Question

Lassaigne extract is acidified with acetic acid and then lead acetate solution is added. It forms a black precipitate of:
$
  A.{\text{ }}PbS \\
  B.{\text{ }}C{H_3}COONa \\
  C.{\text{ }}PbS{O_4} \\
  D.{\text{ }}PbO \\
 $

Answer 1

Hint: We know that the Lassaigne test is the qualitative analysis that is done for the identification of nitrogen, halogen, and sulfur in an organic compound. We can also call it a sodium fusion test.

Complete step by step answer:
First, we see the principle of the Lassaigne test:
The principle behind the sodium fusion test is the conversion of the presented element into its ionic form by the sodium metal.
The conversion nitrogen element to its ionic form by sodium is,
$Na + C + N \to NaCN$
The conversion Sulfur element to its ionic form by sodium is,
$2Na + S \to N{a_2}S$
The conversion halogen element to its ionic form by sodium is,
$Na + X \to NaX$
Now we see the procedure to carry out the Lassaigne test:
Gently heat the small piece of sodium metal in a fusion tube and melt the metal till it forms a shining globule.
After melting, add a given substance and heat strongly and crush the red hot tube in distilled water taken in a china dish.
Boil the contents then cool and filter. The filtrate is known as Lassaigne’s extract.
Divide the extract and perform the respective test for the elements.
Now, see the option (A).
If it is taken for the analysis. It has sodium sulfide in the extraction when the extraction is acidified with $C{H_3}COOH$ and on the addition of lead acetate it forms Lead acetate (black precipitate) which confirms the presence of sulfur.
$N{a_2}S + Pb{\left( {C{H_3}COO} \right)_2} \to PbS \downarrow + 2C{H_3}COONa$
Hence option (A) is correct.

Now, coming to the options (B), (C) and (D) they do not form any precipitate with acidification since they lack sulfur atoms. Hence option (B), (C), and (D) are incorrect.

Note:
Another method to detect the presence of sulfur is the sodium nitroprusside test.
To perform the sodium nitroprusside test add a drop of sodium nitroprusside to the small portion of the Lassaigne extract. The Formation of purple color confirms the presence of sulfur.