While lots of of other things— like air, steam, and yeast— also help give baked goods their lift, today we’re going to focus on chemical leaveners. Specifically, baking soda and baking powder. Both, when used correctly in a recipe, produce carbon dioxide within the structure of a cake or cookie and causes rise.
Carbon dioxide is produced in baked goods when an acid and a base come together with water. Many baking ingredients are naturally acidic— like citrus, buttermilk, yogurt, sour cream, vinegar, molasses, honey, chocolate, etc. Baking soda (aka sodium bicarbonate) is a base and reacts with those ingredients to leaven your baked goods.
If carbon dioxide needs both an acid and base, why not just create a leavener with both? Well, that’s how we got baking powder. Baking powder contains baking soda, an acid like cream of tartar, and cornstarch to absorb moisture in the air.
A baking powder made from strictly baking soda, cream of tartar and cornstarch is a “single-acting” baking powder. Which means that it gives off carbon dioxide just once when mixed with a liquid. Most baking powders are “double-acting”, meaning that they also contain another acid that reacts when heated and give baked goods another lift in the oven.
Recipes that call for both baking soda and baking powder have some acid, but not enough to react with the baking soda and so baking powder is used to pick up the slack.