Baking powders are mixtures of baking soda plus one or more acids to react with it. They also contain starch, which prevents lumping and brings the leavening power down to a standard level. Because baking powders do not depend for their leavening power on acid ingredients in a formula, they are more versatile.
Single-acting baking powders require only moisture to release gas. Like baking soda, they can be used only if the product is to be baked immediately after mixing. For all practical purposes, no single-acting baking powders are sold today, although you can make it yourself, as explained in the sidebar. Practically, single-acting baking powders release gas too quickly to be useful for most products.