The baking soda you use to make cakes, i.e. the commercial product, is (pure?) sodium bicarbonate, i.e. Na+−HCO3.
Baking powder is another leavening agent, but it contains cream of tartar, i.e. potassium ditartrate, KC4H5O6, a salt of a diacid. When you add baking soda to a cake batter, the acid in the batter, the butter and the eggs, is conceived to react with the carbonate to make CO2, which raises the cake, and makes it fluffy and spongy. Yumm.