Baking soda is pure sodium bicarbonate, a base that needs an acidic ingredient—such as buttermilk, yogurt, lemon juice, or vinegar—to produce carbon dioxide and lift batter. Baking powder already combines baking soda with a dry acid (often cream of tartar) plus starch, so it can leaven with moisture and heat alone; many commercial powders are double-acting, reacting once when mixed and again in the oven.
Source: King Arthur Baking — What’s the difference between baking soda and baking powder?