Equilibrium in a system having two (or) more phases is known as heterogeneous equilibrium. Heterogeneous equilibria often involve pure solids (or) liquids.

Consider the thermal decomposition of calcium carbonate in a closed vessel, the equilibrium constant for this reaction can be written as

CaCO_3(s) $\rightleftharpoons$${\text{CaO}}_{\text{(S)}}$$+$${\text{CO}}_{\text{2}}$$$

K_c = $\frac{\text{[CaO][}{\text{CO}}_{\text{2}}\text{]}}{\text{[}{\text{CaCO}}_{\text{3}}\text{]}}$

Since calcium carbonate and calcium oxide are solids, their concentrations remain constant, and hence, their concentration terms in the expression for equilibrium constant can be ignored.

K'c = [CO₂] or K_p = pCO₂

It shows that at a particular temperature, irrespective of the masses of the two solids Calcium oxide and Calcium carbonate, there is a constant concentration or pressure of Carbon dioxide in equilibrium with calcium oxide and calcium carbonate.

Characteristics of equilibrium:

• The value of equilibrium constant is independent of the initial concentrations of the reactants and products.
• The equilibrium constant for a particular reaction at a particular temperature is always constant and changes only with a change in temperature.
• The equilibrium constants of a forward reaction and its backward reaction are reciprocals of each other.
• If a chemical equation is multiplied by a certain factor, then its equilibrium constant must be raised to a power equal to that factor in order to obtain the equilibrium constant for the new reaction.