Energy plays an important role in chemical reactions. A chemical reaction that stores energy is called an endothermic reaction. In photosynthesis, solar energy is stored in the sugar molecules. Most endothermic reactions store energy in the form of heat. Another endothermic reaction is putting a chemical ice pack on an injury. The chemicals inside are stored separately, but when mixed feel cool against the skin.

Exothermic reactions are the opposite of endothermic. These release energy. Burning something is an exothermic reaction. So is the reaction that digests your food. As food molecules are broken down, energy is released that the body uses to function.