Question

Which one of the following elements is not detected by Lassaigne’s test?
A.Sulphur
B.Nitrogen
C.Oxygen
D.Chlorine

Answer 1

We know that, we can rely on Lassaigne's test for the detection of nitrogen, halogens and Sulphur in an organic compound and the compound is heated with sodium metal (Na) to convert the elements present in the organic compound into the water-soluble salts of sodium.

Complete answer:
Lassaigne’s test which is also known as the sodium fusion 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. But, silver fluoride is soluble in water and does not precipitate and thus this method cannot be used for detection of fluorine.
These elements are covalently bonded to the organic compounds. In order to detect them, these have to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal.
Nitrogen. In this test, a small piece of sodium metal is heated in a fusion tube with the organic compound. Na converts all the elements present into ionic form.
$Na + C + N \to NaCN$
$2Na + S \to Na2S$
$Na + X \to NaX$ (Where, X = Cl, Br, or I)
The formed ionic salts are extracted from the fused mass by boiling it with distilled water. This is called sodium fusion extract.
Test for Nitrogen: The extract is boiled with FeSO4 and acidified with concentrated H2SO4. The appearance on Prussian blue colour indicates the presence of nitrogen.
Test for Sulphur: The extract is treated with sodium nitroprusside. The appearance of violet colour indicates the presence of sulphur.
Test for Halogens: The extract is acidified with $HN{O_3}$ and then treated with $AgN{O_3}$. A white precipitate soluble in $N{H_4}OH$ indicates the presence of Cl, a yellowish precipitate sparingly soluble in $N{H_4}OH$ indicates the presence of Br, and a yellow precipitate insoluble in $N{H_4}OH$ indicates the presence of I.
In the given options, only Oxygen cannot be detected by Lassaigne’s test.

Therefore, the correct answer is option (C).

Note:
This test is not given by compounds that contain N but not C atoms. For instance, $N{H_2}$ does not answer this test despite having an N atom. This is because both C and N are required to form $C{N}^{–}$ ions. This test is not given by diazonium salts as they decompose to give nitrogen gas on heating.