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A pure substance is one that has a homogeneous and invariable chemical composition.
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It may exist in more than one phase, but the chemical composition is the same in all phases. Thus, liquid water, a mixture of liquid water and water vapor, and a mixture of ice and liquid water are all pure substances; every phase has the same chemical composition. In constrast, a mixture of liquid air and gaseous air is not a pure substance because the composition of the liquid phase is different from that of the vapor phase.
Sometimes a mixture of gases, such as air, is considered a pure substance as long as there is no change of phase. Strictly speaking, this is not true. A mixture of gases such as air exhibits some of the characteristics of a pure substance as long as there is no change of phase.
The behavior of a pure substance depends on the absolute pressure.