Baking soda has only one ingredient, sodium bicarbonate. Because sodium bicarbonate reacts chemically when combined with moisture and an acid, this baking staple serves as a leavening agent, giving baked goods their light and airy textures.
When pure sodium bicarbonate comes in contact with a moist and acidic ingredient, such as buttermilk or vinegar, a chemical reaction occurs. The chemical reaction results in the release of carbon dioxide. This carbon dioxide comes in the form of bubbles that become trapped in the batter. As the trapped bubbles build up, the mixture expands and becomes lighter and airier. This chemical reaction occurs immediately upon mixture of the components, so baked goods that depend on this chemical reaction must go into the oven immediately.
Another baking staple, baking powder, is often confused with baking soda, as it is similar in form and function. Baking power also contains sodium bicarbonate, but it includes other ingredients as well. Baking powder typically contains cream of tartar and a drying agent, most commonly starch. The addition of these two elements to the sodium bicarbonate modifies the speed with which the chemical reaction occurs. This allows for more control of the leavening process and, in some cases, a delay in baking.