Baking powder is a commercial product used to leaven baked goods. It consists of three ingredients: baking soda, the source of the carbon dioxide gas that causes the dough or batter to rise; an acid, such as cream of tartar, calcium acid phosphate, or sodium aluminum sulphate, which, when the powder is combined with a liquid, causes the baking soda to release its gas; and a starch such as cornstarch (cornflour) or flour, to prevent the powder from absorbing moisture.