Reaction rates. Image 2 of 2. Small pieces of limestone (calcium carbonate, CaCO3) reacting fast with dilute acid to produce carbon dioxide gas bubbles. Here, the large limestone surface exposed to the acid causes a fast 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) & water (H2O): 1) CaCO3 + 2H(+) ----> Ca(2+) + H2CO3 2) H2CO3 ----> CO2 + H2O For a slow reaction rate, see image A500/297.