A carbonate, such as calcium carbonate (CaCO3), does not react with carbonic acid (H2CO3) because carbonic acid does not exist in a free state. When carbon dioxide (CO2) dissolves in water, it establishes an equilibrium involving CO2 molecules and water, alongside aquated protons and hydrogencarbonate ions. The concentration of hydrogen ions in this equilibrium is too low to initiate a reaction with carbonate ions, meaning the pH of dissolved CO2 is insufficiently acidic for a reaction to occur. In acid-base reactions, the acid must be strong enough to react with the base, which is why certain acids, like acetic acid, can react with hydrogencarbonate, while others, like phenols, cannot.
