When an acid mixes with soda (sodium bicarbonate, NaHCO3), the gas produced is carbon dioxide (CO2).

Here's a breakdown of the reaction:

* Acid (e.g., vinegar, citric acid): The acid provides hydrogen ions (H+).

* Sodium bicarbonate (soda): This contains bicarbonate ions (HCO3-).

Reaction:

H+ (from acid) + HCO3- (from soda) → H2CO3 (carbonic acid) → H2O (water) + CO2 (carbon dioxide)

The carbonic acid (H2CO3) formed is unstable and quickly decomposes into water and carbon dioxide gas, which is what you see as bubbles.