Question

Lassaigne’s test is used for the detection of:
(A) Carbon only
(B) Hydrogen only
(C) Oxygen only
(D) Nitrogen, sulphur and halogens

Answer 1

First we have to know that the Lassaigne's test is a general test for the detection of elements which are covalently bonded to the organic compounds. In order to detect them, the compound is heated/fused with a small piece of sodium metal ($Na$) in a fusion tube to convert the elements present in the organic compound into the water-soluble salts of sodium.

Complete Step By Step Answer:
Since halogens, nitrogen and sulphur elements are covalently bonded to the organic compounds. Hence, lassaigne’s test is used for the detection of halogens, nitrogen and sulphur in an organic compound.
When a nitrogenous organic compound is fused with sodium, the nitrogen present in the compound is converted into sodium cyanide. When an organic compound which contains sulphur is fused with sodium, the sulphur present in the compound is converted into sodium sulphide. Sulphide ions are readily identified using sodium nitroprusside. Whereas, Halogens present in an organic compound forms sodium halide on fusion with sodium metal. Sodium halide extracted with water can be easily identified by adding a silver nitrate solution after acidifying it with dil. $HN{O_3}$.
Hence the correct answer is option (D).

Note:
Lassaigne’s test is also known as the sodium fusion test. This test is used in elemental analysis for the qualitative determination of the presence of foreign elements. Also, the ionic compounds formed during the fusion are extracted in aqueous solution and can be detected by simple chemical tests.