They’re both white and powdery, they’re in the same section of the market, they even have almost the same name. Here’s the difference.
What Is Baking Soda?
Baking soda is a simple chemical compound: sodium bicarbonate, which readily reacts with acids.
As soon as it contacts an acidic ingredient, such as buttermilk, it breaks down into smaller molecules, including carbon dioxide gas. In a baked good, that gas inflates tiny bubbles, causing the dough or batter to rise right away. So it’s used in baking recipes that include an acidic ingredient: buttermilk, yogurt, lemon juice, perhaps molasses or brown sugar.
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What Is Baking Powder?
Baking powder, on the other hand, is a mixture of a few different chemical ingredients—one of which is baking soda. Baking powder also contains a dry acid, typically monocalcium phosphate, that reacts with the baking soda as soon as it gets wet, forming carbon dioxide and creating an initial dose of leavening.
So-called double-acting baking powder, which is the most common kind, also contains a second dry acid, one that reacts only when it gets hot, such as sodium aluminum phosphate. So after a baked good made with baking powder goes into the oven, the second reaction begins, releasing a fresh quantity of carbon dioxide and an extra puff of rise.
Baking powder also contains starch, to keep it dry, so that it doesn’t start reacting on its own.
Why Do Some Recipes Call for Both Baking Soda and Baking Powder?
Often a recipe will call for both baking soda and baking powder if there’s a small amount of acid in the recipe, which will react with the baking soda, but we want more leavening than that provides. Occasionally, as in our soda bread, we use baking powder for lift, and add a little baking soda simply because we like the mineral-salty taste it contributes to the bread.