Baking powder

About Baking powder
Baking powder is a dry chemical leavening agent used in baking. There are several formulations; all contain an alkali, typically sodium bicarbonate (baking soda), and an acid in the form of salt crystals, together with starch to keep it dry. When dissolved in water the acid and alkali react and emit carbon dioxide gas, which expands existing bubbles to leaven the mixture. Most modern baking powders are double acting, that is, they contain two acid salts, one which reacts at room temperature, producing a rise as soon as the dough or batter is prepared, and another which reacts at a higher temperature, causing a further rise during baking. Baking powders that contain only the low-temperature acid salts are called single acting. Most recipes call for a mixing procedure that is designed to introduce many tiny air bubbles, for example, "cream the butter and sugar", which the leavening gas from baking powder will expand.