Double-acting baking powder is the chemical raising agent that releases carbon dioxide in two stages — firstly on being moistened, and secondly on being heated. It blends an alkali, usually sodium bicarbonate, with two acids: one that reacts at room temperature the moment liquid hits the batter, giving an early lift, and a second, heat-triggered acid that reacts in the oven’s heat for a powerful second rise. The staging gives batters tolerance — a mixed batter can wait before baking without losing all its lift — which is why double-acting powder dominates home and commercial baking. Single-acting powders, which release everything on moistening, demand the batter reach the oven immediately. The double action is formulation chemistry in service of the cook: two timed bursts of gas, matched to the two phases of baking.