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Qualitative analysis
Concepts tested here
- Lassaigne's test for halogens
All Questions
2011 AIPMT-PRE 1 question
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The Lassaigne's extract is boiled with conc. $HNO_3$ while testing for halogens. By doing so itIf the organic compound also contains nitrogen or sulphur, the sodium fusion extract contains NaCN or $Na_2S$.
These would react with $AgNO_3$ to give a white precipitate of AgCN or a black precipitate of $Ag_2S$, which would interfere with the test for halogens.
Boiling with conc. $HNO_3$ decomposes them into volatile products: $NaCN + HNO_3 \rightarrow NaNO_3 + HCN\uparrow$
$Na_2S + 2HNO_3 \rightarrow 2NaNO_3 + H_2S\uparrow$
After this, $AgNO_3$ gives a precipitate only with the halide ions.
Hence boiling with conc. $HNO_3$ decomposes $Na_2S$ and NaCN, if formed.