Question

In lead acetate test of sulphur detection, Lassaigne extract is acidified with _________ and then lead acetate solution is added.
A. Oxalic acid
B. Hydrochloric acid
C. Sulphuric acid
D. Acetic acid

Answer 1

Hint- In order to deal with this question first we will understand the lassaigne’s test, then we will proceed further by knowing about its parts that are Lead acetate test and Sodium nitroprusside test.

Complete answer:
Lassaigne’s Test:
A small piece of a sodium metal is heated gently 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. 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. This filtrate thus formed is commonly called as sodium extract or Lassaigne ‘s extract.
The extract contains sodium sulphide formed by the reaction between sulphur (present in the compound) and sodium:
$2Na + S \to N{a_2}S$
The extract of the Lassaigne is split into two sections, and the following experiments are carried out:
Lead acetate test: One part of the extract is acidified with acetic acid and then the solution is replaced with lead acetate. Formation of a black precipitate confirms the presence of sulphur in the organic compound
$N{a_2}S + Pb{\left( {C{h_3}COO} \right)_2} \to PbS \downarrow + 2C{H_3}COONa$
Sodium nitroprusside test: A few drops of a solution of sodium nitroprusside are added to another portion of the Lassaigne sample. The appearance of purple colouration confirms the presence of sulphur:
$N{a_2}S + N{a_2}\left[ {Fe{{\left( {CN} \right)}_5}NO} \right] \to N{a_4}\left[ {Fe{{\left( {CN} \right)}_5}NO.S} \right]$
Hence, Lassaigne extract is acidified with Acetic acid.

So, the correct answer is option D.

Note- In order to solve such types of problems students must remember the steps and processes involved in various tests. The sodium fusion test, or Lassaigne 's test, is used in elementary research to assess the qualitative existence of foreign elements in an organic compound, including halogens, nitrogen and sulphur. Lead acetate test is used to detect the poisonous and highly inflammable hydrogen sulfide in a fluid by discoloration of a paper moistened with the lead acetate solution.