It\'s due to the reverse of the formation reaction of CdS CdCl2 + H2S(g) <--> CdS + 2HCl(aq) if we make the HCl strong enough we can force the equilibrium the other way, and CdCl2 is soluble. Solution It\'s due to the reverse of the formation reaction of CdS CdCl2 + H2S(g) <--> CdS + 2HCl(aq) if we make the HCl strong enough we can force the equilibrium the other way, and CdCl2 is soluble..