The position of the equilibrium CaCO3(s) ⇌ CaO(s) + CO2(g) does not depend on the amounts of CaCO3(s) and CaO(s) present.If pure solids or pure liquids are involved in a chemical reaction, their concentrations are not included in the equilibrium expression for the reaction.This simplification occurs only with pure solids or liquids, not with solutions or gases, since in these last two cases the concentrations can vary.