Context: Lassaigne’s extract test for halogens

In sodium extract, if halogens are present, they are tested by adding $AgNO_3$ after acidifying the extract with concentrated $HNO_3$.

Why is acidification needed?

The sodium extract may also contain:

• $S^{2-}$ from sulphur
• $CN^-$ from nitrogen

These ions also react with $Ag^+$ to form precipitates: $2Ag^+ + S^{2-} \rightarrow Ag_2S \downarrow$ $Ag^+ + CN^- \rightarrow AgCN \downarrow$

These precipitates can interfere with the halogen test because silver halides also form precipitates: $Ag^+ + X^- \rightarrow AgX \downarrow \qquad (X = Cl^-, Br^-, I^-)$

Role of concentrated $HNO_3$

Concentrated $HNO_3$ decomposes/removes $S^{2-}$ and $CN^-$ present in the sodium extract, so they do not produce interfering precipitates with $AgNO_3$.

Thus, only halide ions react with silver nitrate in the final test.

Check the options

• A: "Silver halides are totally insoluble in nitric acid."
  This is not the reason for heating sodium extract with concentrated $HNO_3$.

• B: "$Ag_2S$ and $AgCN$ are soluble in acidic medium."
  This is not the correct explanation here. The purpose is to destroy $S^{2-}$ and $CN^-$ before adding $AgNO_3$.

• C: "$S^{2-}$ and $CN^-$, if present, are decomposed by conc. $HNO_3$ and hence do not interfere in the test."
  This is exactly correct.

• D: "Ag reacts faster with halides in acidic medium."
  Incorrect.

Final answer

The correct option is C.