Question

What is the role of dil. $HNO_{3}$ in Lassaigne's extract.

Answer 1

The presence of elements such as Nitrogen, Sulphur, and phosphorus in an “organic compound” is detected by Lassaigne's test. In the Lassaigne's test for detection of halogens in a given sample the “sodium fusion” extract is treated with dilute $HN{ O }_{ 3 }$.

The main role of dilute $HN{ O }_{ 3 }$ in the Lassaigne's extract is to decompose the ions of cyanide as well as sulphide ions of sodium formed during the test. These ions must be decomposed because they will get involved in the reaction and will interfere in the final result derivation.