The main ingredient of gel ice packs is water. Water has a high latent heat capacity, which makes it an ideal heat reservoir for keeping food, drinks and injured body parts cool. Though the composition varies by brand, the water is usually supplemented with either hydroxyethyl cellulose or silica gel, coated with vinyl, to keep the water from freezing into a solid block of ice.
Ice packs fall into two broad categories: those that have to be frozen in advance and those that become instantly cold when activated. People must freeze the water and cellulose and water and silica gel packs in advance of use, but they have a large heat capacity that keeps them cold for extended periods. They are most commonly used in coolers as a refrigerant for food and drinks.
Unlike reusable gel packs, instant ice packs do not have to be refrigerated or frozen before use. Instant ice packs contain quantities of water and ammonium nitrate, which are kept separated by interior partitions. When sufficient force is applied to break the partitions, the water and ammonium nitrate mix and begin an endothermic reaction that drains heat from its surroundings. This instantly cools the pack and keeps it cold for several minutes, making ideal for first aid.