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 analytical procedure designed to ascertain the presence of specific elements within organic compounds. The elements commonly identified by this methodology include nitrogen, sulfur, phosphorus, and halogens (chlorine, bromine, iodine). The test is performed through the following sequential steps:

1. The organic substance undergoes fusion with metallic sodium, a process that decomposes the compound and generates sodium salts corresponding to the constituent elements.
2. These resultant sodium salts are subsequently leached using distilled water. The aqueous extract is then subjected to individual tests for each element:
   • Nitrogen: Following the boiling of the fused mass with water and subsequent acidification of the extract, treatment with iron sulfate precipitates Prussian blue, signaling the presence of nitrogen.
   • Sulfur: Acidification of the extract followed by the addition of lead acetate solution will yield a black precipitate of lead sulfide, indicative of sulfur.
   • Halogens: The addition of nitric acid and silver nitrate solution to the extract will result in the formation of precipitates, either white or colored, signifying the presence of halogens. The precipitate's color is characteristic of the specific halogen: white for chlorine, pale yellow for bromine, and yellow for iodine.
   • Phosphorus: Heating the sodium extract with concentrated nitric acid and subsequently introducing ammonium molybdate will produce a yellow precipitate if phosphorus is present.
3. Based on the described methodology, the test facilitates the detection of nitrogen, sulfur, phosphorus, and halogens, which aligns with an option encompassing all these elements.

Consequently, the definitive identification is: Nitrogen, Sulphur, phosphorus, and halogens.