* Baking powder contains sodium bicarbonate (baking soda) and an acid. When mixed with water, the acid reacts with the baking soda, producing carbon dioxide gas. This is what creates the "rise" in baked goods.

* The reaction is chemical. The chemical reaction between the acid and baking soda creates new substances (carbon dioxide, water, and a salt). These new substances cannot be easily reversed back to their original components.

* The carbon dioxide escapes. The carbon dioxide bubbles that form are what give baked goods their texture. These bubbles escape into the air, so you can't get them back.

In short, once baking powder reacts with water, the process cannot be reversed to get the original ingredients back.