If the recipe calls for both leavening agents, it's likely that there is acid in the recipe. However, this is not enough to cause a full reaction if combined with baking soda alone. Baking powder is used to reinforce the leavening process, while baking soda neutralizes the acid (lessens the astringent flavor) and adds some lifting power. This is teamwork! Baker's tip If it fizzes it is baking powder. If it doesn’t, it’s baking soda.