The equilibrium constant expression only includes species whose concentrations or partial pressures can change. Pure solids and pure liquids have constant activities (= 1) and are excluded. For CaCO3(s) <=> CaO(s) + CO2(g), Kp = P(CO2) only. Aqueous ions and dissolved species are included. For heterogeneous equilibria, focus only on (aq) and (g) species. When a reaction is reversed, K becomes 1/K. When multiplied by n, K becomes K^n. When two reactions are added, the new K = K1 * K2. Kp = Kc(RT)^delta_n, where delta_n = moles of gaseous products - moles of gaseous reactants. If delta_n = 0, Kp = Kc (e.g., H2 + I2 <=> 2HI).