Hydration energy (also hydration enthalpy) is the amount of energy released when one mole of ions undergo hydration which is a special case of solvation. It is a special case of dissolution energy, with the solvent being water.
For example, upon dissolving a salt in water, the outermost ions (those at the edge of the lattice) move away from the lattice and become covered with the neighboring water molecules. If the hydration energy is equal to or greater than the lattice energy, then the salt is water-soluble. In salts for which the hydration energy is higher than the lattice energy, solvation occurs with a release of energy in the form of heat. For instance, CaCl2 (anhydrous calcium chloride) heats the water when dissolving. However, the hexahydrate, CaCl2ยท6H2O cools the water upon dissolution. The latter happens because the hydration energy does not completely over come the lattice energy, and the remainder has to be taken from the water in order to compensate the energy loss.