Pure substance with illustration

A pure substance is a substance that has a homogeneous and invariable chemical composition throughout its mass, in any phase or mixture of phases. It may exist in one or more phases, but each phase must have the same chemical composition.

Illustrations

• Water in any phase: liquid water, a mixture of ice and liquid water, or a mixture of liquid water and steam is a pure substance, because all phases are H₂O.
• Air (a mixture of gases) is regarded as a pure substance as long as it stays in one gas phase (composition is fixed).
• A mixture of liquid air and gaseous air is not a pure substance, since the liquid is richer in oxygen (composition differs between phases, because N₂ and O₂ condense at different temperatures).
• Oil and water, or a mixture of different gases that react, are not pure substances.

 Pure:               Not pure:
 +----------+       +----------+
 | steam    |       | gas air  |  (mostly N2)
 |----------|       |----------|
 | water    |       | liquid   |  (more O2)
 | (all H2O)|       | air      |
 +----------+       +----------+

For a simple compressible pure substance, any two independent intensive properties fix the state.