|Jmol-3D images||Image 1|
|Molar mass||99.424 g/mol|
|Density||3.053 g/cm3, solid|
|Melting point||125 °C (decomposition)|
|Solubility in water||slightly soluble|
|Solubility product, Ksp||3.0×10−16|
|Solubility in alcohol||insoluble|
|Std enthalpy of
|EU Index||not listed|
|Other anions||Zinc oxide|
|Other cations||Cadmium hydroxide|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Like the hydroxides of other metals, such as lead, aluminium, beryllium, tin and chromium, zinc hydroxide (and zinc oxide), is amphoteric. Thus it will dissolve readily in a dilute solution of a strong acid, such as HCl, and also in a solution of an alkali such as sodium hydroxide.
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.
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 to form a colorless, water-soluble ammine complex.
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.
One major use is as an absorbent in surgical dressings.
- Chemistry in Context - By Graham Hill, John Holman (pp. 283,284)
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