A leavener containing a combination of baking soda, an acid (e.g. cream of tartar) and a moisture-absorber (e.g. cornstarch). When mixed with a liquid, baking powder releases carbon dioxide gas bubbles that cause bread or cake to rise. There are three kinds of baking powder; double-acting (most common), which releases some gas when it becomes wet and the rest when exposed to oven heat; single-acting tartrate and phosphate baking powder release their gases as soon as they're moistened. Baking powder is perishable and should be kept in a cool dry place. Always check the date on the bottom of the package before purchase. To test if a baking powder still can be used, combine 1 teaspoon of it with 1/3 cup hot water. If it bubbles energetically, it's fine.