How do cold packs work?

Instant hot and cold packs are used by athletes to quickly and conveniently treat an injury. They last for about 20 minutes. The packs take advantage of chemicals that either absorb a lot of heat or release a lot of heat when dissolved in water. When a chemical process absorbs a lot of heat it is called endothermic. When heat is released, it is called exothermic.

In cold packs, ammonium nitrate is used because it absorbs a lot of heat when it dissolves in water. The water and ammonium nitrate are in separate compartments in the pack. When the cold pack is needed, the chambers are broken and the ammonium nitrate dissolves in the water, absorbing heat and making the pack as cold as 0C.

In hot packs, calcium chloride or magnesium sulphate are used, along with the water. They release heat when the chambers are broken and the chemical dissolves in the water. A hot pack can reach a temperature of 90C.