Aqueous solutions and precipitates of zinc

Zinc only exists in oxidation state +2 in its aqueous solutions. The zinc (II) ion is colorless and amphoteric. In acidic and neutral solutions, it exists as the hexaaqua ion Zn(H2O)62+(aq). When a small amount of hydroxide is added, then the hydrous hydroxide is formed as a coarse precipitate, and when excess of hydroxide is added, then the precipitate redissolves again, forming zincate ion, Zn(OH)3(H2O)(aq). At very high pH, the following ion is said to be formed: Zn(OH)42-(aq).


Oxidation state +2

As stated already, zinc ion is colorless. The precipitate of zinc hydroxide is white and the zincate ion, formed on adding a large excess of hydroxide is colorless again.

The three pictures below show a solution of zinc chloride (at the left), the same solution, to which a small amount of a concentrated solution of sodium hydroxide is added, and finally at the right, the redissolved precipitate after adding much more solution of sodium hydroxide.






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