A white precipitate is obtained when a solution is diluted with H 2 O and boiled. On addition of excess NH 4 Cl/NH 4 OH, the volume of precipitate decreases leaving behind a white gelatinous precipitate. Identify the precipitate which dissolves in ammonia solution or NH 4 Cl.
Text Solution
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[Zn(OH) 4 ] 2– 
Zn(OH) 2 ↓ (white) + 2OH –
Zn(OH) 2 precipitate is readily soluble in excess of ammonia and in solutions of ammonium salts due to the formation of tetraamminezinc(II).
Zn(OH) 2 + 4NH 3 
[Zn(NH 3 ) 4 ] 2+ (soluble complex) + 2OH –
or
Zn(OH) 2 + 4NH 4 OH 
[Zn(NH 3 ) 4 ] 2+ + 4H 2 O + 2OH – .
But Al(OH) 3 , Mg(OH) 2 and Ca(OH) 2 don’t dissolves in excess of NH 3 solution.
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