Chemicals can store energy and release it in the form of heat. A chemical reaction that releases heat is called an exothermic reaction. But chemical reactions can also absorb heat from the environment and get cold. These reactions are called endothermic reactions. When chemicals are dissolved in water, sometimes heat is released, and sometimes heat is absorbed.
Hot/cold packs are used by athletes to minimize swelling of injuries such as muscle and joint sprains. They are constructed of a large pouch containing a dry chemical plus an inner pouch of water. The hot/cold pack is activated by breaking the seal on the pouch of water and shaking the pack vigorously. This action mixes the water with the chemical starting the exothermic or endothermic reaction.
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