baking powder
A dry chemical leavener combining an acid and baking soda in one product -- unlike baking soda alone, it doesn't need a separate acidic ingredient to work. Most supermarket baking powder is double-acting, releasing gas once when it's moistened and again with oven heat -- which is why mixed batter still rises even after sitting a few minutes, though it's still best baked without unnecessary delay.
Buying guidance
Buy double-acting baking powder unless a recipe specifically calls for something else -- that's the standard supermarket product. Buy a small container if you bake infrequently; potency fades within months of opening, well before the printed date.
Substitutions
- Baking soda plus cream of tartar (roughly 1/4 tsp cream of tartar per 1/4 tsp baking soda, in place of each teaspoon of baking powder) -- a working homemade substitute; measure carefully rather than eyeballing it.
- Baking soda alone -- not a direct substitute; it needs its own separate acidic ingredient (buttermilk, vinegar, brown sugar) already in the batter to react, and using it 1:1 without one will under-leaven the result.
Tips & common mistakes
If a container has been open for months, test it before baking with it: stir a little into hot water, and if it doesn't fizz vigorously right away, its lifting power is mostly gone and it needs replacing even if it isn't past the printed date. Baking powder and baking soda aren't interchangeable measure-for-measure -- baking soda is roughly three to four times more powerful, so swapping one for the other over- or under-leavens the result.
Storage
Moisture is what triggers baking powder's reaction, so a wet spoon, or a container left open near a steaming pot, can set off some of its lifting power before it ever reaches the batter -- always measure with a dry spoon into a dry bowl.