<P> Water has a much higher latent heat of fusion than most substances, and a melting temperature which is convenient and easily attained with, for example, a household freezer . Additives to improve the properties of water are often used . For example, substances can be added to prevent bacterial growth in the pack, or to prevent the water from solidifying so it remains a thick gel throughout use . </P> <P> Gel packs are often made of non-toxic materials that will remain a slow - flowing gel, and therefore will not spill easily or cause contamination if the container breaks . Gel packs may be made by adding hydroxyethyl cellulose (Cellusize), sodium polyacrylate, or vinyl - coated silica gel . </P> <P> 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 . </P> <P> Instant cold packs are a convenient direct replacement for crushed ice used as first aid on sport injuries, and can be carried as first aid to remote or wilderness areas where ice is unavailable . </P>

How long does it take for freezer packs to freeze