Which of the following substances is not used for preparing lyophilic sols?

The colour of a colloidal solution is due to

Which of the following colloidal solutions cannot be prepared by Bredig's arc method?

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

The flocculation value of $HCl$ for arsenic sulphide sol is $30 \; mmol \; L^{-1}$. If $H_{2}SO_{4}$ is used for the flocculation of arsenic sulphide,the amount,in grams,of $H_{2}SO_{4}$ in $250 \; mL$ required for the above purpose is (molecular mass of $H_{2}SO_{4} = 98 \; g/mol$)

The colour of the sky is due to