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.