Colloidal solutions of metals like gold,silver and platinum are generally prepared by using

Which of the following reactions leads to the formation of a substance in the colloidal state (in the presence of excess of $HCl$)?

$X$ is a protecting colloid. The following data is obtained for preventing the coagulation of $10 \ mL$ of gold sol to which $1 \ mL$ of $10 \% NaCl$ is added. What is the gold number of '$X$'?

Expt. No.	Weight of $X$ (in $mg$) added to gold sol	Coagulation
$1$	$24$	Not prevented
$2$	$23$	Not prevented
$3$	$26$	prevented
$4$	$27$	prevented
$5$	$25$	prevented

Preparation of Lyophobic sols by chemical method involves

Which one of the following is used in the hardening of leather?

Which of the following can form a colloidal sol with water?