Ammonium nitrate dissolves in water with an endothermic reaction, a chemical reaction that consumes heat rather than releasing it. The temperature of the solution is lower than the starting temperature of either component chemical. Ammonium nitrate's solubility increases as the water temperature rises.
The endothermic reaction between water and ammonium nitrate is used in chemical cold packs. When the two compounds are allowed to mix, the reaction lowers the surrounding temperature. This makes it effective at cooling small insulated containers such as coolers and lunch boxes.
Ammonium nitrate is capable of absorbing water from the air at a relative humidity of 59.4 percent. In conditions more humid than this, it absorbs water until the solution liquefies. This can be prevented by keeping the relative humidity low, storing ammonium nitrate in a sealed container or combining it with other chemical compounds.
In addition to applications that utilize its reaction with water, ammonium nitrate is used in fertilizers and explosives. Ammonium nitrate is only moderately toxic, and is safe to use as a fertilizer on food crops. Its primary hazard is its explosive and oxidizing capabilities. When stored in large quantities and accidentally detonated or ignited, it is capable of causing significant damage.