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.

Heat is given off as a result of a chemical reaction.

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.

If the dissolving of the chemical in water is an endothermic process and absorbs heat energy, it is a good candidate for making a cold pack because this process will lower the temperature of the content of the pack. If the dissolving of the chemical in water is an exothermic process and releases heat energy, it is a good candidate for making a hot pack because this process will raise the temperature of the content of the pack.

1. How do cold packs work?

2. How do hot packs work?

3. How do you know that a reaction is "exothermic?"

4. How do you know that a reaction is "endothermic?"

5. Who uses hot packs and cold packs?

6. What is the chemical that is used in hot packs that use a super cooled fluid? Why is that chemical a good choice for use to manufacture a hot pack?