Mostly the energy it takes to separate the ions of salt.
What factors affect temperature changes when salts dissolve in water
Here's a quick answer.
- It takes energy to break the bonds between the ions in the salt.
- Energy is released when water molecules solvate the ions.
- If it takes more energy to separate the ions of the salt than is released when the ions are solvated, the temperature goes down.
- If it takes less energy to separate the ions of the salt than is released when the ions are solvated, the temperature goes up.
Many ice packs contain a salt like ammonium nitrate that is separated from the water by a thin membrane. When you squeeze the pack, the membrane breaks, and the salt dissolves in the water.
NH₂NO₃(s) + H₂O(l) –> NH₃⁺(aq) + NO₃⁻(aq); Δ
The reaction is endothermic, so it absorbs heat from its surroundings. The ice pack can get cold enough to treat minor headaches and athletic injuries.