The gas produced when an acidic solution reacts with magnesium carbonate is carbon dioxide (CO₂).

Here's the chemical reaction:

MgCO₃ (s) + 2H⁺ (aq) → Mg²⁺ (aq) + CO₂ (g) + H₂O (l)

* MgCO₃ is magnesium carbonate (solid)

* H⁺ represents the hydrogen ions from the acid (aqueous)

* Mg²⁺ is magnesium ions (aqueous)

* CO₂ is carbon dioxide gas

* H₂O is water (liquid)

The reaction produces carbon dioxide gas, which is why you would see bubbles forming in the solution.