The difference between baking soda and baking powder?
Also known as sodium bicarbonate, baking soda is an effective chemical leavening agent. In recipes (like most baked goods) where a liquid and an acid are included, the soda will react and release carbon dioxide. When the CO2 gas is released, the product rises. (Acids in a recipe may come from: yogurt, honey, sour cream, molasses, buttermilk, chocolate/cocoa, juices, etc.) Typically, recipes only use enough baking soda to balance the acid – any more than that is unnecessary. If additional leavening is desired, baking powder is generally added.
Baking powder is a chemical leavener like sodium bicarbonate, but contains an acid (and starch to prevent lumping). Baking powder is useful in a variety of recipes, because (unlike soda) baking powder doesn’t require an acid in the formula to undergo the leavening reaction. Instead, the acid in the powder initiates the leavening reaction. Ever seen a cake rise, then fall before setting? This may be an indicator that too much baking powder was used. Additionally, using too much baking powder can give an off flavor in the finished product.
A Food Science tid-bite: Store both baking soda and baking powder in an air-tight container. If moisture is introduced to either product, the leavening property may be compromised.
Source: Wayne Gisslen. Professional Baking Second Edition. John Wiley & Sons Inc, 1985, 1994.