For a heterogeneous equilibrium involving pure solids and gases, the concentration or partial pressure of pure solids is taken as constant (or unity) . The given reaction is: $\text{MgCO}_3(s) \rightleftharpoons \text{MgO}(s) + \text{CO}_2(g)$ Since $\text{MgCO}_3$ and $\text{MgO}$ are pure solids, their partial pressures are not included in the equilibrium constant expression. Therefore, $K_p = P_{\text{CO}_2}$ .