Question: Mention with reason the colour changes observed when:
(i) silver chloride is exposed to sunlight.
(ii) copper powder is strongly heated in the presence of oxygen.
(iii) a piece of zinc is dropped in copper sulphate solution. [CBSE, OD, Set 1, 2020]
Answer: (i) When silver chloride (white) is exposed to sunlight, it undergoes photochemical decomposition forming black metallic coloured silver and greenish yellow gas chlorine. Thus, the change in colour is observed due to the oxidation of silver chloride in the presence of sunlight.
(ii) Heated copper metal reacts with oxygen to form the black copper oxide. Thus, the change in colour is observed when copper powder is strongly heated.
2Cu (s) + O₂ (g) → 2CuO (s)
(iii) On adding zinc to CuSO₄ solution, zinc displaces copper from copper sulphate and forms zinc sulphate solution. This is indicated by colour change from blue to colourless. CuSO4 solution has a blue colour while ZnSO4 solution is colourless.
Zn(s) + CuSO₄ (aq) → Cu(s) + ZnSO₄ (aq)