For the reaction $\mathrm{CaCO_3(s)} \rightleftharpoons \mathrm{CaO(s)} + \mathrm{CO_2(g)}$ at equilibrium, the equilibrium constant expression $K_c$ is

1. NCERT §6.5 (Heterogeneous equilibria): for a heterogeneous reaction, the activities of PURE solids and PURE liquids are taken as 1 (their molar concentrations are essentially constant — they don't appear in $K$). 2. In this reaction, $\mathrm{CaCO_3}$ and $\mathrm{CaO}$ are PURE SOLIDS. Their concentrations are constants absorbed into $K_c$. 3. Only $\mathrm{CO_2(g)}$ appears explicitly. So $K_c = [\mathrm{CO_2}]$. 4. Equivalently in pressure form: $K_p = p_{\mathrm{CO_2}}$, which is why the equilibrium $\mathrm{CO_2}$ pressure above heated limestone depends only on temperature. 5. Options A and D would be the (incorrect) expression for a homogeneous reaction. Option B includes a solid that shouldn't appear. _Source: NCERT Class 11 Chemistry, Ch 6, §6.5 (Heterogeneous Equilibria — $\mathrm{CaCO_3}$ decomposition example), p. 13–14._