They're definitely not baking soda, that's for sure
If you've ever paid close attention to the container of baking powder that's sitting in your kitchen cupboard, you might've noticed that it has either the phrase double-acting or single-acting written on the side. But what does it mean for baking powder to be "double-acting," and what is the difference between double-acting and single-acting baking powder besides a label on a canister? Well, to best understand the difference between these two types of baking powder, and whether you can substitute one for the other, you need to understand a little bit about what what baking powder does and how it works.
Baking powder, like baking soda, is a leavening agent, meaning it helps doughs and batters rise. But unlike baking soda, baking powder is a complete leavening agent. That means it contains both the alkaline baking soda and the acid needed to create lift in one packaged ingredient. This is why it's so easy to make a baking powder substitute if you have baking soda and an acid, like cream of tartar or even lemon juice or vinegar. And really, the difference between double-acting and single-acting baking powder comes down to which type of acid is paired with the alkaline baking soda to make baking powder.
A double-acting baking powder will react and create gas bubbles twice: once when added to liquid, and again when exposed to heat. "That is, they inflate an initial set of gas bubbles upon mixing the powder into the batter, and then a second set during the baking process," explains food scientist Harold McGee in On Food and Cooking.
Since the alkaline baking soda in baking soda will always immediately react with liquid, creating that initial lift in the batter, whether or not a baking powder is single-acting or double-acting depends on what type of acid is added. "There are several different acids used in baking powders, each with a different pattern of gas production," explains McGee, adding, "Most double-acting supermarket baking powders are a mixture of sodium bicarbonate [baking soda], MCP [monocalcium phosphate], and SAS [sodium aluminum sulfate]." Single-acting baking powders use acids that are primarily heat-activated, not released after mixing with liquid.
Fortunately for the still-confused bakers out there, you're more likely to run into double-acting baking powder in the grocery store, since most single-acting baking powders are only available for commercial use. But if you do stumble upon a canister of the commercial-grade stuff, or have a recipe that calls for single-acting baking powder but only have double-acting, feel free to use the the two types of baking powder pretty much interchangeably. But keep in mind that when people are referring to "regular" baking powder, it's usually the double-acting stuff.