Describe chemical methods of preparation of colloids.

Explanation

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.
i. Oxidation method:- When hydroiodic acid is treated with iodic acid `I_(2)` sol is obtained.
`HIO_(3)+5HI to 3H_(2)O +3I_(2"(sol)")`
ii. Reduction method:- Gold sol is prepared by reduction of auric chloride using formaldehyde.
`2AuCl_(3)+3HCHO+3H_(2)O to 2Au_((sol))+6HCl+3HCOOH`
iii. Hydrolysis:- Ferric chloride is hydrolysed to get ferric hydroxide colloid
`FeCl_(3)+3H_(2)O to Fe(OH)_(3)(sol)+3HCl`
iv. Double decomposition:- When hydrogen sulphide gas is passed through a solution of arsenic oxide, a yellow coloured arsenic sulphide is obtained as a colloidal solution.
`As_(2)O_(3)+3H_(2)S to As_(2)S_(3)+3H_(2)O`
v. 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.
`S_(2)O_(3)^(2-)+2H^(+) to S_((sol))+H_(2)O+SO_(2)`