Question

Freshly prepared solution of sodium nitroprusside is added to the sodium extract. Appearance of a deep violet color indicated the presence of:
A. nitrogen
B. sulfur
C. Both nitrogen and sulfur
D. halogen

Answer 1

Hint: The sodium fusion test, or Lassaigne's test, is used in elemental analysis for the qualitative determination of the presence of foreign elements, namely halogens, nitrogen, and sulphur, in an organic compound. It was developed by J. L. Lassaigne.

Complete answer:
According to the Lassaigne test, a small piece of a sodium metal is heated casually in a fusion tube till it melts to a shining globule. At this stage, a small amount of substance is added and the tube is heated strongly at higher temperatures. The red hot tube is plunged into distilled water contained in a china dish. The contents are boiled for some time, cooled and then filtered. The filtrate is known as sodium extract or Lassaigne extract.
 The extract contains sodium sulfide formed by the reaction between sulfur (present in the compound) and sodium. The reaction is as follows:
$2Na + S \to N{a_2}S$
This Lassaigne extract undergoes sodium nitroprusside test to determine the presence of sulfur in the extract. A few drops of sodium nitroprusside solution are added to another part of the Lassaigne’s extract. The appearance of purple coloration confirms the presence of sulfur. The reaction that takes place here is as follows:
$N{a_2}S + N{a_2}[Fe{(CN)_5}NO] \to N{a_4}[Fe{(CN)_5}NO.S]$

Thus, the correct option is B.

Note:
The Lassaigne extract is also used in the lead acetate test to determine the presence of sulfur in the given compound. One part of the extract is acidified with acetic acid and then lead acetate solution is added. The formation of a black precipitate confirms the presence of sulfur in the organic compound.
The reaction that takes place here is as follows:
$N{a_2}S + Pb{(C{H_3}COO)_2} \to PbS \downarrow + 2C{H_3}COONa$