2.1 Pure substance

A pure substance refers to a matter that has a homogeneous and definite chemical composition. A pure substance may exist in a single phase, such as liquid water, ice, and CO₂ gas. It can also exist as a multi-phase mixture, such as a mixture of liquid water and water vapour in equilibrium.

Air is a common working fluid used in many thermal devices. Can air be treated as a pure substance? Although air is a mixture consisting of roughly 78% of nitrogen, 21% of oxygen, and trace amounts of other gases and moisture, air is homogeneous and has distinct properties without phase changes in its composition in many thermodynamic processes. In such conditions, we can treat air as a pure substance.