What is the difference between baking soda and baking powder?

Baking soda is pure sodium bicarbonate, an alkaline compound that requires an acidic ingredient in the recipe, such as buttermilk, vinegar, or lemon juice, to activate it and produce carbon dioxide bubbles that make baked goods rise. Baking powder contains baking soda plus a dry acid, usually cream of tartar or sodium aluminum sulfate, and a starch. Because it already contains its own acid, it can activate with just moisture and heat, making it more versatile. Most baking powder is double-acting, meaning it releases gas twice: once when wet and again when heated. The two are not directly interchangeable, though baking soda is roughly three times stronger than baking powder.