Law of mass action is applicable to

a)     Chemical reactions [Reversible]

b)     Chemical reactions [Irreversible]

c) Homogeneous  chemical equilibria and heterogeneous chemical equilibria

d)    Physical equilibria

Law of mass action states that the rate of  a reaction is directly propotional to product of active masses of reactants.

 in any physical process, physical properties changes

Thus law of mass action cannot be applied to a physical process.

Law of mass action can be applied to any chemical process which may be reversible (or) irreversible. It can be Homogeneous (or) Heterogeneous.