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Baking soda is pure sodium bicarbonate and needs acid present in your recipe to create lift, while baking powder is baking soda mixed with dry acid already built in - so it can work without you adding anything acidic. What trips people up is that most baking powder sold today is double-acting, meaning it starts releasing gas when wet *and again* when heat hits it, which is why you can let a batter sit for a bit without it totally deflating. I learned this the hard way trying to make pancakes from scratch - kept wondering why my fluffy batter would go flat while I finished cooking the previous batch.
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.
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