An instant cold pack is a device that consists of two bags, one containing water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, which is inside a second bag containing ammonium nitrate. When the inner bag is broken by squeezing the package, the ammonium nitrate is allowed to dissolve in the water in an endothermic

Endothermic

In thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...

 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

Ice

Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

 used as first aid

First aid

First aid is the provision of initial care for an illness or injury. It is usually performed by non-expert, but trained personnel to a sick or injured person until definitive medical treatment can be accessed. Certain self-limiting illnesses or minor injuries may not require further medical care...

 on sport injuries. It is also important not to refrigerate before activation. This may result in frostbite

Frostbite

Frostbite is the medical condition where localized damage is caused to skin and other tissues due to extreme cold. Frostbite is most likely to happen in body parts farthest from the heart and those with large exposed areas...

if used for prolonged periods of time.

For safety reasons, instant cold packs have recently been developed without ammonium nitrate.