The "baking soda" you use to make cakes, i.e. the commercial product, is (pure?) "sodium bicarbonate", i.e. Na^(+)""^(-)HCO_3.
"Baking powder" is another leavening agent, but it contains "cream of tartar", i.e. "potassium ditartrate", KC_4H_5O_6, 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 CO_2, which raises the cake, and makes it fluffy and spongy. Yumm.