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.
Rain with pH < 5, typically caused by SO₂ and NO_x dissolving to form sulfuric and nitric acids.
Greenhouse effect
Examiner keyword
Trapping of infrared radiation in the atmosphere by greenhouse gases (CO₂, CH₄, H₂O vapour).
Catalytic converter
Examiner keyword
Device on vehicle exhausts that uses Pt/Rh catalysts to convert CO, NOₓ and unburnt fuel into CO₂, N₂ and H₂O.
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.
✕Saying CO causes acid rain
0620/42 — recurring
▼
Why it happens
Mixing CO with SO₂.
How to avoid it
CO: toxic. SO₂ and NOₓ: acid rain.
✕Saying the catalyst gets used up
▼
Why it happens
Generalising 'reactant'.
How to avoid it
Catalyst is REGENERATED — not used up.
✕Saying greenhouse effect should be eliminated
▼
Why it happens
Treating it as bad.
How to avoid it
Some greenhouse effect is essential. The PROBLEM is ENHANCED greenhouse effect from rising CO₂.
Air — frequently asked questions
The things students keep getting wrong in this sub-topic, answered.