What's the difference between baking soda and baking powder?

The key difference is that baking soda needs an acid (buttermilk, lemon juice, vinegar) to activate and rise, while baking powder already has the acid built in so it works on its own. Baking soda also reacts immediately, whereas baking powder (especially double-acting) can produce two rises - one when wet and another when heated.

People usually frame this as baking soda needing acid and baking powder being self-contained, which is mostly right but misses something important: double-acting baking powder has *two* acid phases. It reacts once when wet (cold), then again when heat hits it in the oven. So if you swap them 1-to-1 in a recipe, you might get underrise or a weird texture because the timing's different. Baking soda hits hard and fast if there's acid present; powder gives you a slower, steadier lift. The real trap is thinking they're interchangeable just because they both make things fluffy.

If you're baking something and need lift right away without adding acidic ingredients, reach for baking powder - it's got everything mixed in already and starts working the moment it gets wet. Baking soda is just the base chemical, so you have to pair it with something acidic in your recipe (buttermilk, yogurt, vinegar, whatever) to actually get that rise. The guys above nailed the double-acting thing too, where baking powder reacts in stages depending on temperature, which is why it works better in some recipes than baking soda alone.