Limestone in acid. Piece of limestone (calcium carbonate) reacting in a beaker of dilute hydrochloric acid. This reaction releases carbon dioxide as a gas, which causes the bubbles seen here. It also forms water and the soluble salt calcium chloride. The equation for the reaction is CaCO3 + 2HCl ---> CaCl2 + H2O mechanisms by which rainwater (which is slightly acidic) can dissolve limestone buildings and rock formations. If the rain is more strongly acidic, the effect is increased. In areas affected by acid rain, limestone masonry and statues have been badly eroded.