Baking powder is a raising agent that is commonly used in cake-making. It is made from an alkali, bicarbonate of soda, and an acid, cream of tartar, plus a filler like cornflour or rice flour which absorbs moisture.
The powder is activated when liquid is added, producing carbon dioxide and forming bubbles that cause the mixture to expand. For this reason, it is important to get your cake mixture into the oven quickly once the ‘wet’ ingredients have been added to the ‘dry’ ingredients.
Baking powder contains baking soda. It is a mixture of baking soda, cream of tartar (a dry acid), and sometimes cornstarch. Since baking powder already contains an acid to neutralize its baking soda, it is most often used when a recipe does not call for an additional acidic ingredient. Like my sugar cookies. However, this isn’t always the case. You can still use baking powder as the leavening agent in recipes calling for an acidic ingredient.
Another reason to use both baking powder and baking soda is because they affect both browning and flavor. Fine Cooking breaks it down easily.