When baking soda (sodium bicarbonate) reacts with a citrus acid (such as citric acid found in lemons, oranges, and grapefruits), it undergoes a chemical reaction known as an acid-base reaction. This reaction produces carbon dioxide gas, which is responsible for the fizzing and bubbling observed when baking soda and citrus are mixed.

The overall chemical equation for this reaction is:

NaHCO3 (sodium bicarbonate) + H+ (from citrus acid) -> CO2 (carbon dioxide gas) + H2O (water) + Na+ (sodium ion)

Here's a step-by-step explanation of what happens during this reaction:

1. When baking soda (sodium bicarbonate) is dissolved in water, it dissociates into sodium ions (Na+) and bicarbonate ions (HCO3-).

2. Citric acid, when present, donates hydrogen ions (H+) to the solution, forming hydronium ions (H3O+).

3. The hydrogen ions (H+) from the citrus acid react with the bicarbonate ions (HCO3-) from baking soda to form carbonic acid (H2CO3).

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

5. The carbon dioxide gas produced during this reaction is what causes the fizzing and bubbling observed when baking soda and citrus are mixed.

This reaction is often used in baking to create a leavening effect, where the carbon dioxide gas produced helps to make baked goods rise. It is also used in cleaning products and deodorizers, where the fizzing action can help to remove dirt and neutralize odors.