How does aqua-regia act on dissolving black HgS?

Explanation

In dissolving HgS, aqua regia acts as an oxidising agent and also complexing agent. The `NO_(3)^(-)` ions in presence of hydronium ion oxidize the `S^(2-)` ions to free sulphur and the `Cl^(-)` ions with `Hg^(2+)` ions from soluble complex-ion, `HgCl_(4)^(2-)`
`3HgS+2NO_(3)^(-)+8H_(3)O^(+)+12Cl^(-) rarr 3HgCl_(4)^(2-)+2NO+3S+12H_(2)O`