Here's why:

* Carbonates contain the carbonate ion (CO₃²⁻).

* Acids contain hydrogen ions (H⁺).

* When an acid and a carbonate react, the hydrogen ions from the acid react with the carbonate ions. This forms carbonic acid (H₂CO₃).

* Carbonic acid is unstable and quickly decomposes into water (H₂O) and carbon dioxide (CO₂).

The general reaction can be represented as:

Acid + Carbonate → Salt + Water + Carbon Dioxide

Example:

Hydrochloric acid (HCl) + Calcium carbonate (CaCO₃) → Calcium chloride (CaCl₂) + Water (H₂O) + Carbon dioxide (CO₂)