The chemistry behind cold packs and hot packs

A cold pack or hot pack is a compact energy-transfer device. When activated, it changes the temperature at its surroundings by absorbing or releasing heat. This simple effect comes from familiar chemical ideas: dissolution, crystallisation, oxidation and the movement of thermal energy.

For students, athletes and families in Australia, these packs are common in first-aid kits, pharmacies and supermarkets. They are useful after a Saturday football match in Melbourne, during a long-distance hike near Canberra, or when an injury occurs far from a refrigerator or kettle. Understanding the chemistry helps explain why each pack works and how to use it safely.

What endothermic and exothermic mean

An endothermic process absorbs heat from its surroundings. As the material inside a cold pack dissolves or reacts, energy moves from nearby skin, water or air into the chemical system. The surroundings cool because they have lost thermal energy.

An exothermic process releases heat. In a hot pack, chemical bonds form, ions arrange into crystals, or a substance reacts with oxygen. Energy then moves into the surrounding material, raising its temperature. Temperature change is therefore evidence of energy transfer, rather than a separate type of matter.

The enthalpy change, written as ΔH, describes this energy change at constant pressure. Endothermic processes have a positive ΔH, while exothermic processes have a negative ΔH. The total temperature change also depends on the mass of the substances, their specific heat capacities and how well the pack is insulated.

How an instant cold pack absorbs heat

Many single-use cold packs contain ammonium nitrate and water in separate compartments. Squeezing or striking the pack breaks an internal seal, allowing the solid to dissolve:

NH₄NO₃(s) → NH₄⁺(aq) + NO₃⁻(aq)

Separating the ions in the crystal lattice requires energy, and the hydration of those ions releases some energy. For ammonium nitrate, the energy required is greater than the energy released during hydration, so the overall dissolution is endothermic. Heat flows from the surrounding liquid and the user’s skin into the solution.

Some products use urea or other salts instead. The precise temperature drop depends on the formulation, starting temperature and quantity of water. A cold pack does not create “cold” as a substance; it removes heat from the area touching it.

In Australia, instant cold packs are sold through pharmacies, sports suppliers and first-aid retailers, while reusable gel packs are often kept in household freezers. A cloth barrier should be placed between either type and the skin, especially during prolonged use, because excessive cooling can cause skin damage.

Why hot packs release thermal energy

An instant hot pack may contain calcium chloride or magnesium sulfate. When calcium chloride dissolves, hydration of the ions releases more energy than is required to separate the ionic lattice:

CaCl₂(s) → Ca²⁺(aq) + 2Cl⁻(aq)

The solution becomes warmer because the released energy enters the water and the pack material. Calcium chloride is used for this purpose because its dissolution is strongly exothermic, although the product concentration must be controlled to manage the final temperature.

Reusable hot packs commonly use sodium acetate trihydrate. Heating the pack melts the crystals and produces a supersaturated liquid. Bending the small metal disc starts nucleation, allowing sodium acetate to crystallise rapidly. The liquid becomes solid again and releases the stored enthalpy of crystallisation as heat.

This process is a phase change rather than a new substance being created. The pack can be reset by heating it in water until all crystals dissolve, then allowing it to cool without disturbing it. Instructions vary, so Australian users should follow the manufacturer’s heating directions rather than placing a sealed pack directly in a microwave.

Air-activated warmers and reaction rates

Disposable hand warmers use a different exothermic process. They usually contain finely divided iron, salt, water, activated carbon and a porous material. Oxygen enters through the outer packaging and oxidises the iron:

4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)

This reaction releases heat gradually. Salt and water help create an electrolyte pathway, while carbon distributes heat and supports the reaction. The packet controls the rate by limiting oxygen flow, which is why opening it exposes the contents to air and starts the warming period.

The reaction rate depends on surface area, moisture, temperature and oxygen availability. Fine iron powder reacts more readily than a large iron object because more surface is exposed. This illustrates a wider analytical chemistry principle: measurements of ions and concentration can help explain how a system behaves, and resources such as potentiometry methods explore how electrical signals are used to study chemical solutions.

In alpine areas such as the Victorian High Country, warmers may be useful for outdoor activities, but they should not be placed directly against bare skin or used while sleeping. A warmer that becomes unusually hot, leaks or smells strongly should be removed and disposed of according to the product instructions.

Safe use, storage and disposal

Cold and hot packs should remain sealed. Their contents may irritate skin and eyes, and some salts can be harmful if swallowed. A damaged pack should be placed in a container, kept away from children and pets, and handled according to local waste guidance. Do not attempt to reproduce commercial packs by mixing concentrated chemicals at home.

For an injury, a cold pack is generally used for short periods with a fabric layer between the pack and skin. A hot pack should feel comfortably warm rather than painfully hot, particularly for children, older adults or anyone with reduced sensitivity. Medical advice is appropriate when swelling, pain or suspected serious injury persists.

The local climate also matters. In Brisbane or Darwin, a reusable cold pack may warm quickly outside a refrigerator, while in a dry Australian summer a hot pack can add unwanted heat to an already overheated body. Packs should be stored away from direct sunlight, vehicles and damp areas, where packaging can deteriorate.

The same principles apply across school laboratories and everyday first aid: identify the materials, observe the energy change and control contact with the surroundings. To see the chemistry in practice, record the starting temperature of water, activate one commercial pack according to its instructions, and measure the temperature every minute for ten minutes with a thermometer.