Here are two substances that react with an acid to produce carbon dioxide gas:

1. Metal carbonates: When a metal carbonate (like calcium carbonate, CaCO₃) reacts with an acid (like hydrochloric acid, HCl), it produces carbon dioxide gas, a salt, and water.

* Example:

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

2. Bicarbonates (hydrogen carbonates): Bicarbonates (like sodium bicarbonate, NaHCO₃) also react with acids to release carbon dioxide.

* Example:

NaHCO₃ (s) + HCl (aq) → NaCl (aq) + H₂O (l) + CO₂ (g)

Key Principle: These reactions are examples of acid-base reactions. The acid (H⁺ donor) reacts with the carbonate or bicarbonate (base) to form carbonic acid (H₂CO₃), which is unstable and decomposes into carbon dioxide and water.