Question

Aqueous copper sulphate solution (blue in colour) gives:
(i) a green precipitate with aqueous potassium fluoride, and
(ii) a bright green solution with aqueous potassium chloride Explain these experimental results.

Answer 1

Aqueous $CuSO_4$ exists as $[Cu(H_2O)_4]SO_4$. It is blue in colour due to the presence of $[Cu[H_2O)4] ^{2+}$ ions.
(i) When $KF$ is added:
$[Cu(H_2O)_4]^{2+}+4F^{-}\rightarrow[Cu(F)_4]^{2-}+4H_2O$
(green)

(ii) When $KCl$ is added:$[Cu(H_2O)_4]^{2+}+4Cl^{-}\rightarrow[CuCl_4]^{2-}+4H_2O$
(bright green)
In both these cases, the weak field ligand water is replaced by the $F^{-}$ and $Cl^{-}$ ions.