A LEAVENER containing a combination of baking soda, an acid (such as CREAM OF TARTAR) and a moisture-absorber (such as cornstarch). When mixed with liquid, baking powder releases carbon dioxide gas bubbles that cause a bread or cake to rise. There are three basic kinds of baking powder. The most common is double-acting, which releases some gas when it becomes wet and the rest when exposed to oven heat. Baking Powder: Baking powder is normally made of three different parts: " An acid " A base " A filler of some sort All three need to be dry powders that can be mixed together. For example, baking soda (a base), cream of tartar (an acid) and corn starch (the filler) are three common ingredients. In school, you may have done the experiment where you mix baking soda (a base) and vinegar (an acid) and get a bubbling reaction. Baking powder works the same way. When you add water to baking powder, the dry acid and base go into solution and start reacting to produce carbon dioxide bubbles. Single-acting baking powder produces all of its bubbles when it gets wet. Double-acting baking powder produces bubbles again when it gets hot. Many recipes call simply for baking soda rather than baking powder. Usually these recipes use some kind of liquid acid like buttermilk or yogurt to react with the baking soda to produce the bubbles. The reason why people often prefer baking powder to yeast is because yeast takes so long -- usually two to three hours -- to produce its bubbles. Baking powder is instant, so you can mix up a batch of biscuits and eat them 15 minutes later.