Baking powder is a leavening agent that is added to batter to cause it to rise when baked. It neutralizes acids in the recipe while adding tenderness. Baking powder typically contains baking soda, acid salts and cornstarch.
Baking powder is typically double-acting, reacting to liquid and heat. Baking powder reacts to the acid salts to produce carbon dioxide gas. The gas cells then expand once the batter is placed in the oven, causing the batter to rise. Using too much baking powder produces a bitter taste in the batter, while too little powder results in poor volume and causes the finished product to be tough.