muddy water went straight to the filters, the sand would quickly become blocked and would need cleaning constantly [1]

(ii) Disinfecting at the end means that any microbes introduced during the earlier stages are destroyed before the water leaves the plant [1]

(ii) A small amount of chlorine remains in the water, protecting it from microbes on its way to the village [1]

⚠ If you missed marks here: Questions about the order of stages are testing practical reasoning, so refer to what would go wrong if the order were reversed: the filters clogging, or microbes re-entering the water after disinfection. Simply asserting "that is the correct order" earns nothing.

1 mark -- settling removes bulk of mud cheaply using gravity 1 mark -- otherwise sand filters would block quickly 1 mark -- disinfecting last kills microbes added in earlier stages 1 mark -- residual chlorine protects water on way to village

Before the plant was built, villagers boiled their drinking water.

(i) State one advantage and one disadvantage of boiling water compared with chlorination. [2]

(ii) After treatment, the village water boils at 100.2 °C. A villager says this proves the treatment has failed. Evaluate this claim. [2]

Model Answer -- 4(c)

(i) Advantage: boiling kills microbes without adding any chemicals, so there is no chlorine taste [1]

(i) Disadvantage: boiling uses fuel and is expensive or slow for large volumes, and the water is not protected from re-infection afterwards [1]

(ii) The claim is not valid for safety: the raised boiling point shows only that dissolved solids are present, so the water is not chemically pure [1]

(ii) Treatment is designed to make water safe rather than pure, and dissolved salts are harmless; the microbes have still been killed by chlorination [1]

⚠ If you missed marks here: Evaluation questions need both sides. Concede what the data do show — the water is not pure — then explain why that does not support the conclusion, because treated drinking water is meant to be safe rather than pure and dissolved mineral salts do no harm. An answer that simply says "he is wrong" scores nothing.

1 mark -- advantage boiling kills microbes without adding chemicals 1 mark -- disadvantage uses fuel slow and no lasting protection 1 mark -- raised boiling point shows only dissolved solids present 1 mark -- treatment makes water safe not pure microbes still killed

Question 5 -- The Composition of Air

Total: 10 marks

A student passes 500 cm³ of clean, dry air backwards and forwards over heated copper until no further change in volume occurs. The copper turns black and the volume of gas falls.

(a) [4]

(i) State the approximate percentages by volume of nitrogen, oxygen and argon in clean, dry air. [3]

(ii) State the approximate percentage of carbon dioxide in clean, dry air. [1]

Model Answer -- 5(a)

(i) Nitrogen approximately 78% [1]

(i) Oxygen approximately 21% [1]

(i) Argon approximately 0.9% (about 1%) [1]

(ii) Carbon dioxide approximately 0.04% [1]

⚠ If you missed marks here: Learn the four figures as a set: 78, 21, about 1 and about 0.04. The commonest error is giving carbon dioxide a whole percent, which would be more than twenty times its actual level, or leaving argon out altogether because it is chemically unreactive and easy to forget.

1 mark -- nitrogen about 78 percent 1 mark -- oxygen about 21 percent 1 mark -- argon about 1 percent 1 mark -- carbon dioxide about 0.04 percent

(b) [3]

(i) Explain why the copper turns black and why the volume of gas decreases. [2]

(ii) Predict the final volume of gas remaining in the apparatus. [1]

Model Answer -- 5(b)

(i) The hot copper reacts with the oxygen in the air to form black copper(II) oxide [1]

(i) The oxygen is removed from the gas as a solid product, so the volume of gas falls [1]

(ii) About 79% of 500 cm³ remains, which is approximately 395 cm³ [1]

⚠ If you missed marks here: The volume falls because the oxygen is converted to a solid oxide, not because it "disappears" or "is used up" without explanation. For the calculation, 21% of 500 cm³ is 105 cm³ of oxygen removed, leaving about 395 cm³ of nitrogen and argon; an answer of 390 cm³ based on the nitrogen alone is also acceptable if the working is shown.

1 mark -- copper reacts with oxygen forming black copper oxide 1 mark -- oxygen removed as solid so gas volume falls 1 mark -- final volume about 395 cubic centimetres

The student repeats the experiment using air collected beside a busy road instead of clean, dry air.

(i) Name two pollutant gases that might be present in this sample but not in clean air. [2]

(ii) Suggest why the sample was described as "dry" air in the original experiment. [1]

Model Answer -- 5(c)

(i) Carbon monoxide [1]

(i) Oxides of nitrogen (or sulfur dioxide) [1]

(ii) Ordinary air contains a variable amount of water vapour, so the air must be dried for the percentages of the other gases to be reliable [1]

⚠ If you missed marks here: Carbon dioxide is not a good answer in (i) because it is present in clean air too; the question asks for gases that would not normally be there. In (ii) the key word is "variable" — because humidity changes from day to day, quoted compositions are always for dry air.

1 mark -- carbon monoxide named as pollutant 1 mark -- oxides of nitrogen or sulfur dioxide named 1 mark -- water vapour is variable so air is dried for reliable percentages

Question 6 -- Climate Change and Photosynthesis

Total: 12 marks

A national newspaper reports that the concentration of carbon dioxide in the atmosphere has risen from 315 parts per million in 1960 to over 420 parts per million today, and that global average temperature has risen over the same period.

(a) [4]

(i) Explain how carbon dioxide in the atmosphere causes the surface of the Earth to become warmer. [3]

(ii) Name one other greenhouse gas. [1]

Model Answer -- 6(a)

(i) The Earth's surface radiates thermal energy (infrared radiation) outwards [1]

(i) Carbon dioxide molecules absorb this radiation and re-emit it in all directions [1]

(i) Some of the energy is returned to the surface instead of escaping to space, so the temperature rises [1]

(ii) Methane [1]

⚠ If you missed marks here: Three marks require three distinct steps, so avoid compressing the answer into "carbon dioxide traps heat". Name the radiation as thermal or infrared, say the gas absorbs and re-emits it, and finish by explaining that less energy escapes to space. Nitrogen and oxygen are not greenhouse gases, so they cannot be offered in (ii).

1 mark -- Earth's surface radiates thermal infrared energy 1 mark -- carbon dioxide absorbs and re-emits in all directions 1 mark -- less energy escapes to space so temperature rises 1 mark -- methane named as another greenhouse gas

(b) [4]

Plants remove carbon dioxide from the air by photosynthesis.

(i) Write the balanced symbol equation for photosynthesis and state the two conditions required. [3]

(ii) Explain why cutting down large areas of forest increases the amount of carbon dioxide in the atmosphere. [1]

Model Answer -- 6(b)

(i) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ with correct formulae [1]

(i) Equation correctly balanced [1]

(i) The conditions are light and chlorophyll [1]

(ii) Fewer trees remain to absorb carbon dioxide by photosynthesis, and burning the felled timber releases yet more carbon dioxide [1]

⚠ If you missed marks here: The glucose formula and the six-fold balancing are marked separately, so check both. In (ii) the strongest answers give both effects of deforestation: less carbon dioxide is absorbed, and burning or rotting the wood puts stored carbon back into the air.

1 mark -- correct formulae in photosynthesis equation 1 mark -- equation balanced correctly 1 mark -- conditions light and chlorophyll 1 mark -- fewer trees absorb carbon dioxide and burning releases more

(i) Describe two strategies, other than planting trees, that reduce the effects of climate change, and explain how each works. [2]

(ii) A reader writes to the newspaper saying "carbon dioxide cannot be causing warming, because it makes up only 0.04% of the air". Evaluate this argument. [2]

Model Answer -- 6(c)

(i) Generating electricity from renewable sources such as wind or solar power, which produces no carbon dioxide because no fuel is burned [1]

(i) Improving energy efficiency or using electric vehicles, so that less fossil fuel is burned overall [1]

(ii) The argument is weak: a gas can have a large effect at low concentration if it absorbs thermal radiation strongly, as carbon dioxide does [1]

(ii) What matters is the change in concentration; the rise from 315 to over 420 ppm is an increase of more than a third in the amount of the absorbing gas [1]

⚠ If you missed marks here: Each strategy must be paired with a mechanism — a bare list of "wind power, solar power" scores one mark at most. For the evaluation, the point is that a small percentage does not mean a small effect when the gas absorbs infrared radiation strongly, and the relevant figure is how much the concentration has risen, not how small it looks.

1 mark -- renewable electricity no fuel burned so no carbon dioxide 1 mark -- energy efficiency or electric vehicles burn less fossil fuel 1 mark -- small concentration can have large effect strong absorber 1 mark -- the increase from 315 to 420 ppm is what matters

Question 7 -- Comparing Fertilisers

Total: 10 marks

A garden centre sells three nitrogen fertilisers: urea CO(NH₂)₂, ammonium sulfate (NH₄)₂SO₄ and potassium nitrate KNO₃. A gardener wants to know which gives the most nitrogen for her money.

(a) [4]

(i) Calculate the percentage by mass of nitrogen in urea, CO(NH₂)₂. (Aᵣ: H = 1, C = 12, N = 14, O = 16) [3]

(ii) The percentage of nitrogen in potassium nitrate is 13.9%. State which of urea and potassium nitrate is the richer source of nitrogen. [1]

Model Answer -- 7(a)

(i) Mᵣ of CO(NH₂)₂ = 12 + 16 + 2(14 + 2) = 60 [1]

(i) Mass of nitrogen per formula unit = 2 × 14 = 28 [1]

(i) Percentage of nitrogen = 28 ÷ 60 × 100 = 46.7% [1]

(ii) Urea is the richer source, because 46.7% is much greater than 13.9% [1]

⚠ If you missed marks here: In CO(NH₂)₂ the bracket contains NH₂ and everything inside it doubles, giving two nitrogen and four hydrogen atoms in total. Careless expansion is the main source of error here; set the formula mass out term by term rather than trying to total it in your head.

1 mark -- relative formula mass of urea 60 1 mark -- nitrogen mass 28 1 mark -- percentage 46.7 1 mark -- urea richer source than potassium nitrate

(b) [3]

(i) State which element, apart from nitrogen, is supplied to plants by potassium nitrate, and state its effect on the plant. [2]

(ii) Explain why a fertiliser must be soluble in water to be useful. [1]

Model Answer -- 7(b)

(i) Potassium [1]

(i) Potassium promotes the development of flowers and fruit [1]

(ii) The compound must dissolve in soil water so that the ions can be absorbed through the roots of the plant [1]

⚠ If you missed marks here: Naming the element earns one mark and its role earns the other, so both are needed. In (ii) the mark is for absorption through the roots — "so it works better" does not explain the chemistry, which is that only dissolved ions can be taken up.

1 mark -- potassium named 1 mark -- potassium promotes flowers and fruit 1 mark -- must dissolve so ions absorbed through roots

The gardener wants to check that a bag labelled "ammonium sulfate" really contains an ammonium salt.

(i) Describe the test she should carry out and the result she should obtain. [2]

(ii) State one environmental problem caused by using too much fertiliser. [1]

Model Answer -- 7(c)

(i) Warm a sample of the solid with aqueous sodium hydroxide [1]

(i) A gas is released that turns damp red litmus paper blue, showing that ammonia, and therefore the ammonium ion, is present [1]

(ii) Excess fertiliser is washed into rivers and lakes, where nitrates and phosphates cause algal growth that removes dissolved oxygen and kills fish [1]

⚠ If you missed marks here: The test needs warming with an alkali and the damp red litmus turning blue; omitting either detail loses a mark. In (ii) the environmental answer must go beyond "pollution" — state that the run-off feeds algae whose decay strips the water of oxygen.

1 mark -- warm solid with aqueous sodium hydroxide 1 mark -- gas turns damp red litmus blue showing ammonium ion 1 mark -- run-off causes algal growth oxygen loss and fish deaths

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