Question

Lassaigne's test (with silver nitrate) is commonly used to detect halogens such as chlorine, bromine and iodine but not useful to detect fluorine because the product AgF formed is

• A. volatile
• B. reactive
• C. explosive
• D. soluble in water
• E. a liquid

Hint:

AgF is the only silver halide soluble in water → no precipitate → fluorine not detected in Lassaigne’s test.

Answer 1

The Correct Option is D

Concept:
Lassaigne’s test is used for the qualitative detection of halogens in organic compounds. The compound is fused with sodium to convert covalently bonded halogens into ionic sodium halides (NaCl, NaBr, NaI, NaF). These are then treated with silver nitrate (AgNO$_3$) to form silver halide precipitates: \[ \text{NaX} + \text{AgNO}_3 \rightarrow \text{AgX} \downarrow + \text{NaNO}_3 \] The nature of AgX helps identify the halogen:
• AgCl → white precipitate (insoluble)
• AgBr → pale yellow precipitate
• AgI → yellow precipitate

Step 1: Behavior of fluorine in Lassaigne’s test
Fluorine forms sodium fluoride (NaF), which reacts with AgNO$_3$: \[ \text{NaF} + \text{AgNO}_3 \rightarrow \text{AgF} + \text{NaNO}_3 \]

Step 2: Key property of AgF
Unlike other silver halides, AgF is soluble in water. Hence:
• No precipitate forms
• Test fails to detect fluorine

Step 3: Analysis of options

• (A) Volatile → incorrect
• (B) Reactive → not relevant
• (C) Explosive → incorrect
• (D) Soluble in water → correct reason
• (E) Liquid → incorrect Final Conclusion:
Fluorine is not detected because AgF does not precipitate due to its solubility: \[ \boxed{(D)} \]