Marble reacting with an acid. Carbon dioxide gas being produced in a test tube from the reaction of marble with hydrochloric acid. The main component of marble is calcium carbonate. The other products of this reaction are water and calcium chloride. The chemical equation for the reaction is: CaCO3 + 2.HCl => CaCl2 + H2O + CO2. This is the same mechanism by which acid rain erodes statues, and by which weakly acidifed rain erodes limestone rocks to form caves and other structures in limestone landcapes.