What is the difference between baking soda and baking powder?

Baking soda is pure sodium bicarbonate that requires an acid to activate, while baking powder contains both the base and acid needed to rise.

Baking soda reacts immediately when combined with acidic ingredients like buttermilk, lemon juice, or yogurt. Baking powder is more self-contained—it starts working when moistened and activates further when heated.

You can substitute baking powder for baking soda (use about 4 times as much), but not the reverse without adding an acid. Understanding this helps you troubleshoot flat cakes and adjust recipes.