Here's why:

* Acid-Base Reaction: The reaction between sodium bicarbonate and citric acid is an acid-base reaction. Citric acid is a weak acid, and sodium bicarbonate is a base.

* Bicarbonate Decomposition: When the acid (citric acid) reacts with the bicarbonate ion (HCO₃⁻) from sodium bicarbonate, the bicarbonate decomposes into carbon dioxide (CO₂) and water (H₂O).

* Gas Evolution: The carbon dioxide is a gas and bubbles out of the solution, causing the fizzing effect often seen in baking or in homemade soda.

The overall reaction can be summarized as follows:

3 NaHCO₃ (sodium bicarbonate) + C₆H₈O₇ (citric acid) → 3 CO₂ (carbon dioxide) + 3 H₂O (water) + Na₃C₆H₅O₇ (sodium citrate)