Lassaigne's test is used for detection of :
- Nitrogen and Sulphur only
- Nitrogen, Sulphur and Phosphorous Only
- Phosphorous and halogens only
- Nitrogen, Sulphur, phosphorous and halogens
The Correct Option is D
Solution and Explanation
Explanation of Lassaigne’s Test: - Lassaigne’s test involves the conversion of elements such as nitrogen, sulphur, phosphorus, and halogens present in an organic compound into their ionic forms by fusion with sodium metal. - The resulting sodium fusion extract (also known as Lassaigne’s extract) can be tested for these elements using specific reagents.
- Detection of Nitrogen: The sodium fusion extract is treated with ferrous sulphate (FeSO$_4$) and acidified with sulphuric acid. The formation of a Prussian blue colour indicates the presence of nitrogen.
Detection of Sulphur: The extract is treated with lead acetate solution. A black precipitate of lead sulphide (PbS) indicates the presence of sulphur.
Detection of Halogens: The extract is treated with silver nitrate (AgNO$_3$) solution after acidification with nitric acid. The formation of a white, yellow, or pale yellow precipitate indicates the presence of chlorine, bromine, or iodine, respectively.
- Detection of Phosphorous: Phosphorous is detected by the formation of a yellow precipitate of ammonium phosphomolybdate when the extract is treated with ammonium molybdate in the presence of nitric acid.
Conclusion: Lassaigne’s test is used for the detection of nitrogen, sulphur, phosphorus, and halogens.
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