Zinc hydroxide Zn( OH)₂ is an

inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemist ...

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

. It also occurs naturally as 3 rare minerals: wülfingite (orthorhombic), ashoverite and

sweetite Sweetite has a general formula of Zn(OH)2. The name is given after a curator of mineral department of The British Museum, Jessie May Sweet (1901–1979). It occurs in an oxidized vein in limestone bedrock with galena, ashoverite, wülfingite, ang ...

(both tetragonal). Like the hydroxides of other metals, such as

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form m ...

tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...

and chromium, Zinc hydroxide (and

Zinc oxide Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement ...

), is

amphoteric In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used. One type of amphoteric species are amphipro ...

. Thus it will dissolve readily in a dilute solution of a

strong acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions ...

, such as HCl, and also in a solution of an alkali such as sodium hydroxide.

Preparation

It can be prepared by adding sodium hydroxide solution, but not in excess, to a solution of any zinc salt. A white precipitate will be seen: :Zn²⁺ + 2 OH⁻ → Zn(OH)₂. Zn²⁺ is known to form hexa-aqua ions at high water concentrations and tetra-aqua ions at low concentrations of water Sze, Yu-Keung, and Donald E. Irish. "Vibrational spectral studies of ion-ion and ion-solvent interactions. I. Zinc nitrate in water." Journal of Solution Chemistry 7.6 (1978): 395-415. and, thus, this reaction may be better written as the reaction of the aquated ion with hydroxide through donation of a proton, as follows. : Zn²⁺(OH₂)_4(aq) + OH⁻_(aq) → Zn²⁺(OH₂)₃OH⁻_(aq) + H₂O_(l) Subsequent reactions discussed below can also, therefore, be considered as reactions with the aquated zinc ion and one can adjust them accordingly. However, for simplicity, the water molecules are omitted from here-on. If excess sodium hydroxide is added, the precipitate of zinc hydroxide will dissolve, forming a colorless solution of zincate ion: :Zn(OH)₂ + 2 OH⁻ → Zn(OH)₄²⁻. This property can be used as a test for zinc ions in solution, but it is not exclusive, since aluminum and lead compounds behave in a very similar manner. Unlike the hydroxides of aluminum and lead, zinc hydroxide also dissolves in excess aqueous

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

to form a colorless, water-soluble

ammine complex In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia () ligand. "Ammine" is spelled this way due to historical reasons; in contrast, alkyl or aryl bearing ligands are spelt with a single "m". Almost ...

. Zinc hydroxide will dissolve because the ion is normally surrounded by water ligands; when excess sodium hydroxide is added to the solution the hydroxide ions will reduce the complex to a −2 charge and make it soluble. When excess ammonia is added, it sets up an equilibrium which provides hydroxide ions; the formation of hydroxide ions causes a similar reaction as sodium hydroxide and creates a +2 charged complex with a co-ordination number of 4 with the ammonia ligands - this makes the complex soluble so that it dissolves.

Applications

One major use is as an absorbent in

surgical dressings A dressing is a sterile pad or compress applied to a wound to promote wound healing, healing and protect the wound from further harm. A dressing is designed to be in direct contact with the wound, as distinguished from a bandage, which is most oft ...

. It is also used to find zinc salts by mixing sodium hydroxide with the suspect salt.

References

* Chemistry in Context - By Graham Hill, John Holman (pp. 283,284)

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...

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