Final answer:
When citric acid is mixed with baking soda, hydrogen gas is released as a result of an acid-base reaction. This reaction produces carbon dioxide gas, water, and a salt.
Step-by-step explanation:
When citric acid is mixed with baking soda, a chemical reaction occurs that releases hydrogen gas. This reaction is an example of an acid-base reaction. Citric acid, which is an acid, donates a hydrogen ion (H+) to the baking soda, which acts as a base. The result is the formation of carbon dioxide gas, water, and a salt.