Question

Sodium nitroprusside when added to an alkaline solution of sulphide ions produces a colouration which is:

• A. Red
• B. Violet
• C. Brown
• D. Green

Answer 1

The correct option is B Violet

Lassaigne’s test can be carried out with the help of sodium fusion extract.
In this test, the elements present in a compound are converted from covalent to ionic form by fusing the organic compound with Sodium (Na) metal.
$2Na+S\stackrel{\Delta }{⟶}N{a}_{2}S$

Sulphides of sodium formed on sodium fusion are extracted from the fused mass by boiling with water.
This extract is known as Sodium Fusion Extract (Lassaigne Extract).
On treating the Lassaigne extract $\left(N{a}_{2}S\right)$ with sodium nitroprusside, the appearance of violet colour indicates the presence of Sulphur.
$N{a}_{2}S+\underset{\text{Sodium nitroprusside}}{N{a}_2\left[Fe\right(CN{)}_{5}NO]}\to \underset{\text{Violet colouration}}{N{a}_4\left[Fe\right(CN{)}_{5}NOS]}$