What is the difference between baking soda and baking powder?
Baking soda is pure sodium bicarbonate, while baking powder includes an acidifying agent (cream of tartar) and a drying agent (starch).

You can substitute baking soda for baking powder if you already have an acidifying agent in a recipe (like buttermilk).

http://chemistry.about.com/cs/foodchemistry/f/blbaking.htm