Actually, baking powder rarely contains sodium bicarbonate. People imitate baking powder by mixing baking soda, cream of tartar and corn starch. That's only single acting.
Commercial baking powder is double acting. It has a low temp/wet activated acid base reaction and a high temp reaction. From wikipedia:
Most modern baking powders are double acting
, that is, they contain two acid salts
, one which reacts at room temperature, producing a rise as soon as the dough or batter is prepared, and another which reacts at a higher temperature, causing a further rise during baking. Baking powders that contain only the low-temperature acid salts
are called single acting
. Most recipes call for a mixing procedure that is designed to introduce many tiny air bubbles, for example, "cream the butter and sugar", which the leavening gas from baking powder will expand. Common low-temperature acid salts
include cream of tartar
, calcium phosphate
, and citrate
. High-temperature acid salts are usually aluminium
salts, such as calcium aluminum phosphate
Harold McGee gives a more detailed breakdown in his book, but my copy is still on loan...