Air
Section: 10. Chemistry of the Environment | Syllabus: Cambridge IGCSE Chemistry (0620)
Composition of Clean, Dry Air
| Gas | Approximate percentage |
|---|---|
| Nitrogen, N₂ | 78% |
| Oxygen, O₂ | 21% |
| Noble gases (mainly argon) and carbon dioxide, CO₂ | remainder (~1%) |
Sources of Air Pollutants
| Pollutant | Source |
|---|---|
| Carbon dioxide, CO₂ | complete combustion of carbon-containing fuels |
| Carbon monoxide, CO, and particulates | incomplete combustion of carbon-containing fuels |
| Methane, CH₄ | decomposition of vegetation, and waste gases from digestion in animals |
| Oxides of nitrogen | car engines |
| Sulfur dioxide, SO₂ | combustion of fossil fuels which contain sulfur compounds |
Adverse Effects of Air Pollutants
| Pollutant | Adverse effect |
|---|---|
| Carbon dioxide | increased global warming, leading to climate change |
| Carbon monoxide | toxic gas |
| Particulates | increased risk of respiratory problems and cancer |
| Methane | increased global warming, leading to climate change |
| Oxides of nitrogen | acid rain, photochemical smog, respiratory problems |
| Sulfur dioxide | acid rain |
Worked Example
- Question: which equation describes the formation of two atmospheric pollutants that are not present in clean air - one producing only CO₂ and H₂O, one producing CO₂, CO and H₂O, one producing CO₂, C and H₂O, or one producing only CO, C and H₂O?
- Answer: the equation producing only CO, C and H₂O (no CO₂ at all). CO₂ is already a normal part of clean air, so it doesn't count as a pollutant "not present in clean air" - only CO (carbon monoxide) and C (carbon/soot particulates) qualify, and this is the only option that produces both without also producing CO₂.
Common Mistakes
- ❌ Treating carbon dioxide as if it isn't naturally present in clean air
- ✓ CO₂ already makes up about 0.04% of clean, dry air - the problem is the increased level caused by burning fuels, not its mere presence
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