The Phase Rule is given by Willard Gibbs. It is defined as, in heterogenous systems, if equilibrium between phases are not influenced by gravity, magnetic & electrical forces, but are influenced only by pressure, temperature and concentration, then the number of degree of freedom (F) of the system is related to number of components (C) and number of phases(P) by the following phase rule equation.

F = C-P+2

Limitations of phase rule:

1. As the phase rule is applicable to heterogeneous systems in equilibrium, it is therefore of no use for such systems which are slow in reaching the equilibrium state.
2. As the phase rule is applicable to a single equilibrium state, it never tells about the number of other equilibrium possible in the system.
3. In phase rule, various variables are temperature, pressure and composition. This phase rule does not consider the electric or magnetic influences. If such variables are considered, the factor 2 of the phase rule has to be adjusted accordingly.
4. All the phases in the system must be present under the same pressure, temperature and gravitational force.
5. No liquid or solid phases should be finely divided otherwise their vapour pressures will differ from their normal values.