Sodium extract is heated with concentrated \(\mathrm{HNO_3}\) before testing for halogens because :
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- Silver halides are totally insoluble in nitric acid.
- \(\mathrm{Ag_2S}\) and AgCN are soluble in acidic medium.
- \(\mathrm{S^{2-}}\) and \(\mathrm{CN^-}\), if present, are decomposed by conc. \(\mathrm{HNO_3}\) and hence do not interfere in the test.
- Ag reacts faster with halides in acidic medium.
The Correct Option is C
Solution and Explanation
With AgNO3, S2- and CN- would also give precipitates (Ag2S, AgCN) and interfere.
Boiling with conc. HNO3 decomposes them (to HCN/H2S or S, which are expelled), so only halide ions are tested.
Answer: (3) \(\mathrm{S^{2-}}\) and \(\mathrm{CN^-}\), if present, are decomposed by conc. \(\mathrm{HNO_3}\) and hence do not interfere in the test.
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