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Hydration energy (also hydration enthalpy) is a term for energy released upon attachment of water molecules to ions. 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 lower than the lattice energy, solvation occurs with a release of energy in the form of heat. For instance, CaCl2 (anhydrous calcium chloride) cools the water when dissolving. However, the hexahydrate, CaCl2·6H2O heats the water upon dissolution. The latter happens because the hydration energy does not completely cover the lattice energy, and the remainder has to be taken from the water in order to compensate the energy loss.