Dissolution reactions are reactions that occur when a solute either in gaseous, liquid, or solid form dissolves in a solvent such as water to form a solution.
In this case we are given Calcium carbonate (CaCO3) which undergoes dissolution according to the equation;
CaCO3(aq) → Ca2+(aq ) + CO3 2-(aq)
Then the bicarbonate ion combines with two protons from water to form a weak acid H2CO3. The weak acid is then broken down to form CO2 and H2O since its unstable.