There are certain situations when two phases of a pure substance coexist in equilibrium. As a commonly used substance water may be taken up to demonstrate the basic principles involved. However, all pure substances exhibit the same general behaviour.

We shall remember the following definitions:

Saturated State: A state at which a phase change begins or ends

Saturation Temperature: Temperature at which phase change (liquid-vapour) begins or ends at a given pressure

Saturation Pressure: It is the pressure at which phase changebegins or ends at a specified temperature.

Saturated Liquid: It is the substance at which is fully liquid (no-vapour).

Saturated Vapour: It is the substance at which is fully vapour (no-liquid).

Subcooled liquid: If the temperature of the liquid (T) is less then then the liquid is called sub-cooled liquid.

Superheated vapour: If the temperature of the vapour (T) is greater than then the vapour is called superheated vapour.