What is air made of?
Air is a mixture of gases that surrounds the Earth. It is not a single substance, so its parts can be separated by physical methods. By volume, dry air is about 78% nitrogen, 21% oxygen, and roughly 1% other gases such as carbon dioxide, argon and water vapour.
Key idea
Air ≈ 78% nitrogen (N₂) + 21% oxygen (O₂) + about 1% other gases (CO₂, argon, water vapour). It is a mixture, not a compound.
The jobs of the main gases
Oxygen (O₂) is needed for respiration in living things and for combustion (burning). Nitrogen (N₂) is fairly unreactive; it dilutes the oxygen so that burning is not too fast, and it is used by plants to help them grow. Carbon dioxide (CO₂) is used by green plants during photosynthesis to make food.
Showing oxygen is about 21%
A simple experiment burns a candle inside a sealed jar standing over water. As the candle uses up the oxygen, the water rises to fill about one-fifth (≈ 21%) of the jar. This shows how much of the air was oxygen.
- Living things breathe in oxygen and breathe out carbon dioxide.
- Burning fuel also uses oxygen and releases carbon dioxide.
Remember
- Nitrogen is the most abundant gas in air.
- Oxygen supports respiration and combustion.
- Air is a mixture, so its gases keep their own properties.