A pure substance has a uniform chemical composition even when more than one phase is present. Liquid water and water vapor together are still a pure substance.

On a pressure-temperature diagram, solid, liquid and vapor occupy distinct regions separated by coexistence curves. Along a liquid-vapor coexistence curve, pressure and temperature are not independent: each saturation temperature has a corresponding saturation pressure.

The coexistence curves meet at the triple point. The liquid-vapor curve ends at the critical point. Above the critical point there is no sharp boundary between liquid and vapor.

At a fixed pressure below critical, a substance can be a compressed or subcooled liquid, a saturated liquid-vapor mixture, or a superheated vapor depending on temperature.

For example, if water at a given pressure has saturation temperature $T_{sat}$, then a state with $T<T_{sat}$ is liquid-like, a state with $T=T_{sat}$ may be any saturated mixture, and a state with $T>T_{sat}$ is superheated vapor.

Phase diagrams therefore tell you which property models and tables are appropriate before any energy balance is attempted.