Question

Aqueous copper sulphate solution (blue in colour) gives a bright green solution with aqueous potassium chloride. Explain this experimental result.

Answer 1

Aqueous copper sulphate solution exists as [Cu(H₂O)₄]SO₄ and is blue in colour due to the presence of [Cu(H₂O)₄]²⁺ ions.

When KCl solution is added, the Cl^– ligands replace the weak H₂O ligands and form [CuCl₄]²⁻ ions, which are bright green in colour.

$$[\mathrm{Cu}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_4]{\phantom{A}}^{2+}+\underset{[\mathrm{From}\mathrm{KCl}{\phantom{A}}_{(\mathrm{aq})}]}{4\mathrm{Cl}{\phantom{A}}^{-}}⟶\underset{\mathrm{solution})}{\underset{(\mathrm{Bright}\mathrm{green}}{[\mathrm{CuCl}{\phantom{A}}_4]{\phantom{A}}^{2-}}}+4\mathrm{H}{\phantom{A}}_2\mathrm{O}$$