Single-acting baking powder is a chemical raising agent that releases its carbon dioxide in one burst, on contact with moisture, as soon as it is mixed into a batter. Unlike double-acting powder, which gives a second lift in the oven’s heat, the single-acting type starts working immediately, so the batter must go into the oven without delay or the gas escapes and the bake falls flat. It is the older formulation, still used in some traditional recipes and commercial mixes where timing is controlled. Modern home baking has largely moved to double-acting powder for its insurance against slow ovens and dawdling cooks. Understanding which type a recipe assumes matters: they are not interchangeable without adjusting method.