If you've ever tried using baking powder in place of baking soda—or vice-versa—the results may have flopped (quite literally). You see, baking powder and baking soda don't actually work the same way. Baking soda needs an acidic ingredient like lemon juice to activate it. And baking powder is, basically, just baking soda with the acid component already built-in. You can't use the two interchangeably. In fact, substituting one for the other can be disastrous.
The Chemistry of Baking Soda
On its own, baking soda is an alkaline ingredient that needs the presence of an acid to activate a reaction. So when you mix it with an ingredient like vinegar, it releases gas. Use baking soda in recipes that include acidic components like buttermilk, sour cream, lemon juice, and yogurt. Molasses is also acidic, and—believe it or not—so is honey. Any of these ingredients will cultivate a chemical reaction. But if you make a mistake and sub baking soda for baking powder in a recipe without an acid, there will be no release of gas and the dough won't rise.
The Chemistry of Baking Powder
Baking powder, on the other hand, is nothing more than baking soda with some sort of acidic compound already included. Different brands of baking powder use different acids, the most common being cream of tartar (or potassium bitartrate), an acid-salt byproduct of winemaking. Others include cornstarch as an acid. But, even though both a base and an acid are present in the baking powder, the compound won't react until it is moistened. "Double-acting" baking powder activates with both the addition of moisture and the addition of heat (like an oven or a griddle), giving it greater leavening power.
Subbing Baking Powder for Baking Soda
Let's say you were determined to use baking powder instead of baking soda (maybe that's all you have on hand). Doing so will certainly make your recipe rise, but maybe not to the extent you intend it to. Since baking powder is comprised of approximately one-third baking soda and two-thirds other ingredients, you will only essentially be using one-third the amount of baking soda the recipe calls for. Therefore, you'll need to triple the amount of baking powder you use in order to achieve full leavening. However, the additional ingredients in the baking powder may create a bitter flavor and the extra acids in the recipe may cause the batter to quickly rise, and then fall before the aeration has a chance to bake in. Either way, the results are not ideal.
Making Your Own Baking Powder
If your recipe calls for baking powder, yet all you have on hand is baking soda, making your own is easy. Simply combine one teaspoon of baking soda with two teaspoons of cream of tartar to yield one tablespoon of baking powder. Use it right away, as storing it may cause the chemical reaction to happen prematurely. And if you don't have cream of tartar lying around, you might as well just hit the store and buy some baking powder anyway. (Note: Most bakers keep cream of tartar ion hand for stabilizing egg whites when you whip them into a soufflé or meringue.
Storing Leavening Agents
Over time, chemical leavening agents, like baking powder and baking soda, will lose their potency. To prevent this, store them in a cool dry place and in a tightly sealed container. And since they're inexpensive, seasoned bakers suggest replacing both products every six months.