What clean air is made of
Dry air is roughly 78% nitrogen, 21% oxygen, about 1% argon, and a small but crucial 0.04% carbon dioxide, plus traces of other gases and variable water vapour. That tiny slice of CO₂ does an outsized amount of work — for plants, and for the climate.
Air pollutants and where they come from
Burning fuels — mostly in vehicles and power stations — adds gases that clean air does not contain:
| Pollutant | Source | Problem it causes |
|---|---|---|
| Carbon monoxide (CO) | incomplete combustion of fuels | toxic — binds to blood, stops it carrying oxygen |
| Sulfur dioxide (SO₂) | burning sulfur-containing fuels | acid rain |
| Oxides of nitrogen (NOₓ) | high-temperature combustion in engines | acid rain and smog |
| Particulates (soot) | incomplete combustion, diesel | respiratory disease, dirties surfaces |
The greenhouse effect
Some gases — chiefly carbon dioxide and methane — let sunlight in but trap the heat the Earth radiates back, warming the surface. A natural amount of this keeps the planet habitable; the problem is the extra we add.
Burning fossil fuels (coal, oil, gas) releases CO₂ faster than it is removed, raising its concentration and enhancing the greenhouse effect. The consequences — a warming climate, shifting weather, rising seas — are why cutting fossil-fuel use and its emissions matters.
Water
Water for drinking is made safe by treatment: filtration through beds of sand and gravel to remove solids, then chlorination to kill bacteria (and sometimes other steps). It matters because untreated water spreads disease.
Water is also a fine solvent, which is why pollution dissolves and spreads in it — fertiliser run-off, industrial waste and sewage all damage rivers and the life in them.
Test for water
Anhydrous copper(II) sulfate turns from white to blue in the presence of water, and cobalt chloride paper turns from blue to pink. To confirm it is pure water, check it boils at 100 °C and freezes at 0 °C.
A worked example
A car engine emits carbon monoxide and oxides of nitrogen. Name the problem each causes, and what reduces them.
- Carbon monoxide comes from incomplete combustion and is toxic — it stops the blood carrying oxygen.CO binds to haemoglobin more strongly than oxygen.
- Oxides of nitrogen form in the hot engine and cause acid rain and smog.They dissolve in atmospheric water to form acids.
- A catalytic converter reduces both, turning CO and NOₓ into CO₂ and N₂.It catalyses reactions that convert the pollutants to harmless gases.
CO is toxic (blocks oxygen transport); NOₓ causes acid rain and smog; a catalytic converter changes both into CO₂ and N₂.
How pollutants form, and what they do
Most air pollution comes from burning fuels. Carbon monoxide (a toxic gas that stops blood carrying oxygen) forms from incomplete combustion where oxygen is limited. Sulfur dioxide comes from sulfur impurities in fuels, and oxides of nitrogen form when the intense heat of an engine makes nitrogen and oxygen from the air combine.
Sulfur dioxide and nitrogen oxides dissolve in rain to make acid rain, which kills trees and fish, and corrodes limestone buildings and metals. Soot (unburnt carbon) blackens buildings and harms lungs.
Reducing the impact
A catalytic converter in a car's exhaust turns pollutants into harmless gases — for example 2CO + O₂ → 2CO₂, and it reduces nitrogen oxides back to nitrogen. Power stations remove sulfur dioxide by flue-gas desulfurisation with calcium oxide or carbonate.
Carbon dioxide is different — it is not toxic, but as a greenhouse gas it traps outgoing heat and drives climate change: rising sea levels, more extreme weather and shifting habitats. Cutting it means burning fewer fossil fuels, using renewable energy and improving efficiency.
Greenhouse gases trap heat
Gases like CO₂ and methane let the Sun's energy in but absorb and re-emit the heat the Earth radiates back out. More of them means more trapped heat — a warmer planet.
Treating and testing water
Making water safe to drink takes several stages: sedimentation and filtration to remove solids, then chlorination to kill microbes. This makes it potable, but not chemically pure — dissolved salts remain.
To test for water, add anhydrous copper(II) sulfate (white → blue) or blue cobalt chloride paper (→ pink). To show water is pure, check that it boils at exactly 100 °C and freezes at 0 °C — dissolved substances would shift those points.