Baking powder is used in baking as a leavening agent. Quick breads, cakes and cookies usually use baking powder as an ingredient.
Single acting: Single acting baking powder can be made from several different chemical formulations: Single-acting baking powders react immediately when liquid is added to the mixture.
Those that contain cream of tartar and tartaric acid (C4H606) create gas rapidly when mixed with baking soda and a liquid. These batters must be cooked quickly or they will go flat. Slower single-acting baking powders are phosphate baking powders that contain either calcium phosphate (Ca3O8P2) or disodium pyrophosphate (H2Na2O7P2). Aluminium sulfate (AI2012S3) powders react more slowly at room temperature but give a bitter taste to the batter.
Double Acting: A quick acting acid provides an initial rise during the mixing process followed by a second rise when heat is applied. 
Baking Powder Substitute
You can make your own baking powder equivalent at home.
- 2 tablespoons cream of tartar
- 1 tablespoon baking soda
- 1 tablespoon cornstarch
Store baking powder sealed in a cool, dry place no longer than one year.
Proofing the Baking Powder
To check, or proof, the baking powder add 1 teaspoon baking powder to 1/2 cup warm water. The baking powder should immediately bubble.
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- Joy of Baking.com: Baking Powder (English). Retrieved on 2010-09-29.