1. It is a statement given by J. Willard Gibbs which explains the
behaviour of a heterogeneous system at equilibrium under the changes in
the variables such as temperature, pressure and concentration.
2. It is only applicable to macroscopic(large) systems.
3. It can be applied to both physical and chemical equilibrium systems.
4. It predicts the appearance and disappearance of various substances
present in the equilibrium under different sets of variable.
Before discussing the phase rule and its applications to various equilibria, it is
desirable to define and explain the various terms contained in it.
Explanation of different terms:
It is a part of the universe which on under thermodynamic study.
Types of system
It is of two types : (1) Homogeneous system (2) Heterogeneous system
Homogeneous system: The systems in which the physical and chemical
properties are same throughout are called Homogeneous systems.
Example-all pure substances, salt solutions, acid solutions, etc.
Heterogeneous system: The systems in which the physical and chemical
properties are not same throughout are called heterogeneous systems.
Example-oil in water,benzene in water,decomposition of CaCO3 solid,etc.
Phase of a System
It is any homogeneous and physically distinct segment of a system which remains
bounded by a surface and is mechanically separable from other part of the
system. It is represented by ‘P’
1. Pure Substance: It forms a single phase.i.e P=1
2. Mixture of gases: It forms a single phase. i.e P=1
3. Miscible liquids: It forms a single phase. i.e P=1