A hot pack and a cold pack are examples of miniature chemical reactions, specifically endothermic and exothermic reactions.
An endothermic reaction is a chemical reaction that absorbs heat from its surroundings, causing the temperature to decrease. Hot packs use an endothermic reaction to generate heat. They typically contain a substance, such as calcium chloride, that absorbs water and releases heat as it does so. This heat is then transferred to the injury, helping to increase blood flow and reduce pain and swelling.
On the other hand, an exothermic reaction is a chemical reaction that releases heat to its surroundings, causing the temperature to increase. Cold packs use an exothermic reaction to generate cold. They typically contain a substance, such as ammonium nitrate, which when mixed with water produces a cooling effect by releasing heat. This cold is then transferred to the injury, helping to decrease blood flow and reduce inflammation and pain.