Limestone reacting with acid. Pipette being used to add dilute acid to a lump of limestone (calcium carbonate, CaCO3). Bubbles of carbon dioxide gas are being produced in the resulting reaction. This is an acid-base reaction, where calcium carbonate (the base) reacts with the acidic hydrogen (H+) to form aqueous calcium ions & carbonic acid (H2CO3). This acid is unstable, and rapidly decomposes to form carbon dioxide (CO2) and water (H2O). This is how rainwater erodes limestone. The equations for this reaction are: 1) CaCO3 + 2H(+) ----> Ca(2+) + H2CO3 2) H2CO3 ----> CO2 + H2O