Question

Lassaigne's extract is prepared as described in the case of nitrogen. The extract contains sodium sulfide formed by the reaction between ________ and _________.
(a)- sodium nitrate, sodium
(b)- sulfur, sodium
(c)- sodium chloride, hydrogen sulfide
(d)- sodium carbonate, hydrogen sulfide

Answer 1

Hint: Lassaigne’s test is done in two parts: Lead acetate test and sodium nitroprusside test. Both the tests are used to detect the presence of sulfur in the compound. The Lassaigne's extract is prepared from 2 elements.

Complete step by step answer:
Lassaigne’s test is performed in the following steps:
First, a small solid piece of sodium metal is gently heated in a fusion tube till it melts and it forms a shining globule. At this stage, a small amount of the substance which is to be tested is mixed and then the tube is strongly heated. The red hot tube is pushed into distilled water. This distilled water is contained in a china dish. The contents are boiled for some time. After boiling the content is cooled and then filtered. This filtrate is known as sodium extract or Lassaigne’s extract.
This extract is sodium sulfide and it is formed by the reaction between sodium and sulfur. The reaction is given below:
$2Na+S\to N{{a}_{2}}S$

This filtrate is divided into 2 parts for further testing.
Lead acetate test: To the one part of the divided extract acetic acid is added and to it, lead acetate solution is added. On adding all the components, the formation of a black precipitate confirms the presence of sulfur in the compound. The reaction is given below:
$N{{a}_{2}}S+Pb{{(C{{H}_{3}}COO)}_{2}}\to PbS\downarrow +2C{{H}_{3}}COONa$

Sodium nitroprusside test: To the second part of the Lassaingne’s extract, a few drops of sodium nitroprusside solution is added. If the purple color appears, it confirms the presence of sulfur. The reaction is given below:
$N{{a}_{2}}S+N{{a}_{2}}[Fe{{(CN)}_{5}}NO]\to N{{a}_{4}}[Fe{{(CN)}_{5}}NO.S]$

So, the correct answer is “Option B”.

Note: The Lassaigne’s test is also used for the detection of halogens, and nitrogen in the compound. The sodium used in this test must be pure. This test was developed by J.N.Lassaigne.