Baking powder is made through the combination of an acid, a base and a filler. Baking soda, cream of tartar and corn starch are a common base, acid and filler combination, respectively, used to make baking powder.
Baking powder is often used in recipes as a chemical leavening agent. Chemical leavening agents release carbon dioxide air pockets that cause dough to stretch. Producing these bubbles is important in baking because a greater number of bubbles released means a fluffier product.
Baking powder can be single-acting or double-acting. Single-acting baking powder releases carbon dioxide bubbles only once. This baking powder contains cream of tartar as its base and releases carbon dioxide only when dough becomes wet. Double-acting baking powder contains sodium aluminum sulfate or calcium acid phosphate instead of cream of tartar as its base, and it releases carbon dioxide bubbles twice. It releases them the first time when mixed into dough and again when heated. Because baking powder begins to release carbon dioxide bubbles when it becomes wet, many recipes suggest adding baking powder last, which keeps baking powder from reacting until the end of the mixing process. Another suggestion made in many recipes with baking powder is to mix lightly, which helps to prevent the escape of carbon dioxide. Mixing thoroughly can be problematic because it may bring the reaction to completion, letting all the bubbles escape.