Question

Describe about condensation methods of preparation of colloids. (OR) Describe chemical methods of preparation of colloids. When the substance for colloidal particle is present as smalL sized particle, molecule or ion, they are brought to the colloidal dimension by condensation methods.

Answer 1

1. Oxidation method: When hydroiodic acid is treated with iodle acid J, sol is obtained.

HIO₃ + 5HI → 3H₂O + 3I_2(sol)

2. Reduction method: Gold sol is prepared by reduction of aune chloride using formaldehyde. 

2 AuCl₃ + 3HCHO + 3H₂O → 2 Au_(sol) + 6HCI + 3HCOOH

3. Hydrolysis: Ferric chloride is hydrolysed to get ferric hydroxide colloid

FeCl₃ + 3H₂O → Fe(OH)₃(sol) + 3HCI 

4. Double decomposition: When hydrogen suiphide gas is passed through a solution of arsenic oxide, a yellow coloured arsenic sulphide is obtained as a colloidal solution.

As₂O₃ – 3H₂S → As₂S₃ + 3H₂O

5. Decomposition: When few drops of an acid is added to a dilute solution of sodium thiosulphate, sulphur colloid is produced by the decomposition of sodium thio sulphate.

S₂O₃^2- + 2H⁺ → S_(sol) + H₂O + SO₂