A leavening agent is a substance used to produce carbon dioxide gas in dough or batter, causing it to rise. YEAST is a common leavening agent, consisting of single-celled microorganisms that feed on sugars and starches, producing carbon dioxide and alcohol as byproducts. Baking soda is another leavening agent, which is a combination of sodium bicarbonate and an acid, such as cream of tartar. When combined with liquid, baking soda produces carbon dioxide gas, which helps dough or batter to rise.