Question

Identify the element that cannot be detected by Lassaigne’s test.

• A. Nitrogen
• B. Fluorine
• C. Sulfur
• D. Phosphorous

Answer 1

The correct option is B Fluorine

Lassaigne's test is a general test for the detection of nitrogen, sulfur, phosphorus and halogens present in an organic compound. This method is done by fusing the organic compound with sodium metal. The ionic compound formed during the fusion are extracted in aqueous solution and can be used for detection of nitrogen, sulfur, phosphorus and halogens by simple chemical tests.
Fluorine cannot be detected by lassaigne’s test. Even though lassaigne’s test is used for the detection of halogens, all halogens except fluorine can be detected. This is because, in lassaigne’s test, the sodium extract is treated with silver nitrate. Only in the case of fluorine, the silver fluoride formed is soluble, unlike the others which is insoluble. Thus, the precipitate will not be formed and so, this method cannot be used for fluorine detection.