Baking powder can be substituted for baking soda in a recipe by adding 2 teaspoons of double-acting baking powder for every half teaspoon of baking soda and replacing the acidic liquid in the recipe with a non-acidic liquid. Baking powder is composed of baking soda, a dry acid, such as cream of tartar, and an ingredient to reduce moisture, such as cornstarch.
Baking soda is usually used as a leavening agent in a baking recipe and allows the batter to rise once cooked. Baking soda is a base ingredient that is mixed with another acidic liquid in the recipe such as yogurt or buttermilk to create the rising effect. Baking powder is used to achieve the same results, but does not require an additional acidic liquid as it is already included in it.