Mercuric sulphide dissolves in aqua regia to form: (a) \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}\) (b) \(\mathrm{HgCl}_{2}\) (c) \(\mathrm{Hg}\left(\mathrm{NO}_{3}\right) \mathrm{Cl}\) (d) \(\mathrm{Hg}_{2}\left(\mathrm{NO}_{3}\right)_{2}\)

Aqua regia is a powerful chemical concoction renowned for its ability to dissolve noble metals, such as gold and platinum, which are otherwise resistant to single acids. This capability also extends to mercury compounds like mercuric sulphide (HgS). The term 'aqua regia' is Latin for 'royal water', aptly named for its kingly power over such noble substances.

Its composition is a carefully mixed solution of two strong acids: hydrochloric acid (HCl) and nitric acid (HNO3). It's not a mere mixture but a potent chemical blend, usually prepared in a volume ratio of 3 parts hydrochloric acid to 1 part nitric acid. When these acids combine, they produce a series of chemical reactions that unleash the unique dissolving power of aqua regia.

When dealing with this corrosive liquid, safety is paramount. It fumes and can cause severe burns, thus proper handling with gloves and eye protection in a well-ventilated area is essential. Understanding its composition is the first key step in utilizing aqua regia to dissolve mercury in a controlled and safe manner.

Chemistry hinges on the principle of the conservation of mass, which is elegantly showcased in a balanced chemical equation. A balanced chemical equation respects this law by showing that the number of atoms for each element is the same on both the reactant and product sides of the reaction. This balance is crucial for correctly portraying the stoichiometry of the reaction – the quantitative relationship between reactants and products in a chemical reaction.

For the dissolution of mercuric sulphide (HgS) in aqua regia, the balanced chemical equation is:
\[\begin{equation}\text{HgS} + 4 \text{HNO}_{3} + 12 \text{HCl} \rightarrow \text{HgCl}_{2} + 2 \text{NO}_{2} + 2 \text{H}_{2}\text{O} + 3 \text{Cl}_{2}\end{equation}\]

This equation shows the reactants (HgS, HNO3, and HCl) on the left side, transforming into products (mercuric chloride, nitrogen dioxide, water, and chlorine gas) on the right side, with each element's atom count remaining constant across the reaction. The ability to write and balance such equations is an essential skill in chemistry, as it lays out the blueprint of the chemical process involved.

Mercuric chloride (HgCl2) is a significant product derived from the reaction of mercuric sulphide (HgS) and aqua regia. This compound is of interest due to its applications and toxicity. As seen in the balanced chemical equation from the previous section, mercuric sulphide is converted into a more soluble form when it reacts with aqua regia.

The formation of HgCl2 is noteworthy because it is a soluble mercury salt, making it easier to handle and process compared to HgS, which is insoluble. Mercuric chloride is a white crystalline solid with applications ranging from antiseptics to preservatives. However, its hazardous nature cannot be understated, as it poses risks of mercury poisoning if not managed with appropriate precautions.

It's important for students to recognize the practical application of chemistry concepts. Understanding how HgCl2 is produced in the reaction with aqua regia not only helps solve textbook exercises but also provides insight into industrial chemical processes and the overarching importance of safety in chemical handling.