Explain chemical methods,Bredig's Arc method and peptization method for the preparation of colloids.

(N/A) Chemical methods : Colloidal dispersions can be prepared by chemical reactions leading to formation of molecules by double decomposition,oxidation,reduction or hydrolysis. These molecules then aggregate leading to formation of sols.
$As_{2}O_{3} + 3 H_{2}S \xrightarrow{\text{Double decomposition}} As_{2}S_{3 \text{ (sol)}} + 3 H_{2}O$
$SO_{2} + 2 H_{2}S \xrightarrow{\text{Oxidation}} 3 S_{\text{(sol)}} + 2 H_{2}O$
$2 AuCl_{3} + 3 HCHO + 3 H_{2}O \xrightarrow{\text{Reduction}} 2 Au_{\text{(sol)}} + 3 HCOOH + 6 HCl$
$FeCl_{3} + 3 H_{2}O \xrightarrow{\text{Hydrolysis}} Fe(OH)_{3 \text{ (sol)}} + 3 HCl$
Electrical disintegration or Bredig's Arc method: This process involves dispersion as well as condensation. Colloidal sols of metals such as gold,silver,platinum etc.,can be prepared by this method. In this method,electric arc is struck between electrodes of the metal immersed in the dispersion medium. The intense heat produced vapourises the metal,which then condenses to form particles of colloidal size.
Peptization : Peptization may be defined as the process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this purpose is called peptizing agent.
This method is applied to convert a freshly prepared precipitate into a colloidal sol. During peptization the precipitate adsorbs one of the ions of the electrolyte on its surface. This causes the development of positive or negative charge on precipitates,which ultimately break up into smaller particles of the size of a colloid.