Baking powder is a chemical leavener. White and soluble, it produces carbon-dioxide gas when mixed with a liquid; the resulting bubbles cause pancakes, waffles, breads, and cakes to rise. Unlike baking soda, which needs an acidic ingredient added to it in order for it to react, baking powder contains both baking soda and an acid, which means it can be mixed with any type of liquid and create its own bubble-producing reaction.
Double-acting baking powder is the most common baking powder on the market. It contains two types of acid (usually cream of tartar and sodium aluminum sulfate), one that reacts when the batter is first moistened, and one that reacts later in the heat of the oven.
Sodium aluminum sulfate can create a bitter metallic aftertaste, especially when used in high amounts. The downside of using aluminum-free baking powder is that you need to get your baked goods into the oven quickly, since most of the leavening power is unleashed once the wet ingredients are added. Its shelf life can also be shorter, as it’s more vulnerable to ambient moisture if not sealed properly.
Whether you’re using aluminum-free baking powder or not, its a good idea to test your baking powder for freshness if it’s been stored for a while. To do this, check for caking or lumping, an indication that the powder has absorbed moisture and is past its prime. If no lumping is apparent, mix a teaspoon of the baking powder with hot water; if it bubbles vigorously, then the powder is still active.
Per 1 tsp. baking powder, substitute 1/4 tsp. baking soda plus 5/8 tsp. cream of tartar
Baking powder will keep for quite a while kept in a cool, dry place. But its leavening powers will less over time, so if it's been a while, check the expiration date, you may need to open a new can. To test for freshness, combine 1 tsp. baking powder with 1/3 cup hot water; it it bubbles enthusaistically, it's still got oomph.