Earth and atmosphere · GCSE Chemistry
Atmospheric pollutants
GCSE Chemistry revision on atmospheric pollutants: complete and incomplete combustion, carbon monoxide, soot, sulfur dioxide, oxides of nitrogen and acid rain.
Complete combustion: CO₂ and H₂O. Incomplete: CO (toxic) and C (soot). Sulfur in fuel → SO₂ → acid rain. Hot engines: N₂ + O₂ → NOₓ, also acid rain and breathing problems.
The important bits
What you need to know
- 1
Complete combustion of a hydrocarbon in plenty of oxygen: hydrocarbon + O₂ → CO₂ + H₂O. Example: CH₄ + 2O₂ → CO₂ + 2H₂O. A blue bunsen flame is the school picture.
- 2
Incomplete combustion in limited oxygen produces carbon monoxide, C (particulates/soot), and still some water. Less energy is released than in complete combustion. Yellow sooty flame.
- 3
Carbon monoxide is colourless, odourless and toxic: it binds to haemoglobin and reduces oxygen transport in the blood. Blocked boiler flues are the safety story.
- 4
Particulates (soot, unburnt carbon) cause respiratory problems and reflect sunlight, contributing to global dimming. They are not the same as the greenhouse effect.
- 5
Sulfur impurities in some fossil fuels burn to sulfur dioxide: S + O₂ → SO₂. SO₂ oxidises and dissolves to form acid rain (sulfuric acid). Wet scrubbers and low-sulfur fuels reduce it.
- 6
Oxides of nitrogen (NO and NO₂, called NOₓ) form when nitrogen and oxygen from the air react at the high temperatures in engines: N₂ + O₂ → 2NO. They also cause acid rain and breathing difficulties.
- 7
Acid rain damages limestone (CaCO₃ + acid → salt + water + CO₂), lakes, trees and metals. Catalytic converters, cleaner fuels and burning less fuel are linked solutions.
- 8
Catalytic converters on cars convert CO, NOₓ and unburnt hydrocarbons into CO₂, N₂ and H₂O. They do not remove CO₂ from complete combustion, so they are not a climate-neutral device.
Quotations worth analysing
Short evidence. Real method.
“CH₄ + 2O₂ → CO₂ + 2H₂O (complete) 2CH₄ + 3O₂ → 2CO + 4H₂O (incomplete example)”
Same fuel, different oxygen supply, different carbon product. Incomplete equations vary (CO or C); the idea is carbon is not fully oxidised to CO₂.
“S + O₂ → SO₂ then acid rain”
The sulfur was in the fuel, not in the air. Nitrogen oxides are the opposite: the nitrogen came from the air, not from the fuel.
“CO binds to haemoglobin; CO₂ is a greenhouse gas.”
Carbon monoxide kills by starving tissues of oxygen. Carbon dioxide changes climate. Different molecules, different harms. Both can come from burning carbon fuels.
Go deeper
Name the molecule and a specific harm — never “fumes”
Examiners award marks for CO — toxic / haemoglobin; particulates — lungs / dimming; SO₂ — acid rain / limestone / lakes; NOₓ — acid rain / respiratory irritation. A sentence that only says “pollution is bad for the environment” scores zero. Incomplete combustion also wastes energy, which is a resource point as well as a health point. A well-serviced boiler with a clear flue and a blue flame is complete combustion: you still make CO₂, so you have traded a toxic gas for a greenhouse gas. That honesty is good Chemistry. Global dimming from particulates can slightly mask warming; cleaning the air without cutting CO₂ can reveal more of the greenhouse warming. You only need that extra if the paper goes there.
Go deeper
Sulfur versus nitrogen: different sources, similar acid rain
Sulfur starts in the fuel. Remove it at the refinery (low-sulfur diesel) or scrub SO₂ from power-station flue gases with calcium carbonate or calcium oxide, making gypsum. Nitrogen starts in the air. You cannot take N₂ out of the engine intake easily, so you reduce peak temperature, use catalytic converters (2NO + 2CO → N₂ + 2CO₂ is a typical conversion), or drive less. Both SO₂ and NO₂ dissolve to make acids. Acid rain on limestone is CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g) — the same carbonate reaction as on the acids page. Link topics; do not treat atmosphere as a separate subject.
Go deeper
Catalytic converters are surface chemistry you already know
Platinum, palladium and rhodium provide an alternative pathway with lower activation energy. Hot exhaust gases adsorb, bonds rearrange, products leave. CO is oxidised; NOₓ is reduced to nitrogen. Unburnt hydrocarbons are oxidised to CO₂ and H₂O. The converter does not absorb carbon dioxide. It also needs to be hot, which is why short journeys pollute more per mile. Lead in old petrol poisoned the catalyst — another reason for unleaded fuel. This is catalysts plus pollutants in one applied paragraph, which is how six-marks are built. Name the metals if the question asks for an example of a catalyst.
See the idea in action
A gas fire with a blocked flue can kill even though methane is “clean”. Incomplete combustion: 2CH₄ + 3O₂ → 2CO + 4H₂O. CO is colourless and odourless and binds to haemoglobin. In a good oxygen supply the same methane makes CO₂ and water, which is not toxic in the same way but is a greenhouse gas. Separately, sulfur in some heating oils makes SO₂ and acid rain. Reducing harm means ventilation and complete combustion for CO, low-sulfur fuel and scrubbers for SO₂, converters for NOₓ, and burning less fuel for CO₂.
Exam technique
Turn knowledge into marks
For each pollutant write formula, how it forms, and one specific effect. Contrast complete and incomplete combustion with equations. Say NOₓ comes from air in hot engines, SO₂ from sulfur in the fuel.
Common mistakes
Do not give these marks away
- 01
Mixing up carbon monoxide toxicity with carbon dioxide’s greenhouse effect.
- 02
Saying sulfur dioxide comes from nitrogen in the air, or that NOₓ comes from sulfur in petrol.
- 03
Claiming catalytic converters remove carbon dioxide, or that incomplete combustion is endothermic.
Which pollutant from incomplete combustion is toxic because it binds to haemoglobin?
ACarbon dioxide
BCarbon monoxide
CSulfur dioxide
DWater vapour
Show the answer
Carbon monoxide. CO reduces oxygen transport in the blood. CO₂ is a greenhouse gas from complete combustion. SO₂ comes from sulfur in fuels and causes acid rain.
Quick questions
If this is the bit you searched
What is the difference between complete and incomplete combustion GCSE?
Complete combustion in plenty of oxygen gives carbon dioxide and water. Incomplete combustion gives carbon monoxide and/or carbon (soot) as well, and releases less energy.
How is acid rain formed?
Sulfur dioxide from sulfur in fuels, and oxides of nitrogen from hot engines, dissolve in clouds to make acids that fall as acid rain.
Why is carbon monoxide dangerous?
It is colourless and odourless and binds to haemoglobin more strongly than oxygen, so less oxygen is carried in the blood.
How do catalytic converters reduce pollution?
They catalyse reactions that turn carbon monoxide, oxides of nitrogen and unburnt hydrocarbons into carbon dioxide, nitrogen and water. They do not remove CO₂ from complete combustion.