the lassaigne's extract is boiled with dil. hno3 before testing for halogens because
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While testing the Lassaigne’s extract for the presence of halogens, it is first boiled with dilute nitric acid. The solution is cooled and reacted with silver nitrate and a white/yellow precipitate indicates presence of halogen.
N or S in organic compounds is present as NaCN and Na2S in the Lassaigne’s extract. These also give a white precipitate with silver nitrate and thus can give a false test for presence of halogen. Hence NaCN and Na2S need to be removed from the sample. This is done by heating the sample with dilute nitric acid. This decomposes NaCN to HCN and Na2S to H2S. Both are gases and are removed from the sample.
N or S in organic compounds is present as NaCN and Na2S in the Lassaigne’s extract. These also give a white precipitate with silver nitrate and thus can give a false test for presence of halogen. Hence NaCN and Na2S need to be removed from the sample. This is done by heating the sample with dilute nitric acid. This decomposes NaCN to HCN and Na2S to H2S. Both are gases and are removed from the sample.
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