Colloidal solution of arsenious sulphide is coagulated by

Muddy water can be purified through coagulation using

What type of reaction is used to obtain $Au$ (sol)?
Reaction: $2AuCl_3 + 3HCHO + 3H_2O \rightarrow 2Au (sol) + 3HCOOH + 6HCl$

Fog is an example of ...

Among the following methods:
$(i)$ Addition of an electrolyte
$(ii)$ Electrophoresis
$(iii)$ Addition of a protective colloid
$(iv)$ Addition of an oppositely charged sol
the coagulation of a lyophobic sol can be carried out by:

The Tyndall effect is observed only when the following conditions are satisfied $:$
$1$. The diameter of the dispersed particles is much smaller than the wavelength of the light used.
$2$. The diameter of the dispersed particle is not much smaller than the wavelength of the light used.
$3$. The refractive indices of the dispersed phase and dispersion medium are almost similar in magnitude.
$4$. The refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude.