(1) Detection of nitrogen
Upon fusion with sodium metal, the covalently bonded nitrogen is converted to ionic sodium cyanide. When sodium cyanide is treated with ferrous sulphate, it is converted to sodium ferrocyanide. Further treatment with ferric chloride results in ferric ferrocyanide, a Prussian blue complex.

(2) Detection of sulphur
Upon fusion with sodium metal, the sulphur atom present in the organic compound is converted to ionic sodium sulphide, which can be extracted by boiling the fused mass with distilled water and is used for sulphur detection. When reacted with sodium nitroprusside, it gives a purple colour due to the formation of sodium thionitroprusside.

(3) Detection of halogens
Upon fusion with sodium metal, the halogens in the organic compound are converted to ionic sodium halides, which can be extracted by boiling the fused mass with distilled water and are used for the detection of halides.
1) Silver nitrate test
(a) Chlorine:
While preparing Lassaigne’s extract, chlorine from the organic compound reacts with sodium to form sodium chloride. Sodium chloride forms a white precipitate of silver chloride with silver nitrate solution, which is soluble in ammonium hydroxide.

(b) Bromine:
Sodium bromide formed during the preparation of Lassaigne’s extract reacts with silver nitrate to form a pale yellow precipitate of silver bromide, which is sparingly soluble in ammonium hydroxide.

(c) Iodine:
Sodium iodide formed during the preparation of sodium fusion extract reacts with silver nitrate solution to form a yellow precipitate of silver iodide, which is insoluble in ammonium hydroxide.
2) Carbon disulphide test 
When sodium bromide and sodium iodide in Lassaigne’s extract are treated with chlorine water, the bromide and iodide oxidise to the corresponding halogens. This halogen dissolves in carbon disulphide. Bromine imparts an orange colour, and iodine imparts a violet colour in the carbon disulphide layer.

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