Colloidal solution cannot be obtained from two such substances which are:

Which one of the following does not involve coagulation?

Gold number is defined as:

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$ : The diameter of colloidal particles in solution should not be much smaller than the wavelength of light to show Tyndall effect.
Reason $R$ : The light scatters in all directions when the size of particles is large enough.
In the light of the above statements,choose the correct answer from the options given below:

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

Give an example of a solid sol in which the dispersed phase is in a gaseous state.