Instant cold pack
An instant cold pack is a device that consists of two bags; one containing water, inside a bag containing ammonium nitrate, calcium ammonium nitrate or urea. When the inner bag of water is broken by squeezing the package, it is allowed to dissolve the solid in an endothermic reaction. This reaction absorbs heat from the surroundings, quickly lowering the pack's temperature.
Instant cold packs are a convenient direct replacement for crushed ice used as first aid on sport injuries. It is also important not to refrigerate before activation. This may result in frostbite if used for prolonged periods of time.
For safety reasons, instant cold packs have recently been developed without ammonium nitrate.