phase rule For a heterogeneous system in equilibrium, the number of phases, P the number of degrees of freedom, F and the number of chemical components, C are related as,

P+F=C+2

The number of physically and chemically homogeneous, physically distinct and mechanically separate portions of a system are called phases. A homogeneous mixture of gases or liquids or liquid solution constitutes a single phase.

The number of degrees of freedom of system is defined as the number of variable factors such as temperature, pressure and relative concentration of the components which can be altered without destroying the nature of equilibrium of the system.

The number of independently variable chemical species (elements and compounds) by means of which composition of each phase can be completely specified is the number of chemical components.