Baking powder is a dry chemical leavening agent used to increase the volume and lighten the texture of baked goods such as muffins, cakes, scones and American-style biscuits. Baking powder works by releasing carbon dioxide gas into a batter or dough through an acid-base reaction, causing bubbles in the wet mixture to expand and thus leavening the mixture. It is used instead of yeast for end-products where fermentation flavors would be undesirable or where the batter lacks the elastic structure to hold gas bubbles for more than a few minutes. Because carbon dioxide is released at a faster rate through the acid-base reaction than through fermentation, breads made by chemical leavening are called quick breads. Most commercially-available baking powders are made up of an alkaline component (typically sodium bicarbonate, also known as baking soda), one or more acid salts(like Tartaric Acid), and an inert starch (cornstarch in most cases, though potato starch may also be used).
Contributions by Lou Sander, BW95, and Mdwyer.