Two common leavening agents—baking soda and baking powder—have similar names and similar roles, but they also have significant differences.
Baking soda is a white soluble compound also known as sodium bicarbonate or bicarbonate of soda. It’s extremely alkaline and will spur a chemical reaction that produces carbon-dioxide gas when mixed with an acid—or more simply, it creates bubbles that cause a batter to rise. 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 powder performs on the same principle of creating carbon-dioxide gas bubbles to raise baked goods, but unlike baking soda, baking powder contains its own catalyst for this reaction. Baking powder contains both baking soda and an acid so that it can be mixed with any type of liquid and create its own bubble-producing reaction.
Double-acting baking powder is the most common baking powder on the market. It contains two types of acid (usually cream of tartar and sodium aluminum sulfate), one that reacts when the batter is first moistened, and one that reacts later in the heat of the oven.