`FeSO_4` solution mixed with `(NH_4)_2SO_4` solution is `1:1` molar ratio gives the test of `Fe^(2+)` ion but `CuSO_4` solution mixed with aqueous ammonia in `1:4` molar ratio does not give the test of `Cu^(2+)` ion. Explain why?

Explanation

`FeSO_(4)` solution mixed with `(NH_(4))_(2)SO_(4)` solution in 1 : 1 molar ratio forms a double salt, `FeSO_(4). (NH_(4))_(2)SO_(4).6 H_(2)O` (Mohr salt) which ionizes in the solution to give `Fe^(2+)` ions. Hence, it gives the tests of `Fe^(2+)` ions.
`CuSO_(4)` solution mixed with `(NH_(4))_(2)SO_(4)` solution in 1 : 1 molar ratio forms a complex salt, with the formula `[Cu(NH_(3))_(4)]SO_(4)`. The complex ion, `[Cu(NH_(3))_(4)]^(2+)` does not ionize to give `Cu^(2+)` ions. Hence, it does not give the tests of `Cu^(2+)` ion.