Baking powder is a mixture of one or more acidic salts and baking soda, an alkali. These two compounds react when they get wet and release carbon dioxide gas bubbles. These, in turn, cause baked goods to rise.
Ingredients: Sodium acid pyrophosphate, sodium bicarbonate, corn starch, monocalcium phosphate.
Shelf life: Baking powder is perishable, and lasts about 18 months. To test a batch, add 1 teaspoon to ½ cup hot water. If it doesn't bubble, throw it out.
Aluminum-free baking powder is preferred by many cooks; powders made with aluminum lend an unpleasant flavor to delicately-flavored baked goods.
Substitutes: (for 1 teaspoon of baking powder): Combine 5/8 teaspoon cream of tartar plus 1/4 teaspoon baking soda OR Combine two parts cream of tartar plus one part baking soda plus one part cornstarch.