IGCSE Chemistry: Chemistry of the Environment Practice Questions
Clean dry air is approximately 78 per cent nitrogen, 21 per cent oxygen and small amounts of noble gases and carbon dioxide. Pollutants including carbon monoxide, sulfur dioxide, oxides of nitrogen and particulates are added mainly by burning fossil fuels.
Topic 10 of Cambridge IGCSE Chemistry 0620 asks for source, effect and remedy for each pollutant, and it also carries two very easy practical marks for the tests for water. The questions below cover the pollutants, acid rain and the water tests.
What you need to know for Chemistry of the Environment
- Composition of clean dry airApproximately 78 per cent nitrogen, 21 per cent oxygen, and the remaining 1 per cent mainly argon with a small proportion of carbon dioxide.
- Carbon monoxideFormed by incomplete combustion of carbon containing fuels when the oxygen supply is limited. It is toxic because it binds to haemoglobin and reduces oxygen transport in the blood.
- Sulfur dioxideFormed when fuels containing sulfur compounds are burnt. It dissolves in rainwater to form acidic solutions, contributing to acid rain.
- Oxides of nitrogenFormed when nitrogen and oxygen from the air react together at the high temperatures inside an engine. They contribute to acid rain and to photochemical smog.
- Catalytic converterFitted to a vehicle exhaust. It converts carbon monoxide and oxides of nitrogen into carbon dioxide and nitrogen, reducing harmful emissions.
- Tests for waterAnhydrous copper(II) sulfate turns from white to blue, and anhydrous cobalt(II) chloride turns from blue to pink. These show water is present. To show water is pure, check that it boils at exactly 100 degrees Celsius and freezes at 0 degrees Celsius at standard pressure.
IGCSE Chemistry Chemistry of the Environment questions and answers
4 exam-style questions written to the 0620 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Name two gaseous pollutants produced by a petrol engine, state the source of each and give one harmful effect of each.
Show the worked answer
- Pollutant 1: carbon monoxide. It is produced by incomplete combustion of the fuel when there is not enough oxygen for complete burning.
- Carbon monoxide is toxic because it binds to haemoglobin in red blood cells, reducing the amount of oxygen the blood can carry.
- Pollutant 2: oxides of nitrogen. Nitrogen and oxygen from the air react together at the very high temperatures inside the engine.
- Oxides of nitrogen dissolve in rainwater to form acids, contributing to acid rain, and they also contribute to photochemical smog and breathing difficulties.
Explain how acid rain forms and describe two of its effects.
Show the worked answer
- Burning fossil fuels that contain sulfur compounds releases sulfur dioxide, and high engine temperatures produce oxides of nitrogen.
- These gases dissolve in water droplets in the atmosphere and are oxidised, forming sulfuric and nitric acids, so the rain has a lower pH than normal.
- Effect 1: acid rain reacts with carbonate rocks such as limestone and marble, damaging buildings and statues, and it corrodes metal structures.
- Effect 2: it lowers the pH of lakes and rivers, killing fish and other aquatic organisms, and it damages trees and leaches nutrients from soil.
Describe how you could show that a colourless liquid contains water, and how you could then show that it is pure water.
Show the worked answer
- Add a small amount of the liquid to anhydrous copper(II) sulfate. If it turns from white to blue, water is present. Anhydrous cobalt(II) chloride turning from blue to pink is an acceptable alternative test.
- These tests show only that water is present. They say nothing about purity, because a solution of a salt in water also contains water.
- To test purity, measure the boiling point. Pure water boils at exactly 100 degrees Celsius at standard atmospheric pressure.
- A dissolved impurity raises the boiling point above 100 degrees Celsius and lowers the freezing point below 0 degrees Celsius, so a sharp boiling point at exactly 100 confirms purity.
State what a catalytic converter does in a car exhaust and write a word equation for one reaction that takes place in it.
Show the worked answer
- A catalytic converter contains a catalyst with a very large surface area over which the exhaust gases pass.
- It converts carbon monoxide and oxides of nitrogen into less harmful products before they leave the exhaust.
- A word equation for one reaction is: carbon monoxide + nitrogen monoxide gives carbon dioxide + nitrogen.
- This removes the toxic carbon monoxide and the acid rain forming nitrogen monoxide in a single reaction.
Common mistakes in this topic
- Confusing complete with incomplete combustion. Complete combustion gives carbon dioxide and water, incomplete gives carbon monoxide or carbon.
- Attributing acid rain to carbon dioxide rather than sulfur dioxide and nitrogen oxides.
- Saying a test for water proves the water is pure.
- Forgetting that oxides of nitrogen come from the nitrogen in the air, not from the fuel.
- Mixing up the two water test colour changes. Copper(II) sulfate goes white to blue, cobalt(II) chloride goes blue to pink.
Exam tips
- Answer every pollutant question with three parts: source, effect, and where relevant the method of reduction.
- Learn the air composition figures. They are easy marks and are often the first part of a question.
- Remember the sequence for the greenhouse effect: gases absorb reradiated energy from the Earth's surface and re-emit it, warming the atmosphere.
- Distinguish the natural greenhouse effect, which makes the Earth habitable, from the enhanced greenhouse effect caused by additional emissions.
- For NPK fertilisers, link nitrogen to protein and growth, phosphorus to roots and potassium to flowering and fruiting.
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Chemistry of the Environment FAQs
What is the composition of clean dry air?
Approximately 78 per cent nitrogen and 21 per cent oxygen, with the remaining 1 per cent made up mainly of argon together with a small proportion of carbon dioxide and traces of other noble gases.
What causes acid rain?
Sulfur dioxide from burning fuels containing sulfur compounds, and oxides of nitrogen formed when nitrogen and oxygen react at high temperatures inside engines. These gases dissolve in atmospheric water and are oxidised to sulfuric and nitric acids, lowering the pH of rainfall.
How do I test for water, and how do I test that water is pure?
Add the liquid to anhydrous copper(II) sulfate, which turns from white to blue, or to anhydrous cobalt(II) chloride, which turns from blue to pink. Both show water is present but not that it is pure. To test purity, check that the sample boils at exactly 100 degrees Celsius and freezes at 0 degrees Celsius at standard pressure.
Why is carbon monoxide dangerous?
Carbon monoxide binds to haemoglobin in red blood cells in place of oxygen, reducing the amount of oxygen the blood can transport around the body. It is colourless and odourless, so it cannot be detected by the senses, which makes it especially hazardous.
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Written to the published Cambridge IGCSE Chemistry (0620) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.