Air is a mixture of gases. About 78% is nitrogen and about 21% is oxygen; the last ~1% includes carbon dioxide, argon and water vapour.
Oxygen supports burning and breathing. Carbon dioxide turns limewater cloudy and is used by plants to make food. Polluted air harms health and the environment.
Remember
- Air ≈ 78% nitrogen + 21% oxygen + ~1% other gases.
- Oxygen supports burning; plants use carbon dioxide.