MCQ Detailed View

The solubility of a compound in nitric acid depends on its chemical nature and the stability of its ions. Many metal sulfides, such as CuS, CdS, and PbS, are insoluble in water but can dissolve in strong oxidizing acids like nitric acid (HNO₃). This is because concentrated nitric acid acts not only as an acid but also as a strong oxidizing agent. It can oxidize sulfide ions (S²⁻) into elemental sulfur or sulfur dioxide (SO₂), allowing the metal cations to go into solution as their corresponding nitrates.

For example:

CuS + 2 HNO₃ → Cu(NO₃)₂ + S ↓

Similarly, cadmium sulfide and lead(II) sulfide also react with nitric acid to form cadmium nitrate and lead nitrate, along with sulfur or other oxidized sulfur products.

However, silver chloride (AgCl) behaves differently. AgCl is a sparingly soluble salt and is well-known for its insolubility in water and many acids. Nitric acid does not dissolve AgCl because it does not provide ions that can break the strong lattice of silver chloride. The nitrate ion does not form a stable complex with Ag⁺ ions, so the compound remains undissolved.

The only reagent that effectively dissolves AgCl is aqueous ammonia (NH₃), where AgCl forms a soluble complex ion [Ag(NH₃)₂]⁺. This is why in qualitative analysis, nitric acid is often used to wash precipitates of silver halides, since AgCl does not dissolve in it, but other impurities might.

Therefore, among the given options, AgCl is the compound that is not soluble in nitric acid, while CuS, CdS, and PbS can dissolve due to oxidation reactions.