Gold Sol can be prepared by
Preparation of Gold sol
2AuCl3(s)Gold(III)Chloride+3HCHO(aq)Formaldehyde+3H2O(l)Water→Reduction2Au(s)Solidsol+3HCOOH(aq)Formicacid+6HCl(aq)Hydrochloricacid
Therefore, gold Sol can be prepared by reduction of Gold(III) Chloride (AuCl3).