They both create gas, which makes baked goods rise. Each contains sodium bicarbonate, an alkaline chemical that gives off carbon dioxide when mixed with an acid. Baking soda consists purely of sodium bicarbonate, so recipes using it must include an acidic ingredient like lemon juice, buttermilk, or brown sugar (the molasses in brown sugar is acidic) to activate it. Baking powder contains some baking soda, cornstarch to keep it from clumping, and one or more acidic salts, which act as the activating/neutralizing agents for the bicarbonate.
The two leavening agents work at different speeds. Baking soda produces gas immediately upon contact with liquid acid. Remember when you were a kid and you’d mix it with Coke to get a crazy foaming effect? So your dough or batter begins rising the minute you mix in the soda. Baking powder, on the other hand, creates a little gas when you first mix it in (that’s the baking soda working), and then more when the acidic salts have had a chance to fully dissolve, and yet a little more when your product is put in the oven.
That’s because the acidic salts most commonly used in baking powder need heat to work fully. The names of these salts are cream of tartar—also known as potassium bitartrate, a by-product of winemaking that has nothing to do with tartar sauce—and calcium aluminum phosphate. That’s why the vast majority of baking powders sold in grocery stores today are what’s known as double-acting: They’re rising once, then again.
So why use one over the other? Baking powder is often called for in recipes in which there is no acidic ingredient, as the powder contains its own acid component. And that built-in acid ensures that the soapy flavor of unneutralized bicarbonate will not be present in what you’re making. Baking soda is often called for in recipes in which color is an issue, says baking expert Shirley Corriher, author of the book CookWise. Cookies that are more alkaline will brown better, says Corriher. Dark chocolate cakes will be darker the more alkaline they are.
But beware of confusing the two. If you use baking soda in a recipe that calls for baking powder, and there isn’t an acid among the ingredients, your product won’t rise. Even if there are acidic elements in your recipe, it may still not rise, because you made the batter too alkaline. (Eggs need the proper acidity to set, for instance.) If you switch out powder for soda, you may not get enough gas, because baking powder contains so much less bicarbonate per volume than soda.
But the most interesting part of this story is where baking soda comes from. It’s mined from the earth. Most of the baking soda in North America comes from trona, a sodium bicarbonate–containing mineral whose largest deposit is underneath Green River, Wyoming. The trona is cleaned and milled to a powder, and you’ve got baking soda. In places without extensive trona deposits, like Europe, baking soda is made using the Solvay process, a reaction involving table salt and ammonia.
Another side note: Some people don’t like to use baking powder with aluminum because they believe it gives food a vaguely metallic taste, and because it has been suggested that there may be a link between aluminum consumption and Alzheimer’s disease. Studies have not proven this, however.
Want to make your own baking powder and ensure it doesn’t have any aluminum in it? It’s easy. Simply add two parts cream of tartar to one part each of baking soda and cornstarch.