10.2.1Air and the Environment

Air — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 6 of 7 Unit quiz

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Part 1 of 5What clean, dry air contains

Gas	Approximate share	Comment
Nitrogen, N₂	78%	The most abundant gas and very unreactive.
Oxygen, O₂	21%	Needed for respiration and combustion.
Carbon dioxide and noble gases	Remaining 1% or so	Only trace amounts of carbon dioxide, argon and other noble gases.

Carbon dioxide

Carbon dioxide Made by the complete combustion of carbon-containing fuels, for example CH₄ + 2O₂ → CO₂ + 2H₂O.
It is a greenhouse gas, so more of it increases global warming and leads to climate change.

Carbon monoxide

Carbon monoxide Made by incomplete combustion when the oxygen supply is limited.
It is toxic because it binds to haemoglobin in red blood cells and stops them carrying oxygen round the body.

Particulates (soot)

Particulates (soot) Tiny particles of unburnt carbon are also produced by incomplete combustion, for example CH₄ + O₂ → C + 2H₂O.
Breathing them in causes respiratory problems and can contribute to cancer.

Methane

Methane Released by the digestion of farm animals, from rotting vegetation, and from bacteria in swamps, rice paddies and landfill sites.
Like carbon dioxide it is a greenhouse gas that increases global warming.

Oxides of nitrogen

Oxides of nitrogen Nitrogen and oxygen react together at very high temperatures, in car engines, furnaces and lightning strikes.
- Bacteria in soil also make some.
They cause photochemical smog, irritate the eyes, throat and lungs, and dissolve in rain to make acid rain.

Sulfur dioxide

Sulfur dioxide Formed when fuels containing sulfur compounds burn: S + O₂ → SO₂.
- Power stations are a major source.
It dissolves in rainwater to form acid rain.

Burning a hydrocarbon

Plenty of oxygen

Too little oxygen

Too little oxygen The products are carbon monoxide and water, and sometimes carbon particles as well.

Example

Example Incomplete combustion of octane can be written C₈H₁₈ + 10O₂ → 5CO + 3CO₂ + 9H₂O, which is one possible balanced version.

1Gases released

2Dissolve in rain

3Damage on the ground

Damage on the ground Metal structures corrode and limestone buildings and statues are eaten away because carbonates react with acid.

4Damage to life

Damage to life Aquatic organisms are harmed and crops and water supplies become polluted.

Term	Meaning
Air pollutant	A gas or particle in the atmosphere, usually from human activity, that harms health or the environment.
Complete combustion	Burning in plenty of oxygen to produce carbon dioxide and water.
Incomplete combustion	Burning in limited oxygen, producing carbon monoxide and sometimes soot.
Particulates	Tiny solid particles, such as carbon soot, suspended in the air.
Acid rain	Rain made acidic by dissolved sulfur dioxide and oxides of nitrogen.
Greenhouse gas	A gas such as carbon dioxide or methane that traps heat in the atmosphere.

Saying air is 80% oxygen or that carbon dioxide is a major component. Clean dry air is about 78% nitrogen and 21% oxygen.

Stating that carbon monoxide is made by complete combustion. It forms only when the oxygen supply is limited.

Writing that carbon monoxide 'stops you breathing'. It binds to haemoglobin so blood cannot carry oxygen.

Linking carbon dioxide to acid rain. Acid rain is caused by sulfur dioxide and oxides of nitrogen, whereas carbon dioxide causes global warming.

Saying oxides of nitrogen come from nitrogen in fuel. They form when nitrogen and oxygen from the air react at high temperature.

Last updated: 20 October 2026