Sarah - from Tustin, CA
No, baking powder and baking soda cannot be used interchangably. Even though they are both leavening agents, they do very different jobs.
Baking soda requires an acid to activate, so you will see it used in recipes that have an acidic ingredient, like citrus, chocolate, or sour cream. If an acidic element is not present in your recipe, baking soda will not give lift to the baked good.
Baking powder consists of bicarbonate of soda, an acidic powder, and a starchy filler like cornstarch or arrowroot. The starchy filler is why more baking powder is used than baking soda in recipes that call for both. Most brands of baking powder bought at groceries stores are double acting, which means that they are activated first with moisture and second with heat.