(i) A colloidal solution of sulphur can be obtained by passing hydrogen sulphide gas into a solution of sulphur dioxide in water.
SO2 + 2H2S → 3S + 2H2O
(ii) A colloidal solution of gold in water can be prepared by reducing auric chloride with stannous chloride.
2AuCl3 + 3SnCl2 → 2Au + 3SnCl4
