hydroxide

hydroxide, chemical compound that contains the hydroxyl (-OH) radical. The term refers especially to inorganic compounds. Organic compounds that have the hydroxyl radical as a functional group are called alcohols; the hydroxyl radical is also present in the carboxyl group of organic acids. Most metal hydroxides are bases—they form solutions that have an excess of OH^- ions and a pH greater than 7, neutralize acids, and change the color of litmus from red to blue. Alkali metal hydroxides such as sodium hydroxide are strong bases and are very soluble in water; alkaline-earth metal hydroxides such as calcium hydroxide (slaked lime) are much less soluble in water and are not as strongly basic. Magnesium hydroxide (milk of magnesia) is only slightly basic. Some hydroxides (e.g., aluminum hydroxide) exhibit amphoterism, having either acidic or basic properties depending on the reaction in which they are involved. The hydroxides of some nonmetallic elements are acidic; the hydroxide of sulfur, S(OH)₆, spontaneously loses two molecules of water to form sulfuric acid, H₂SO₄. Ammonium hydroxide, NH₄OH, is a weak base known only in the solution that is formed when the gas ammonia, NH₃, dissolves in water.

Any compound with one or more functional groups made up of one atom each of hydrogen and oxygen, bonded together and acting as the hydroxide anion (OH⁻). Hydroxides include the familiar alkalies of laboratory and industrial processes. Those of the alkali metals (lithium, sodium, potassium, rubidium, and cesium), the strongest bases, are the most stable and soluble; those of the alkaline earth metals (calcium, barium, and strontium), also soluble strong bases, are less stable. The hydroxides of most other metals are only slightly soluble but neutralize acids; some are “amphoteric,” reacting with both acids and bases. In compounds in which OH is un-ionized and covalently bonded (e.g., in methanol, CH₃OH), it is known as a hydroxyl group.

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In chemistry, hydroxide is the most common 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:

• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Calcium hydroxide (Ca(OH)₂)

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 called neutralization.

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.

Solubility

Most inorganic hydroxide salts are insoluble in water, except for those with cations from Group I, NH₄⁺, Ba²⁺, Sr²⁺, Ca²⁺ (little) or Tl⁺.

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.

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]⁻.

See also

• Hydronium
• Oxide
• Hydroxyl
• Hydroxy

Notes

• Solvation and Transport of H₃O⁺ and OH⁻ Ions in Water (JCP 99, 5749 (1995)