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...
thymeless