Why does Diet Coke and Mentos make a chemical reaction?

Diet coke and Mentos do not undergo a chemical reaction. When Mentos is dropped in diet coke, the rough surface of the candy serves as a nucleus for bubbles to attach to. As more and more bubbles in the diet coke attach to the Mentos they get larger and eventually are released from the surface of the candy. They rise to the surface forming the large, bubbly reaction we are familiar with. Chemical reactions only happen when there is a rearrangement of atoms, not a change in the physical state of the molecules. Since the bubbles were already present in the diet coke and simply stick together in larger numbers, there is no chemical reaction.