baking soda [beyk-ing soh-duh] noun
Also bicarbonate of soda. A chemical leavener that, when combined with an acid ingredient such as buttermilk, yogurt, sour cream, etc., produces carbon dioxide gas bubbles that will help the bread, cookie or cake rise and have a light texture. Baking soda reacts immediately when it comes in contact with the acid, so it should always be mixed with the other dry ingredients before combining with remaining ingredients.
A famous exception is the classic Toll-House Chocolate Chip Cookies recipe where Ruth Wakefield directs us to mix baking powder with water. Once a batter or dough containing baking soda is prepared, it is typically baked immediately to take advantage of the leavening action. Many chocolate chip cookie recipes are refrigerated first; another exception. The theory is that the small bit of acidity provided by the molasses in the brown sugar takes a while to affect the baking soda.