Equilibrium in Chemical process:-

Let us take some CaCO₃ in a vessel & heated upto 1073K, we observe that level of mercury in right limb increase first, then become constant at constant temp. It does not mean that decomposition of CaCO₃ stops, but  rate of decomposition = rate of formation of CaCO₃ & the state is known as equilibrium state.

Law of Mass Action:-

It states that rate of a chemical reaction is directly proportional to the product of the molar concentration of the reactant at a constant temperature at any given time.

For example:- A + B = Product

Rate ∝ [A][B]

                Rate = k [A][B]

                aA + bB= Product                                             Rate = k [A]^a[B]^b

Hence, law of mass action may also be state that rate of a chemical reaction at any particular temperature is proportional to the product of the molar concentration of the reactant with each concentration term raised to the power of  no. of molecules of respective reactant taking part in the reaction.