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. Single-acting tartrate and phosphate baking powders (hard to find in most American markets because of the popularity of double-acting baking powder) release their gases as soon as they're moistened. Because it's perishable, baking powder should be kept in a cool, dry place. Always check the date on the bottom of a baking-powder can before purchasing it. To test if a baking powder still packs a punch, combine 1 teaspoon of it with 1⁄3 cup hot water. If it bubbles enthusiastically, it's fine.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.