In heterogeneous equilibria, the reactions often involve pure solids or pure liquids. The molar concentration of a pure solid or a pure liquid is constant at a given temperature, which means it is independent of the amount present.

If a substance 'X' is involved in the equilibrium, its concentrations $[X(\text{s})]$ and $[X(\text{l})]$ remain constant, whatever the amount of 'X' is taken.

Since these concentrations are constant, they do not change the equilibrium state and are effectively incorporated into the equilibrium constant itself. Therefore, pure liquids and solids can be ignored while writing the equilibrium constant expression.