Animated Solution for Chemistry - s and p-Block Elements: Consider the sulphides $\text{HgS}$, $\text{PbS}$, $\text{CuS}$, ${\text{Sb}}_2{\text{S}}_3$, ${\text{As}}_2{\text{S}}_3$ and $\text{CdS}$. Number of these sulphides soluble in $50\%{\text{ HNO}}_3$ is

When dealing with the solubility of heavy metal sulphides, the choice of acid is everything. Dilute, non-oxidizing acids like $\text{HCl}$ often fail to dissolve these sulphides because their solubility products ($K_{sp}$) are incredibly low.

However, $50\%{\text{ HNO}}_3$ is a completely different beast. Nitric acid is a powerful oxidizing agent. Instead of just providing $H^{+}$ ions, it actively attacks the sulphide ion ($S^{2-}$), oxidizing it to elemental sulphur ($S$) or even sulphate ($S{O}_4^{2-}$). This oxidation removes the sulphide ions from the equilibrium, driving the dissolution reaction forward.

For example, the dissolution of copper sulphide looks like this:

Looking at our given list, most of the sulphides readily surrender to the oxidizing power of nitric acid.

Lead sulphide ($\text{PbS}$), copper sulphide ($\text{CuS}$), arsenic trisulphide (${\text{As}}_2{\text{S}}_3$), and cadmium sulphide ($\text{CdS}$) all dissolve in $50\%{\text{ HNO}}_3$. When they dissolve, you will typically observe a pale yellow turbidity in the test tube, which is the precipitated elemental sulphur.

But chemistry is never without its exceptions! There are two sulphides in our list that stubbornly resist $50\%{\text{ HNO}}_3$.

Mercury(II) sulphide ($\text{HgS}$) has an exceptionally low $K_{sp}$ (around $10^{-52}$). The oxidizing power of nitric acid alone is not enough to break its lattice. To dissolve $\text{HgS}$, we must bring in Aqua Regia (a 3:1 mixture of concentrated $\text{HCl}$ and ${\text{HNO}}_3$). In Aqua Regia, nitric acid oxidizes the sulphide, while the chloride ions from $\text{HCl}$ form a highly stable, soluble complex with mercury: $[{\text{HgCl}}_4{]}^{2-}$.

Similarly, antimony trisulphide (${\text{Sb}}_2{\text{S}}_3$) is also insoluble in $50\%{\text{ HNO}}_3$. It prefers to dissolve in concentrated $\text{HCl}$ or yellow ammonium sulphide.

By filtering out the two insoluble exceptions ($\text{HgS}$ and ${\text{Sb}}_2{\text{S}}_3$), we are left with exactly 4 sulphides that are soluble in $50\%{\text{ HNO}}_3$. Mastering these qualitative analysis exceptions is a surefire way to secure marks in inorganic chemistry!