Baking soda, or sodium bicarbonate, is an alkaline or "base" ingredient. Each molecule of baking soda contains a sodium atom, a hydrogen atom, an oxygen atom, and a carbon dioxide molecule. Its chemical formula is NaHCO₃.

When baking soda is combined with an acidic ingredient, it reacts to form bubbles of carbon dioxide. So, acidic ingredients react to baking soda. When you mix an acid and a base in a solvent, which in this case is the water, usually the acid transfers hydrogen to the base to form a new compound. In this case, the new compound is called carbonic acid, which then decomposes to water and carbon dioxide gas. (A more precise definition of acids and bases states that a base is a compound that wants to donate electrons, and an acid is a compound that wants to accept electrons.)