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Which one of the following elements cannot be detected by “Lassaigne’s

June 1, 2025 by rawan239

Which one of the following elements cannot be detected by “Lassaigne’s test”?

I

Cl

S

F

This question was previously asked in

UPSC CDS-1 – 2019

Download PDF Attempt Online

Lassaigne’s test (Sodium Fusion Test) is used to detect the presence of elements like nitrogen, sulfur, and halogens (chlorine, bromine, iodine) in an organic compound. The organic substance is fused with metallic sodium, converting these elements into ionic compounds (e.g., NaCN, Na2S, NaCl, NaBr, NaI) which are then extracted with water and tested for using specific reagents. Fluorine (F) is a halogen, but Lassaigne’s test is generally not used or is unreliable for its detection because sodium fluoride (NaF) is less soluble than other sodium halides, and organic fluoro compounds may not completely decompose to ionic fluoride upon fusion. Therefore, fluorine cannot be reliably detected by Lassaigne’s test.

– Lassaigne’s test detects N, S, Cl, Br, and I in organic compounds.
– Fluorine (F) is typically not detectable by Lassaigne’s test due to difficulties in forming and dissolving ionic fluoride.

For detecting fluorine in organic compounds, alternative methods like combustion followed by fluoride ion-selective electrode measurement or gravimetric analysis as calcium fluoride are used.

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