Asimple but effective application of chemistry can be found in a well-stocked first aid kit. Cryotherapy (the application of something cold) can help to reduce swelling and pain from certain injuries of the soft tissues, for example sprains. For some injuries more than 2 days old, heat can be soothing and aid healing by causing blood vessels to dilate (get wider), bringing more blood to the damaged area. If you do not happen to have a freezer full of ice handy or a convenient hot water bottle, chemistry can provide relief in the form of cold packs and heat packs by employing knowledge of enthalpy changes when substances dissolve in water.
When an ionic solid dissolves in water, the ions are separated from the lattice and spread out in the solution, with each ion surrounded by water molecules (i.e. the ions are hydrated). Energy has to be supplied to break up an ionic lattice, i.e. it is an endothermic process. The hydration of ions is an exothermic process (i.e. energy is released). The enthalpy change of solution (ΔHsolution) is the net result of these two processes. If the hydration of the ions provides more energy than is needed to break up the lattice, the extra energy is released as heat — the process is exothermic. If slightly more energy is needed to dissolve the lattice than is provided by the hydration of the ions, then that extra energy comes from the surroundings, the solution cools down and the process is said to be endothermic.
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