When CaCO3 is dissolved in water, it dissociated to give Ca2+ as well as CO3 2-. The CO3 2- will eventually gains two protons from water and forms a very weak Carbonic Acid (H2CO3). This acid is highly unstable and quickly dissociated to form Water (H2O) as well as Carbon Di-Oxide (CO2). The CO2 is evolved out in the form of bubble from water.
