How do instant hot and cold packs work?
(Lansing State Journal, March 15, 1995)
In cold packs, the chemical ammonium nitrate is often used because it absorbs a lot of heat when it dissolves in water. In other words, ammonium nitrate dissolves in water endothermically. Water and ammonium nitrate are kept in separate compartments in the pack until the pack is needed. Then the chambers are broken and the ammonium nitrate dissolves in the water, absorbing heat and making the pack as cold as 0 degrees Celsius.
In hot packs, calcium chloride or magnesium sulfate frequently are used because these chemicals dissolve in water exothermically. In other words, they release a lot of heat when they are dissolved in water. Hot packs can reach temperatures around 90 degrees Celsius. Hot and cold packs generally last about 20 minutes.