Originally posted by meowzart
I should probably leave this to Jane Dough or lleechef, but baking soda is a leavener that needs to be activated with an acid to work, such as buttermilk. This creates bubbles, which makes your batter rise. Baking powder is already a combination of the two, baking soda and an acid.
You'll need the above two people two tell you about single-acting and double-acting stuff, though. That's getting a little too scientific for me.
partial credit to meowzart, but extra credit for my honorable mention.
Baking being primarily a scientific art, here's the technical scoop on baking soda v. baking powder...
Baking soda is merely good old-fashioned sodium bicarbonate, a powdered chemical base (as opposed to its chemical opposite, acid) that is needed to neutralize acids in your recipe, such as lemon or orange juice, molasses, etc. In addition to seafarer john's battery acid application, it also serves as a great heartburn remedy when mixed w/ water, if you happen to be out of whatever you usually use from the pharmacy.
Baking powder is actually a mixture of both acid powders and baking soda (to neutralize each other), plus either starch or flour to stabilize the mix. It serves as a CHEMICAL leavener because it releases carbon dioxide at two different stages of production: when exposed to liquid, and then again when exposed to heat (hence that curious phrase on the label, "double-acting.")
An alternate form of leavening is a PHYSICAL leavener, such as whipped egg whites. Whipping adds thousands of little air pockets into the whites, which expand when exposed to heat, hence single-acting. That's the basis behind the classis souffle (translation "blown"), and most Passover items (including the omnipresent sponge cake), because adding leavening agents (e.g., yeast & baking powder) are forbidden during that holiday (c.f., matzo as opposed to bread).
A recipe using baking powder can be made in advance and held until baking because of the double-action, but physically-leavened products must be baked immediately upon gently folding in the whipped whites because the air pockets degenerate quickly.
I hope nibiru's biscuits get "a rise" out of this lesson.