Somewhere in your cupboard, chances are you have a can of baking soda and a box of baking powder. But do you really know the difference between these two products? While both are leavening agents, they are used for entirely different purposes in recipes.
Baking soda causes food to spread, as when cookies spread out on a baking sheet. To have that effect, the baking soda must be mixed with a liquid acid, such as buttermilk. Because baking soda is activated immediately, the food you are making must be baked right away.
Baking powder causes foods like bread to puff up. Baking powder consists of baking soda plus a dry acid that reacts with any liquid added to the recipe. Double-acting baking powder reacts twice, once when wet and then again when heated; this causes the dough to rise twice.