Baking powder consists of a mixture of baking soda and a substance such as cream of tartar. Baking soda is sodium hydrogen carbonate, NaHCO3. In the production of carbon dioxide gas, baking powder acts very differently from baking soda. When water is added to baking soda, it just dissolves. When water is added to baking powder, carbon dioxide gas is produced. What is the purpose of cream of tartar in baking powder?