what’s the difference between baking powder and baking soda?

The short answer is acidity. Baking soda is just sodium bicarbonate, and is alkaline in nature. In order for it to react and create carbon dioxide it needs moisture and acid. So just about all recipes that call for baking soda as a leavener will also call for an acidic liquid too, like buttermilk. The other important factor with baking soda is that it reacts quickly. Think science fair volcano. This isn’t always a good thing when your product needs time to develop or rest before baking.

Baking powder, on the other hand, has a longer reaction time. That’s because baking powder is the same base of sodium bicarbonate but with the addition of two acids and a starch. The acids each work at slower rates and in different stages, and the starch helps to prolong full hydration of the sodium bicarbonate and acid by attract water to itself instead.