Question

State why Aqua Regia dissolves Gold which is not soluble in any other acid?

Answer 1

Aqua regia is 1 part of concentrated HNO₃ by volume and 3 part of concentrated HCl by volume.
HNO₃ ia a very strong oxidizing agent which oxidises a very small amount of gold atoms to gold ions (Au³⁺).
$Au+6H^{+}+3N{O}_3^{-}\to A{u}^{3+}+3N{O}_2+3H_{2}O$
The gold ions further react with chloride ions from HCl to form chloroaurate anions (AuCl₄⁻).
$A{u}^{3+}+4C{l}^{-}\to AuC{l}_4^{-}$
Due to this, more and more gold is oxidized. Thus, gold dissolves to become chloroauric acid (HAuCl₄).