Bicarbonate of Soda is a salt composed of a sodium cation and a bicarbonate anion . It is a white solid that is crystalline, but often appears as a fine powder. In cooking, baking soda is primarily used in baking as a leavening agent. When it reacts with acid, carbon dioxide is released, which causes expansion of the batter and forms the characteristic texture and grain in cakes, quick breads, soda bread, and other baked and fried foods.