Baking Powder: Chemical leaveners, such as baking powder, produce carbon dioxide when an acid and alkali are combined in the presence of a liquid. Baking powder includes both the alkali and the acid: The alkali is almost always baking soda. Starch, usually cornstarch, is also mixed into baking powder, both to stabilize it and to neutralize the chemical reaction, as well as to absorb the excess moisture in the air, which would cause caking and lack of potency. Almost all the baking powder available today is double-acting, meaning that it creates two chemical reactions that promote rising: one when the baking powder is combined with liquid, and the second when it is exposed to heat. Keep baking powder in a dry spot and replace the container every six months or so, as it does lose potency as it ages.