How does baking powder differ from baking soda?

Baking powder and baking soda are both leavening agents used to create a porous structure in baked goods, but they function differently in a recipe.1

Baking soda is pure sodium bicarbonate. It requires the presence of an acidic ingredient—such as buttermilk, yogurt, molasses, or lemon juice—to trigger the chemical reaction that releases carbon dioxide. If used without sufficient acid, it can result in undesirable flavors, yellowing, and poor leavening.213

Baking powder is a pre-mixed combination of sodium bicarbonate, dry acids, and starch for stability. Because it already contains the necessary acid, it does not require additional acidic ingredients to activate. Most commercial versions are double-acting, meaning they release a small amount of gas during mixing and the remainder when heated in the oven. This provides more flexibility and stability for batters that may sit before baking, whereas mixtures using baking soda or single-acting powders must be handled quickly to prevent gas loss.415