Chemical reactions either release or store energy. In endothermic reactions, energy is stored as heat, and you might notice a decrease in temperature. In exothermic reactions, heat energy is released, and temperature will increase. Chemical reactions store or release chemical energy, too. Chemical energy is stored in chemical bonds, and when bonds break, that energy is released. According to the passage, what evidence can show if a reaction is endothermic or exothermic?

a. the presence of bubbles
b. a temperature change
c. glass breaking