Recipes that include such acidic ingredients as buttermilk, honey, cocoa powder, or molasses call for baking soda, which is a base mineral and works with the acid to cause rising. This process produces carbon dioxide, which looks like bubbles to the naked eye and is what gives a baked good its light texture. Remember that science experiment with baking soda and vinegar you did as a kid? Same thing, except now you get a batch of cookies out of it!
Baking powder is a combination of baking soda, acid, and a moisture-absorbing agent like cornstarch. You can use baking powder whether or not the recipe contains acidic ingredients. The most commonly available baking powder is called double-acting, meaning it works in two stages: It's activated when exposed to moisture as the batter is being mixed, and again when exposed to heat. It's mostly used for cakes, cookies, pancakes, and muffins.
How to Store
Both baking soda and baking powder need to be stored away from humidity, in a very dry place. Before buying and using, make sure to check the expiration or use-by date.
In a pinch and only have baking soda or baking powder on hand? Watch our Kitchen Conundrums expert Thomas Joseph explain how to substitute one for the other: