WAEC Chemistry 2021 Paper

Concept:Aqueous ammonia acts first as a precipitating agent and then as a complexing agent for copper(II) ions.Explanation:When ammonia solution is added in drops to a solution containing copper(II) ions, a pale blue precipitate of copper(II) hydroxide is formed.For example, with copper(II) sulfate:$$\mathrm{CuSO_4(aq) + 2NH_3(aq) + 2H_2O(\ell) \rightleftharpoons Cu(OH)_2(s) + (NH_4)_2SO_4(aq)}$$This blue precipitate is $\mathrm{Cu(OH)_2}$.On adding excess ammonia solution, the blue precipitate dissolves to give a deep blue solution.The reaction is:$$\mathrm{Cu(OH)_2(s) + 4NH_3(aq) \longrightarrow [Cu(NH_3)_4]^{2+}(aq) + 2OH^-(aq)}$$The deep blue colour is due to the formation of the tetraamminecopper(II) complex ion, $\mathrm{[Cu(NH_3)_4]^{2+}}$.Thus, the observation is a blue precipitate that dissolves in excess ammonia.Answer:Option A: blue precipitate is formed which is soluble in excess ammonia.