Because recipes often call for such small amounts of baking powder and baking soda, you may think these ingredients are not essential for success. They are! These ingredients are leavening agents -- without them, the baked products will not rise. And, because the chemical properties of the two ingredients are different, one cannot be substituted for another. It's best to keep them both on hand.
When using baking soda, keep in mind that the soda and acid begin to react as soon as a liquid is added.