Baking soda and baking powder are both chemical leavening agents that give height to baked goods through acid-base reactions that release carbon dioxide, but they do so in slightly different ways. To best understand how to use them, it helps to grasp what they are.
Baking soda — also known as “bicarbonate of soda” in the UK and some commonwealth countries — is pure sodium bicarbonate, an ionic salt with the chemical formula NaHCO3. It’s naturally alkaline (basic) with a pH of eight, and provides the “base” portion of the acid-base reaction. If you’re using baking soda to leaven something, you need to introduce an acid for it to react with, such as vinegar, lemon juice, cream of tartar, or (depending on how it’s processed) cocoa powder.
If your recipe doesn’t contain any naturally occurring acids, it probably calls for baking powder, which provides both baking soda and one or more acids for the baking soda to react with. It may seem like a bad idea to store acids and bases in one container, but the acids in baking powder are in their crystalline forms, and can only interact with the baking soda once it sees some water. Almost all baking powder you buy at the grocery store is “double-acting,” which just means that there are two acids in the mix — one that reacts immediately when it’s mixed into a wet batter, and one that is later activated by the heat from your oven.
Now for the mnemonic. Even if you understand the theory behind the two leavening agents and how they work, it can still be hard to keep the names straight in your head. If you grasp the concept but have trouble with names, just remember: Baking SOda is SOlely SOdium bicarbonate, while baking Powder is Perfectly complete. And, if all you have is baking soda, don’t fret — you can make your own complete chemical leavening agent by adding a little cream of tartar.