Colloidal solution of gold cannot be prepared by:

Which of the following statements is incorrect about colloidal solutions?

Why do we add alum to purify water?

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.

$A$ negatively charged suspension of clay in water will need for precipitation the minimum amount of

The density of gold is $19 \, g/cm^3$. If $1.9 \times 10^{-4} \, g$ of gold is dispersed in $1 \, L$ of water to form a sol of spherical gold particles of radius $10 \, nm$,find the number of gold particles per $nm^3$ in the sol.