Question

Gold and platinum do not dissolve either in conc. $HCl$ or conc.$H{NO}_3$ but dissolves in the mixture of 3 parts of conc.$HCl$ and 1 part of conc.$H{NO}_3$. Give reason and write the necessary chemical equations.

Answer 1

The dissolution of nobel metals in aquaregia hydrochloric acid to liberate nascent chlorine.
$\underset{Nitrosylchloride}{H{NO}_{3(conc.)}}+3{HCl}_{(conc.)}⟶\underset{Nascentchlorine}{NOCl}+2H_{2}O+2\left[C{l}^{\ast }\right]$
(ii) Nascent chlorine reacts with noble metals to form their soluble chlorides which pass into the solution
$\underset{Gold}{Au}+3\left[C{l}^{\ast }\right]⟶\underset{Gold\left[III\right]chloride}{Au{Cl}_3}$
$\underset{Gold}{Pt}+4\left[C{l}^{\ast }\right]⟶\underset{Platinum\left[IV\right]chloride}{Pt{Cl}_4}$