When you use a medical ice pack, you want it to be cold immediately. For this reason, many ice packs do not require previous freezing but instead create the cold temperatures with a reaction between an ammonium compound and water. To activate the medical ice pack, you simply break the compartment that separates the two substances. They mix and cause the pack temperature to drop.
Water
Water makes up the bulk of the volume of a standard medical ice pack. In order to prevent leaking, the water is mixed with a silica gel to create a malleable solid. Water reacts with ammonium compounds, such as ammonium nitrate or ammonium chloride to create a chemical reaction that cools the ice pack quickly. The water is likely to freeze into ice while the ice pack is in use, which makes the pack more rigid.
Ammonium Nitrate
Ammonium nitrate is an ionic compound with the chemical formula NH4NO3. Also known as ammonium salt, the compound consists of colorless and odorless crystals that can irritate the skin if released from the medical ice pack. When mixed with water, a cooling reaction occurs when the ionic compound dissolves. This lowers the temperature of the overall ice pack. This reaction is not reversible, so medical ice packs are only good for a single use.
Ammonium Chloride
While ammonium nitrate is more common, some medical ice packs use ammonium chloride, or NH4Cl, instead. This ionic compound provides less irritation to the skin if it escapes from the ice pack and is somewhat safer for use as a result. When released into the water gel, ammonium chloride dissolves and removes heat from the solution. The result is the rapidly cooling ice pack. Ammonium chloride-based ice packs may also only be used one time.
