The reaction of an acid with calcium carbonate is an example of an acid-base reaction. In these reactions, an acid will react with a base to form a salt and water. The acid was neutralized by the base.

There are really two steps in the reaction of a carbonate (or a bicarbonate) with an acid. The first step is the acid-base reaction, which forms carbonic acid as a product. Carbonic acid is unstable and tends to form carbon dioxide and water. That is one of the reasons that the lowering of the pH in rainwater by dissolved carbon dioxide is not very significant.

CaCO3   +   2 H+      Ca+2   +   H2CO3

 H2CO3      CO2   +   H2O