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Hydroxide lone pairs-2D.svg

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

Inorganic compounds that contain the hydroxyl group are referred to as hydroxides. Alkali metal (Li+, Na+, K+) and alkaline earth (Mg2+, Ca2+, Ba2+) salts containing hydroxide are common bases. Common hydroxides include


Chemical properties

When hydroxide ion is mixed with hydrogen gas, water forms. This is why water has a neutral acidity. The H+ ion cancels the basic nature of the OH- ion.


Hydroxide as a base

When hydroxides are used as bases, they usually react with hydrogen ions in the acid and form water.

HCl + NaOH → NaCl + H2O

Hydroxide as nucleophile

Hydroxide is also a strong nucleophile, converting halides to hydroxides. Characteristically hydroxides absorb carbon dioxide to give carbonates and bicarbonates:

NaOH + CO2 → NaHCO3

Esters are saponified (converted into soap-like products) by the action of hydroxides:

NaOH + RCO2R' → RCO2Na + R'OH

As a 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].


Many inorganic hydroxide salts are insoluble in water. Exceptions include hydroxide salts of the Group I, Ba2+, Sr2+, Ca2+ (slightly) or Tl+.


Hydroxides and hydroxide ions are extremely useful and are the coproduct in the large chlor-alkali process. Sodium hydroxide (lye) and potassium hydroxide are particularly large scale intermediates. Many hydroxides are useful minerals such as the aluminium ore bauxite is composed largely of aluminium hydroxides. Goethite and limonite are low grade brown iron ores.

Hydrated hydroxide ion H3O2

The mono-hydrated hydroxide ion,H3O2, the bihydroxide ion, has been found in a small number of compounds. 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.

See also


  1. ^ Kamal Abu-Dari, Kenneth N. Raymond, Derek P. Freyberg (1979). "The bihydroxide (H3O2-) anion. A very short, symmetric hydrogen bond". J. Am. Chem. Soc. 101: 3688–3689. doi:10.1021/ja00507a059. 

External links

Simple English

Hydroxide in chemistry is a name given to the diatomic anion OH, made up of oxygen and hydrogen atoms. Most chemical compounds that contain hydroxides are bases.

An Arrhenius base is a substance that when dissolved in an aqueous solution, it produces hydroxide ions. Therefore, hydroxide ions are heavily involved in acid-base reactions.

Many useful chemical reactions or processes involve hydroxide or hydroxide ions. Sodium hydroxide 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 iron ore. The aluminium ore bauxite is made mainly out of aluminium hydroxides.

Most inorganic hydroxide salts do not dissolve in water.

Uses of hydroxides

Many useful chemicals and chemical processes involve hydroxides or hydroxide ions. Sodium hydroxide is used in factories as an alkali, potassium hydroxide is used in farming, and iron hydroxide have been used as low grade brown iron ore.

List of hydroxides


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