Hydration: definition in chemistry
Hydration: Solvation in which the solvent is water, or a reaction in which water is added to a compound.
When an ionic compound dissolves in water, each ion becomes surrounded by water molecules: the slightly negative oxygen ends face cations and the slightly positive hydrogen ends face anions. Some solids hold water inside their crystals, forming hydrates such as copper(II) sulfate pentahydrate (CuSO₄·5H₂O). In organic chemistry, hydration means adding water across a double bond, as when ethene is converted to ethanol.
Examples
- Na⁺ and Cl⁻ ions surrounded by water molecules in salt water
- Blue CuSO₄·5H₂O turning white when heating drives off its water
- Ethene + water → ethanol, an industrial hydration reaction
- Plaster of Paris setting as it takes up water to form gypsum
On the periodic table
Hydration energy is largest for small, highly charged ions, so it is greater at the top of a group and much greater for Al³⁺ than for Na⁺.