Question

How are the following colloidal solutions prepared?
i.Sulphur in water
ii. Gold in water

Answer 1

A colloid is a heterogeneous system that consists of one substance as a dispersed phase that has fine particles. It is dispersed on another surface called a dispersion medium. The size of colloidal particles is from 1 to 1000 nanometer. Here the colloidal solutions contain water as dispersion medium and sulphur and gold as dispersed phase.

Complete answer:
A colloidal solution is a solution made of a dispersed phase and the dispersed medium. The particles of a dispersed phase are thin and fine, thus they are allowed to mix with the dispersed medium making a colloid. There are various methods of preparation of colloids, like peptisation, hydrolysis, oxidation, reduction, etc. Colloidal solutions can be termed as sol. The given sols can be made as:
i.Sulphur in water
The sulphur in water colloidal solution or sulphur sol can be prepared by the oxidation of ${{H}_{2}}S$ and$S{{O}_{2}}$ as:
$S{{O}_{2}}+2{{H}_{2}}S\to 3S(sol)+2{{H}_{2}}O$
ii. The gold in water colloidal solution that is termed as gold sol is prepared by the reduction of$AuC{{l}_{3}}$ with formaldehyde (methanal) as:
$2AuC{{l}_{3}}+3HCHO+3{{H}_{2}}O\to 2Au(sol)+3HCOOH+6HCl$
Hence, the sulphur and gold sol are prepared by oxidation and reduction respectively.

Note:
Colloids are of 2 types, lyophobic sol, where dispersion phase repels the dispersion medium, they are formed by chemical and special methods. Lyophilic sols that have dispersion phase attracting the dispersion medium, they can be simply formed by mixing. Sulphur and gold sol are examples of lyophobic sols.