Most baking powders are made up of an alkaline ( usually baking soda ) , one or two acid salts , and a starch ( cornstarch or sometimes potato starch ) . Baking soda is where the carbon dioxide comes from , and the acid releases it , like this : Baking powder is generally just baking soda mixed with an acid , and a number of kitchen acids may be mixed with baking soda to simulate commercial blends of baking powder .