Chemically speaking, baking soda is a base -- alkaline -- and produces a gas when mixed with an acid. This is the source of both its leavening action and the more explosive effect that makes it a popular drain-cleaner and component of baking-soda-and-vinegar volcanos. Lemon juice, buttermilk, honey, molasses, and chocolate are all acidic enough to trigger leavening. Do not let baking-soda-leavened batter sit too long before baking it or the chemical reaction will run its course and the batter will start to deflate.
Baking powder pre-combines baking soda with a dry acid or two to produce the same effect. The most common kind, double-acting baking powder, contains two acids, one that starts leavening as soon as it gets wet and another that doesn't start working until it's heated. Some recipes will advise you to let baking-powder-leavened batter sit briefly before baking it, in order to, as it were, get the maximum lift from the first stage before heat sets off the second stage.
Baking powder loses strength as it ages. If you're not sure how long that can has been sitting in the cupboard, test it by mixing 1/2 tsp. of it into 1/4 cup of warm water; it will fizz if it's still good.
A possible substitute for baking powder is to add 2 tsp. cream of tartar, 1 tsp. baking soda, and 1 tsp. salt PER CUP OF FLOUR (not in direct proportion to the recipe's baking powder). Keep in mind, though, that you're edging from cooking into chemistry here and this work-around won't work for every recipe.
