Baking cookies is an example of a chemical change. Unlike boiling water or mixing cabbage and carrots, baking cookie dough involves reactions that create a new item with a chemical composition that is different than that of the original parts.
A physical reaction, on the other hand, does not involve an alteration in a substance's chemical makeup. Instead, it involves only a change in shape or phase. To help make the distinction, one important indicator of a chemical reaction is the presence of heat or light energy, either added to make the reaction occur or generated by the process itself.