How an instant cold pack gets cold
Dissolution of ammonium nitrate · Dissolution · Endothermic
An instant cold pack has no ice and no battery; a dissolving salt pulls heat out of the water around it.
Reactants approach
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ΔH = +28 kJ · endothermic, absorbs heat
ΔH per mol NH4NO3 at infinite dilution (measured about +26 kJ in real packs).
Meet the molecules
- Reactantsolid
NH4NO3
ammonium nitrate
- Productdissolved in water
NH4+
ammonium ion
- Productdissolved in water
NO3−
nitrate ion
What you notice
Sports first-aid kits carry instant cold packs: squeeze the pack, shake it, and within seconds it feels icy, staying cold for around 15 to 20 minutes, long enough to reduce swelling on a sprained ankle. Inside is a pouch of water and solid ammonium nitrate; some brands use urea instead. Squeezing breaks the inner pouch so the solid dissolves. No ice or electricity is involved: dissolving this salt absorbs heat from the water, so the water and the pack cool down and then draw heat from your skin.
Try it
Dissolve several spoons of Epsom salt in a small glass of room-temperature water and feel the glass cool; a kitchen thermometer shows a drop of a few degrees.
Safety: Ammonium nitrate is an oxidizer used in fertilizer and explosives, so throw a used pack away rather than heating it or storing it near fuels.
Step by step
- 1
Breaking the lattice
Pulling NH4+ and NO3– ions out of the crystal costs energy, the lattice enthalpy.
- 2
Water surrounds the ions
Water molecules cluster around each ion and release energy as ion–dipole attractions form, but slightly less energy than the lattice cost.
- 3
Net heat is absorbed
Overall, about 26–28 kJ is absorbed per mole (80 g) of salt. It comes from the water's own thermal energy, so the temperature drops.
- 4
Entropy pays the bill
The salt still dissolves on its own because ions spreading through the solution increase entropy enough to make ΔG negative.