Question

Although $Au$ is soluble in aqua regia but $Ag$ is not. Explain.

Answer 1

Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions ($A{u}^{3+}$). The hydrochloric acid provides a ready supply of chloride ions (Cl−), which react with the gold ions to produce tetrachloroaurate(III) anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors formation of chloroaurate anions ($AuC{l}_4^{-}$).

$Au+3HN{O}_3+4HCl\rightleftharpoons [AuC{l}_4{]}^{-}+3[N{O}_2]+[H_{3}O{]}^{+}+2H_{2}O$

Silver actually forms a silver chloride layer that protects the metal from further attack. Silver can only be attacked by Aqua regia when the silver chloride layer is ruptured.