Question

The formation of violet colour by the addition of sodium nitroprusside to the Lassaigne’s solution is the indication of, element in the organic compound.

Answer 1

On treating sodium fusion extract with sodium nitroprusside, appearance of a violet colour due to formation of [Fe(CN)5NOS]4 indicates the presence of sulphur.

Complete step by step answer:
In Lassaigne’s Test, a small piece of a sodium metal is heated gently in a fusion tube until it melts to a shining globule. At this stage, a small amount of substance is added and the tube is heated strongly. The hot tube is shoved into distilled water present in a china dish. The contents are then boiled for some time, cooled and then filtered. The filtrate is known as sodium extract.
The extract consists of sodium sulphide formed by the reaction between sulphur which is present in the compound and sodium:
$2Na + S \to Na2S$
The following tests are performed on the Lassaigne’s extract:
In Lead acetate test, one part of the extract is acidified with acetic acid and after that the lead acetate solution is added. The formation of a black precipitate confirms the presence of sulphur in the organic compound.
$Na2S + Pb\left( {CH3COO} \right)2 \to PbS \downarrow + 2CH3COONa$
In the Sodium nitroprusside test, a few drops of sodium nitroprusside solution are added to another part of the Lassaigne’s extract. The appearance of purple colouration confirms the presence of sulphur:
$Na2S + Na2\left[ {Fe\left( {CN} \right)5NO} \right] \to Na4\left[ {Fe\left( {CN} \right)5NOS} \right]$

Note: Nitrogen, halogens, sulphur, and phosphorus present in an organic compound are detected by Lassaigne’s test. The elements present in the compound are converted from covalent form to ionic formed by fusing the compound with sodium metal.