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In chemistry, hydroxide is the name for the diatomic anion OH−, consisting of oxygen and hydrogen atoms, usually derived from the dissociation of a base. It is one of the simplest diatomic ions known.

Inorganic compounds that contain the hydroxyl group are referred to as hydroxides. Common hydroxides include:
Hydroxide as a base

Most compounds containing hydroxide are bases.

An Arrhenius base is a substance that produces hydroxide ions when dissolved in aqueous solution. One example would be ammonia, NH3:

NH3(g) + H2O(l) ⇌ NH4+(aq) + OH−(aq)

Thus, hydroxide ions are heavily involved in acid-base reactions as well as the special double displacement reaction

Salts containing hydroxide are called base salts. Base salts will dissociate into a cation and one or more hydroxide ions in water, making the solution basic. Base salts will undergo neutralisation reactions with acids. In general acid-alkali reactions can be simplified to

OH−(aq) + H+(aq) → H2O(l)

by omitting spectator ions.

[edit] Solubility

Most inorganic hydroxide salts are insoluble in water, except for those with cations from Group I, Ba2+, Sr2+, Ca2+ (little) or Tl+.

[edit] Applications

Hydroxides and hydroxide ions are relatively common. Many useful chemicals and chemical processes involve hydroxides or hydroxide ions. Sodium hydroxide (lye) is used in industry as a strong base, potassium hydroxide is used in agriculture, and iron hydroxide minerals such as goethite and limonite have been used as low grade brown iron ore. The aluminium ore bauxite is composed largely of aluminium hydroxides.

[edit] Ligand

The hydroxide ion is a kind of ligand. It donates lone pairs of electrons, behaving as a Lewis base. Examples of complexes containing such a ligand include the aluminate ion [Al(OH)4]− and aurate ion [Au(OH)4]−.

[edit] Hydrated hydroxide ion H3O2−

The mono-hydrated hydroxide ion,H3O2−, the bihydroxide ion, has been found in the crystals of a small number of compounds. First found in 1979,in the salt Na2[N(C2H5)3CH3][Cr(PhC(S)=N-(O))3].½NaH3O2.18H2O[1] The H3O2− ion in this compound is centrosymmetric and has a very short hydrogen bond, (114.5 pm), which is similar to the length (114 pm) in the bifluoride, HF2− ion.