Detailed notes on Chemistry of the Environment for Cambridge IGCSE Chemistry, covering key concepts, explanations, examples, and exam-focused revision points.
Air — Cambridge IGCSE 0620 Chemistry Extended (2026)
Composition of clean air, the major air pollutants, their sources, effects, and how catalytic converters reduce them.
At a glance
Clean dry air: ∼78% N₂, ∼21% O₂, ∼1% Ar, traces of CO₂, etc.
Pollutants: CO (incomplete combustion), SO₂ (sulfur in fuel), NOₓ (high-T air in engines), particulates.
SO₂ + NOₓ → acid rain.
CO: poisonous; binds haemoglobin in place of O₂.
Catalytic converter: Pt/Pd/Rh on ceramic honeycomb. Converts CO + NOₓ + unburnt hydrocarbons to CO₂, N₂, H₂O.
2CO+2NO→2CO2+N2 in catalytic converters.
Greenhouse gases: CO₂, CH₄, water vapour. Cause global warming.
What you’ll learn
Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).
12.2 — State the composition of clean dry air.
12.2 — Name common air pollutants, their sources and effects.
12.2 — Describe how catalytic converters reduce car emissions.
12.2 — Describe the greenhouse effect.
Composition of clean air
78% N₂, 21% O₂, ∼1% Ar, traces of CO₂, water vapour and noble gases.
Clean dry air composition.
Gas
Percentage
Nitrogen (N₂)
∼78%
Oxygen (O₂)
∼21%
Argon (Ar)
∼1%
Carbon dioxide (CO₂)
∼0.04%
Other noble gases (Ne, Kr, Xe)
trace
(Real air also contains water vapour, but composition tables usually quote DRY air.)
Nitrogen and oxygen together make up about 99% of clean dry air.
Memorise. N₂ at 78%, O₂ at 21% — these two together are 99% of clean dry air. Ar is the next 1%. CO₂ a tiny fraction.
Worked qualitative. Why is nitrogen the major gas? It's relatively inert and doesn't react readily with most things. Atmospheric N₂ is mostly unreactive — that's why oxygen, despite being highly reactive, doesn't get used up.
Cambridge tip. Pollutants are EXTRA gases — CO, SO₂, NOₓ, etc. — added to clean air by human activity (or natural sources like volcanoes).
N₂ ∼78%, O₂ ∼21%, Ar ∼1%.
CO₂ trace (∼0.04%).
Pollutants are EXTRA gases on top of this.
Air pollutants — sources and effects
CO, SO₂, NOₓ, particulates. From combustion, mainly. Each has specific harmful effects.
Carbon monoxide (CO).
Source: incomplete combustion of fuels (insufficient oxygen).
2C+O2→2CO (limited O₂).
Effect: poisonous. Binds to haemoglobin ∼250× more strongly than O₂ → blood can't carry oxygen → suffocation.
Pt/Pd/Rh on ceramic honeycomb. Converts toxic CO, NOₓ, and unburnt fuel to CO₂, N₂, H₂O.
Catalytic converter. A device fitted in car exhaust systems containing a ceramic honeycomb coated with platinum, palladium, and rhodium catalysts. Hot exhaust gases pass through; the catalysts speed up reactions that convert harmful pollutants to less harmful gases.
Reactions catalysed.
1. CO oxidation.2CO+O2→2CO2.
2. Hydrocarbon combustion. Unburnt CxHy+O2→CO2+H2O.
3. NOₓ reduction.2CO+2NO→2CO2+N2 — CO reduces NO to N₂.
Net effect. Toxic outputs (CO, NO, hydrocarbons) → less harmful (CO₂, N₂, H₂O).
The honeycomb maximises catalyst surface area, converting toxic gases as the exhaust passes through.
Why honeycomb shape? Maximises surface area for catalyst contact with gases. More surface = more effective.
Why precious metals? Pt/Pd/Rh catalyse the reactions effectively even at relatively low exhaust temperatures.
Limitations.
Doesn't work until warm (cold-start emissions are higher).
Catalysts can be poisoned by lead → why leaded fuel was phased out.
Doesn't remove CO₂ — only changes which carbon-containing molecule is emitted.
Doesn't remove SO₂ (driver: switch to low-sulfur fuels).
Worked qualitative. Why doesn't a catalytic converter solve the global warming problem? It STILL produces CO₂ — just from previously-CO-or-hydrocarbon carbon. Total carbon emitted unchanged. Only fuel-source change (e.g. electric cars) reduces total CO₂.
Pt/Pd/Rh catalysts on ceramic honeycomb.
CO + O₂ → CO₂.
NO + CO → CO₂ + N₂.
Reduces toxicity, NOT total carbon.
Poisoned by leaded fuel.
Quick recap
Air: 78% N₂, 21% O₂, 1% Ar, trace CO₂.
CO: poisonous; incomplete combustion.
SO₂: acid rain; sulfur in fuel.
NOₓ: acid rain + smog; high-T engines.
Catalytic converter: converts CO/NOₓ/HC to CO₂/N₂/H₂O.
CO₂, CH₄: greenhouse gases.
Memorise this
Verbatim phrases and definitions Cambridge mark schemes credit.
Pollutant — substance that contaminates the environment, here air.
Catalytic converter — emissions-control device using Pt/Pd/Rh catalysts.
Greenhouse gas — gas that absorbs infrared radiation and warms the atmosphere.
Acid rain — rain made acidic by SO₂ and NOₓ pollution.
How it’s examined
Air composition + pollutants is examined every Paper 4 (6-8 marks). Examiner reports flag students confusing the role of CO and NOₓ in catalytic converters — note that CO is OXIDISED to CO₂ AND ALSO REDUCES NOₓ to N₂.
These are the four figures Cambridge expects. The percentages should add to ~100%, so do not write 21% argon — argon is only ~1%.
2Sources of carbon monoxide and sulfur dioxide
Getting started• pollutants, sources
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Question
State the source of (a) carbon monoxide and (b) sulfur dioxide in polluted air.
Step-by-step solution
Step 1
(a) Carbon monoxide, CO, forms during the incomplete combustion of carbon-containing fuels, e.g. in car engines where there is insufficient oxygen.
Step 2
(b) Sulfur dioxide, SO2, forms when fossil fuels containing sulfur (e.g. coal, some petrol/diesel) are burned and the sulfur impurity is oxidised.
Answer
(a) CO — incomplete combustion of carbon-containing fuels. (b) SO₂ — combustion of fossil fuels that contain sulfur.
Examiner tip
Be specific: CO comes from incomplete combustion, and SO₂ comes from burning fuels that contain sulfur — not from combustion in general.
3Pollutants, sources and adverse effects
Building confidence• Adapted from 0620/42 May/Jun 2024 Q15• pollutants, effects
▼
Question
For each pollutant — CO, SO2, oxides of nitrogen (NOx) and particulates — give one source and one adverse effect.
Step-by-step solution
Step 1
CO — source: incomplete combustion of fuels in engines. Effect: toxic; it binds to haemoglobin, reducing the blood's ability to carry oxygen.
Step 2
SO₂ — source: burning sulfur-containing fossil fuels. Effect: dissolves in rainwater to cause acid rain (and respiratory irritation).
Step 3
NOₓ — source: formed in car engines at high temperatures, where nitrogen and oxygen from the air react (N2+O2). Effect: acid rain and respiratory problems.
N2+O2→2NO
Step 4
Particulates (e.g. soot) — source: diesel exhaust / incomplete combustion. Effect: respiratory disease and they can worsen global dimming.
A common trap is matching the wrong effect to the wrong gas. CO is toxic (not acid rain); SO₂ and NOₓ cause acid rain.
4Reaction in a catalytic converter
Building confidence• catalytic converter, equation
▼
Question
Write a balanced equation for a reaction that occurs in a catalytic converter, and state how it reduces pollution.
Step-by-step solution
Step 1
A catalytic converter on a car exhaust uses precious-metal catalysts (Pt/Rh) to convert the toxic gases CO and NO into harmless gases.
Step 2
Carbon monoxide is oxidised and nitrogen monoxide is reduced in a single reaction:
2CO+2NO→2CO2+N2
Step 3
The products, CO2 and N2, are far less harmful than the toxic CO and NO that entered.
Answer
2CO+2NO→2CO2+N2 — the toxic CO and NO are converted into the less harmful CO₂ and N₂.
Examiner tip
Check the balance: 2 C, 2 N, and 4 O on each side. The catalyst speeds the reaction but is not used up, so it does not appear in the equation.
5Greenhouse gases and global warming
Stretch• greenhouse, climate change
▼
Question
Name two greenhouse gases produced by human activity and explain, in terms of radiation, how an increase in them leads to global warming.
Step-by-step solution
Step 1
Two greenhouse gases are carbon dioxide (CO2, from combustion of fossil fuels) and methane (CH4, from livestock, rice paddies and decomposition).
Step 2
Energy from the Sun reaches the Earth's surface as short-wavelength radiation; the warmed surface re-emits energy as longer-wavelength infrared (IR) radiation.
Step 3
Greenhouse gases absorb this outgoing IR radiation and re-emit some of it back towards the surface, so heat is retained in the atmosphere.
Step 4
A higher concentration of these gases traps more heat — the enhanced greenhouse effect — raising average global temperatures (global warming) and causing climate change.
Answer
CO₂ and CH₄ absorb infrared radiation re-emitted from the Earth's surface and re-radiate it back, trapping heat. Rising concentrations trap more heat, raising global temperatures (enhanced greenhouse effect) and driving climate change.
Examiner tip
Two marks-worthy ideas: gases absorb infrared radiation, and the trapped heat raises temperature. Mention the radiation is infrared (re-emitted by Earth), not the incoming sunlight.
6Evaluating methods to reduce pollutants
Stretch• Paper 4 (Extended) structured style• reduction methods, evaluation
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Question
A coal-fired power station and a busy road both produce sulfur dioxide and oxides of nitrogen. Explain one method to reduce the SO₂ from the power station and one method to reduce the gaseous pollutants from the cars.
Step-by-step solution
Step 1
Power station SO₂ — use flue-gas desulfurisation: the waste gases are passed through an alkali such as calcium oxide or calcium carbonate, which neutralises and removes the acidic SO₂ before it escapes.
Step 2
Alternatively, the station can burn low-sulfur fuel, so less sulfur is available to form SO₂ in the first place.
Step 3
Cars — fit a catalytic converter, which converts toxic CO and NOₓ into less harmful CO₂ and N₂ on a precious-metal catalyst.
2CO+2NO→2CO2+N2
Step 4
So the power station targets SO₂ at source (low-sulfur fuel) or after combustion (desulfurisation), while the car removes CO and NOₓ from the exhaust after combustion.
Answer
Power station SO₂: flue-gas desulfurisation (react SO₂ with an alkali such as CaO/CaCO₃) or burn low-sulfur fuel. Cars: a catalytic converter changes CO and NOₓ into CO₂ and N₂ (2CO+2NO→2CO2+N2).
Examiner tip
Match the method to the pollutant: desulfurisation/low-sulfur fuel for SO₂; catalytic converters for CO and NOₓ. A catalytic converter does NOT remove SO₂.
Model Answers — Air
High-scoring sample answers for air on the Cambridge IGCSE 0620 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Question 1
Paper 4 short-answer style1 mark
State the approximate percentage of carbon dioxide in clean, dry air. (1 mark)
Model answer
Approximately 0.04%.
Why this scores
One mark for ~0.04% (accept 0.03–0.04%). Do not confuse this with oxygen (21%) — CO₂ is only a trace gas.
Question 2
Paper 4 short-answer style2 marks
Name two greenhouse gases. (2 marks)
Model answer
Carbon dioxide (CO2) and methane (CH4).
Why this scores
Two marks for the two gases named in the syllabus: carbon dioxide and methane. Water vapour is also a greenhouse gas, but CO₂ and CH₄ are the human-activity gases Cambridge expects.
Question 3
Paper 4 structured style3 marks
Carbon monoxide is a pollutant gas. State how it is formed and describe one harmful effect on humans. (3 marks)
Model answer
Carbon monoxide is formed by the incomplete combustion of carbon-containing fuels, for example when fuels burn in a limited supply of oxygen in car engines. It is toxic: it binds to haemoglobin in red blood cells (more strongly than oxygen does), reducing the amount of oxygen the blood can carry around the body.
Why this scores
Three marks: (1) incomplete combustion / limited oxygen; (2) toxic; (3) binds to haemoglobin and reduces oxygen transport.
Question 4
Paper 4 structured style4 marks
Explain how a catalytic converter reduces the pollution from a car exhaust. Include a balanced equation. (4 marks)
Model answer
A catalytic converter contains a catalyst (such as platinum/rhodium) with a large surface area. The toxic gases carbon monoxide and nitrogen monoxide pass over it and react together, so the catalyst converts the toxic CO and NOₓ into the less harmful gases CO₂ and N₂. The catalyst speeds up the reaction without being used up. The reaction is:
2CO+2NO→2CO2+N2
Why this scores
Four marks: (1) toxic CO and NOₓ are removed; (2) converted to less harmful CO₂ and N₂; (3) catalyst speeds the reaction / is not used up (large surface area); (4) a correctly balanced equation.
Question 5
Paper 4 (Extended) structured style5 marks
Sulfur dioxide and oxides of nitrogen are released when fuels are burned. Explain how each gas forms and how they lead to acid rain, and state one method of reducing sulfur dioxide emissions. (5 marks)
Model answer
Sulfur dioxide (SO2) forms when fossil fuels that contain sulfur are burned and the sulfur is oxidised. Oxides of nitrogen (NOx) form in car engines at high temperatures, where nitrogen and oxygen from the air react together. Both gases dissolve in rainwater to form acidic solutions (sulfuric acid and nitric acid), producing acid rain, which damages trees, lakes, aquatic life and buildings. Sulfur dioxide emissions can be reduced by flue-gas desulfurisation — passing the waste gases through an alkali such as calcium oxide or calcium carbonate to neutralise and remove the SO₂ — or by burning low-sulfur fuels.
Why this scores
Five marks across: SO₂ from sulfur in the fuel; NOₓ from N₂ + O₂ at high temperature in engines; both dissolve in rain to form acids → acid rain; a named effect of acid rain; and a valid SO₂ reduction method (desulfurisation or low-sulfur fuel).
Question 6
Paper 4 (Extended) structured style6 marks
Explain how carbon dioxide from the combustion of fossil fuels contributes to climate change, referring to the greenhouse effect and the consequences of global warming. (6 marks)
Model answer
Burning fossil fuels for energy and transport releases carbon dioxide (CO2), which is a greenhouse gas, into the atmosphere. The Sun's radiation warms the Earth's surface, and the surface re-emits energy as infrared (long-wavelength) radiation. Carbon dioxide absorbs this outgoing infrared radiation and re-emits some of it back towards the surface, so heat is trapped in the atmosphere — the greenhouse effect. As humans burn more fossil fuels, the concentration of CO₂ rises, so more heat is trapped (the enhanced greenhouse effect), causing the average global temperature to increase — global warming. This drives climate change, with consequences such as melting polar ice and glaciers, rising sea levels and flooding of low-lying land, and more extreme or unpredictable weather.
Why this scores
Six marks: (1) combustion of fossil fuels releases CO₂; (2) CO₂ is a greenhouse gas; (3) it absorbs infrared radiation re-emitted by the Earth; (4) heat is trapped → temperatures rise; (5) more CO₂ → enhanced greenhouse effect / global warming; (6) a named consequence (e.g. melting ice / rising sea levels / extreme weather).
Key Definitions and Keywords — Air
Definitions to memorise and the exact keywords mark schemes credit for air answers — sharpened from recent examiner reports for the 2026 0620 sitting.
Composition of clean dry air
Examiner keyword
Approximately 78% nitrogen, 21% oxygen, about 1% noble gases (mainly argon) and about 0.04% carbon dioxide.
Air pollutant
Examiner keyword
A harmful substance released into the air — the key ones are carbon monoxide (CO), sulfur dioxide (SO₂), oxides of nitrogen (NOₓ) and particulates.
Acid rain
Examiner keyword
Rain made acidic by dissolved SO₂ and NOₓ, which form sulfuric and nitric acids; it damages trees, lakes, aquatic life and buildings.
Catalytic converter
Examiner keyword
A device on vehicle exhausts that uses a metal catalyst (Pt/Rh) to convert toxic CO and NOₓ into less harmful CO₂ and N₂.
Enhanced greenhouse effect
Examiner keyword
The extra trapping of infrared radiation caused by rising concentrations of greenhouse gases (CO₂ and methane), leading to global warming and climate change.
Common Mistakes and Misconceptions — Air
The traps other students keep falling into on air questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.
✕Writing the wrong percentages, e.g. 21% argon or 1% oxygen.
0620/42 — recurring
▼
Why it happens
Confusing the order of abundance of the gases.
How to avoid it
Memorise in order: N₂ ~78%, O₂ ~21%, noble gases ~1%, CO₂ ~0.04%. The four figures must add to ~100%.
✕Saying carbon monoxide causes acid rain.
▼
Why it happens
Mixing up CO with SO₂ and NOₓ.
How to avoid it
CO is TOXIC (binds to haemoglobin). SO₂ and NOₓ cause ACID RAIN.
✕Saying the catalyst in a converter is used up or appears in the equation.
▼
Why it happens
Treating the catalyst as a reactant.
How to avoid it
A catalyst SPEEDS the reaction but is REGENERATED — it is not used up and is not written in the balanced equation.
✕Saying greenhouse gases trap the Sun's incoming radiation, or that the greenhouse effect should be eliminated.
▼
Why it happens
Not distinguishing incoming sunlight from outgoing infrared, and treating the greenhouse effect as entirely bad.
How to avoid it
Greenhouse gases absorb the INFRARED radiation re-emitted BY THE EARTH. Some greenhouse effect is essential for life; the problem is the ENHANCED effect from rising CO₂ and methane.
Air — frequently asked questions
The things students keep getting wrong in this sub-topic, answered.