Equilibrium is that state in which the rate of forward reaction equal to rate of backward reaction.

The rate of changes of concentration of the reactant become equal to that of product state is known as equilibrium state.

Equilibrium are of two types :-

1. Physical equilibrium:- The equilibrium attained in physical process are known as physical equilibrium.
2. Chemical Equilibrium:- The equilibrium stage involving chemical changes is known as chemical equilibrium.