Topic 1 of 3
What is in air, and where do pollutants come from?
Separate the normal mixture from substances causing harm.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
What is in air, and where do pollutants come from?
Separate the normal mixture from substances causing harm.
Dry air contains about 78% nitrogen and 21% oxygen by volume. Most of the remaining 1% is noble gases, mainly argon, with carbon dioxide and other trace gases. Dry air excludes water vapour; the water-vapour content of actual air varies.
| Substance | Source or formation | Main concern |
|---|---|---|
| Carbon monoxide, CO | Incomplete combustion of carbon-containing fuels when oxygen supply is insufficient | Toxic; reduces oxygen transport in blood |
| Nitrogen oxides, NO and NO2 | High-temperature reactions involving air during lightning and in internal combustion engines | Respiratory irritation and acid rain |
| Sulfur dioxide, SO2 | Volcanoes and burning fossil fuels containing sulfur compounds | Respiratory irritation and acid rain |
| Methane, CH4 | Livestock, decomposition without oxygen, and natural-gas leakage | Greenhouse gas |
| Ozone, O3 | Can form near ground level through sunlight-driven reactions involving other pollutants | Harmful to respiration at ground level |
| Unburned hydrocarbons | Fuel that escapes combustion, including engine exhaust and evaporation | Contribute to photochemical air pollution |
Distinguish CO from CO2. Both contain carbon and oxygen, but CO is a toxic incomplete-combustion product. Complete combustion produces CO2, which is important in the carbon cycle and greenhouse effect. A fuel containing no sulfur cannot directly supply sulfur dioxide simply by burning; identify which elements the source provides.
Check your understandingA sample of humid air contains water vapour. Should the quoted 78% nitrogen and 21% oxygen be applied without qualification?Think it through, then reveal the answer
Those figures refer approximately to dry air. Adding water vapour changes the proportions of the other gases in the whole humid sample.