sodium bicarbonate; bicarbonate of soda
An extremely alkaline a white, water-soluble powder. When mixed with an acid, it spurs a chemical reaction that produces carbon dioxide gas—or put more simply, it creates bubbles that cause baked goods to rise. You'll always see baking soda in recipes that also include some sort of acidic ingredient, such as vinegar, yogurt, buttermilk, or cream of tartar.
Because it begins to react as soon as it meets an acidic liquid, combine baking soda with the dry ingredients (not wet) and cook the batter soon after the liquid is added. In order for baking soda to produce light, airy pancakes and muffins, the batter must have the correct amount of acidity (from buttermilk, yogurt, lemon juice, applesauce, vinegar, or honey, for example). If there isn't enough acidity in the batter (if you substitute fresh milk for buttermilk, for instance), the baking soda won't be converted to carbon-dioxide gas. The resulting batter won't rise properly and the unconverted baking soda will leave behind an unpleasantly soapy taste.
Baking soda will keep for quite a while kept in a cool, dry place. But its leavening powers lessen over time, so if it's been a while, check the expiration date; you may need to open a new box.