Question

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

(a) Nitrogen

(b) Fluorine

(c) Sulfur

(d) Phosphorous

This question was posed to me in homework.

This intriguing question comes from Qualitative Analysis of Organic Compounds topic in section Organic Chemistry – Basic Principles and Technique of Chemistry – Class 11

Answer 1

Right option is (b) Fluorine

Best explanation: 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.