Question

Lassaigne's test is used for detection of :

• A. Nitrogen and Sulphur only
• B. Nitrogen, Sulphur and Phosphorous Only
• C. Phosphorous and halogens only
• D. Nitrogen, Sulphur, phosphorous and halogens

Answer 1

The Correct Option is D

Lassaigne's test, also known as the Sodium Fusion Test, is a qualitative test used to detect the presence of various elements in an organic compound. The elements typically identified with this test include nitrogen, sulfur, phosphorus, and halogens (chlorine, bromine, and iodine). Here's how the test works step-by-step: 

1. The organic compound is fused with sodium metal, which breaks down the compound and forms sodium salts of the elements present.
2. These sodium salts are then extracted using distilled water and tested separately for each element:
   • Nitrogen: When the fused mass is boiled with water and the extract is treated with iron sulfate and an acid (sulfuric acid), the formation of Prussian blue indicates the presence of nitrogen.
   • Sulfur: The extract when acidified and treated with lead acetate solution forming a black precipitate of lead sulfide indicates sulfur.
   • Halogens: When nitric acid and silver nitrate solution are added to the extract, the formation of white or colored precipitates indicates the presence of halogens. The color varies depending on the halogen: white (chlorine), pale yellow (bromine), or yellow (iodine).
   • Phosphorus: Heating the sodium extract with concentrated nitric acid, then adding ammonium molybdate, results in a yellow precipitate if phosphorus is present.
3. Given the description, the test conducts detection for nitrogen, sulfur, phosphorus, and halogens, which is consistent with option involving all these elements.

Therefore, the correct answer is: Nitrogen, Sulphur, phosphorus, and halogens.

Answer 2

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.