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Question:

Sodium extract is heated with concentrated HNO3 before testing for halogens because:

Ag2S and AgCN are soluble in acidic medium.

silver halides are totally insoluble in nitric acid.

S2− and CN−, if present, are decomposed by conc. HNO3 and hence do not interfere in the test.

Ag reacts faster with halides in acidic medium.

Solution:

Sodium extract is heated with concentrated HNO3 before testing for halogens because S2− and CN− ions, if present, are decomposed by conc. HNO3 and hence do not interfere in the test.