What state of matter is air?

Air is a gas at ordinary temperatures and pressures.

Unlike a solid, air has no fixed shape or volume. It expands to fill whatever container holds it and can be compressed, which is why air can be pumped into a tyre or stored in a tank. Its particles are widely spaced and move freely compared with particles in liquids and solids.

Air is not a single pure substance. It is a mixture of gases, consisting mainly of nitrogen and oxygen, with smaller amounts of argon, carbon dioxide, water vapour, and other trace gases. Dry air near Earth’s surface is roughly 78% nitrogen and 21% oxygen by volume, although humidity changes the proportions.

A common confusion is treating “air” as the same thing as oxygen. Oxygen is only one major component of air. Another subtle point is that the state of matter depends on conditions: air can be cooled and compressed until its components liquefy, but in everyday classroom conditions it is correctly classified as a gas.