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JEE Main 2023 · 29 Jan · Shift 1

JEE Main 2023 Chemistry question on Practical Chemistry

Compound that will give positive Lassaigne's test for both nitrogen and halogen is:
  1. A.
    NH4Cl\text{NH}_4\text{Cl}
  2. B.
    NH2OH⋅HCl\text{NH}_2\text{OH}\cdot\text{HCl}
  3. C.
    CH3NH2⋅HCl\text{CH}_3\text{NH}_2\cdot\text{HCl}
  4. D.
    N2H4⋅HCl\text{N}_2\text{H}_4\cdot\text{HCl}
Show answer and solution

Correct option: C

Solution

In Lassaigne's test, the compound must be fused with sodium metal to convert nitrogen into NaCN (detected by Prussian blue test) and halogen into NaX (detected by AgNO₃ test). For both tests to be positive, the compound must contain nitrogen and halogen covalently bonded within an organic molecule — specifically, carbon must be present to form the cyanide ion (CN⁻).

CH₃NH₂·HCl (methylamine hydrochloride) is an organic compound containing C, N, and Cl. When fused with Na, it gives NaCN (positive nitrogen test with FeSO₄/FeCl₃) and NaCl (positive halogen test with AgNO₃). The other compounds (NH₄Cl, NH₂OH·HCl, N₂H₄·HCl) either lack carbon or are inorganic/ionic salts where nitrogen cannot form CN⁻ during the sodium fusion, so they fail the nitrogen Lassaigne's test.

[JEE Main 2023 · 29 Jan · Shift 1]

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