A chemical reaction occurs when a cake bakes. The materials in the batter are changed into a different substance. If the cake burns, that is a separate chemical reaction. In fact, any type of burning, such as a piece of paper or a stick of wood, is a chemical reaction.
In a chemical reaction, the chemical composition of matter changes from one thing to another. When this occurs, there is typically a noticeable change. For example, heat, light or gas is given off, a solid forms inside a liquid, or there is a color shift. With a chemical reaction, it is impossible for materials to go back to the way they were before the change occurred.