Animated Solution for Chemistry - s and p-Block Elements: With reference to aqua regia, choose the correct option(s).

Aqua Regia, historically known as "Royal Water" by alchemists, is a highly corrosive and fuming mixture. It earned its majestic name because it possesses the unique ability to dissolve noble metals like gold and platinum, which are otherwise completely unreactive towards single strong acids.

The secret to its power lies in its precise composition. It is prepared by mixing concentrated hydrochloric acid ($\text{HCl}$) and concentrated nitric acid (${\text{HNO}}_3$) in a strict $3:1$ volume ratio. This specific formulation is crucial for its chemical properties, making option (B) a correct statement.

When you mix these two colorless (or slightly yellow) concentrated acids, a fascinating reaction occurs immediately. They don't just mix; they react to form new chemical species.

The reaction is given by:

The products of this decomposition are nitrosyl chloride ($\text{NOCl}$) and chlorine gas (${\text{Cl}}_2$). Both of these substances are highly volatile and possess a distinct yellow-orange color. It is the presence of these dissolved gases that gives freshly prepared Aqua Regia its characteristic vibrant yellow hue. This confirms that option (D) is also correct.

Now, let's explore what happens when we introduce gold ($\text{Au}$) into this potent mixture. Nitric acid acts as a powerful oxidizing agent, while hydrochloric acid provides a high concentration of chloride ions (${\text{Cl}}^{-}$) which act as ligands.

The overall balanced chemical equation for the dissolution of gold is:

Notice the gas that is evolved during this process. It is nitric oxide ($\text{NO}$), a colorless gas, which may later react with atmospheric oxygen to form brown ${\text{NO}}_2$. However, the question specifically states "in the absence of air", meaning the final gaseous product remains $\text{NO}$. Therefore, option (A) is incorrect because it claims ${\text{NO}}_2$ is produced.

Finally, let's analyze the gold-containing product, chloroauric acid (${\text{HAuCl}}_4$). In solution, this exists as the complex anion, the tetrachloroaurate ion, $[{\text{AuCl}}_4{]}^{-}$.

To find the oxidation state of gold in this complex, we set up a simple algebraic equation. Let the oxidation state of $\text{Au}$ be $x$. We know that each chloride ligand carries a $-1$ charge, and the overall charge of the complex ion is $-1$.

Gold is oxidized from an oxidation state of $0$ to $+3$. This mathematical proof confirms that option (C) is absolutely correct. By systematically analyzing the composition, decomposition, and reaction mechanism of Aqua Regia, we have successfully validated options (B), (C), and (D).