1. Baking soda + cream of tartar
¼ tsp
baking soda
½ tsp
cream of tartar
Best swap: Baking soda + cream of tartar. Use ¼ tsp baking soda plus ½ tsp cream of tartar per 1 tsp.
Baking powder is just baking soda plus a dry acid, so you can build it yourself if you have the parts. Mix it right before baking, since it starts working the moment it gets wet.
Mix a quarter teaspoon of baking soda with half a teaspoon of cream of tartar for every 1 teaspoon of baking powder, and use it right away.
Not on its own, because baking soda needs an acid to work. Combine it with cream of tartar or an acidic liquid like buttermilk to replace baking powder.