Baking soda and baking powder look and sound the same. To make matters even more confusing, they're often used in the same recipes. However, knowing what sets these two popular ingredients apart could mean the difference between the perfect baked goods or a smooshed fiasco.
What's baking soda
Baking soda is, essentially, ground up rock -- which means it can last indefinitely (if properly stored). More specifically, baking soda is the colloquial term for sodium bicarbonate, a base that reacts quite energetically when encountering an acid such as buttermilk, yogurt, or vinegar (our brains register acid substances as 'sour'). Mixing baking soda with an acid will produce carbon dioxide -- because in this kitchen this reaction usually takes place in a liquid, it also produces bubbles. This property is what makes the substance so useful for bakers. Mix some baking soda into the proper dough, and it will generate carbon dioxide. As the mixture stiffens and the gas escapes, enlarged air pockets are left behind, making the end product fluffy and soft.
Due to this behavior, you'll often see baking soda mentioned in recipes which include many acidic ingredients like molasses, maple syrup, lemon juice, and pumpkin. In such cases, baking soda works as a leavener, helping the dough rise.
When added to a mixture, baking soda will raise the pH, slowing down protein coagulation -- the process that leads to the stiffening of a food as it cooks or bakes. This helps the bake good spread before it sets, helping the food bake more evenly.
Baking soda is also an excellent cleaning agent. It's a super-effective (but gentle) abrasive and is a great natural deodorizer, so it's helpful in all sorts of cleaning emergencies, from unclogging drains to deodorizing the carpet.
Someone who sure loves baking soda...
What's baking powder
Sometimes, you don't want the rising to take place all at once, which is where baking powder comes in. Baking powder is a mix made from baking soda (sodium bicarbonate) and two acids for it to interact with and produce CO2 gas at different stages of the baking process. This is "double acting" baking powder; single-acting baking powder contains only one acid, which reacts fully when you combine it with another liquid.
One of the acids in baking powder is monocalcium phosphate, which unlike most acids -- like, say, vinegar -- doesn't immediately react with the sodium bicarbonate while it's dry. It's only when the sodium bicarbonate is wet, such as when it's stirred into a wet dough, that the two ingredients begin to react, releasing CO2 bubbles and causing chemical leavening.
Baking powder usually contains a second acid, typically sodium acid pyrophosphate or sodium aluminum sulfate (soda alum), which extends the chemical leavening process. Neither of the two acids will react with the base until the sodium bicarbonate is both wet and hot -- in other words, not until you put the dough in the oven. This way, the batter can rise for a longer period of time, leading to a fluffier cake or muffin. Without the two special kinds of acids, baking powder's heavy lifting powers in the oven would be gone -- and we'd all end up with some pathetic, saggy bake goods.
Since baking powder is only one-fourth baking soda, it is also just one-fourth as powerful as baking soda. The upside, when using baking powder, it that it isn’t necessary to add an acid. Instead, baking powder starts to work when any liquid is added.
Both baking powder and baking soda need to be stored in a cool and dry place. The extra moisture in the air can start the reaction between the acids and base. And like baking soda, it is important to bake the mixture right away, or else the mixture will collapse.
So, there you have it: baking soda is made out of a single ingredient, while baking powder is a mix of baking soda and at least one acid. But which of the two should you use in the kitchen? That's simple: when baking a recipe which already contains an acid as one of the ingredients, use baking soda. If there are no acids in your recipe, use baking powder instead.