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Detection of Elements: Lassaigne's Test - Detection of Halogen

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Estimated time: 3 minutes
CBSE: Class 12

Key Points: Detection of Halogens

SFE is first acidified with HNO₃ (to remove CN⁻ and S²⁻ by boiling with conc. HNO₃/glacial acetic acid), then treated with AgNO₃:

  • NaCN + HNO₃ → NaNO₃ + HCN (removed)
  • Na₂S + 2HNO₃ → 2NaNO₃ + H₂S↑ (removed)
Halogen Precipitate with AgNO₃ Solubility in NH₄OH
Cl⁻ AgCl↓ — white, curdy Soluble
Br⁻ AgBr↓ — pale yellow Partially soluble
I⁻ AgI↓ — yellow Insoluble
 

Fluorine cannot be detected by Lassaigne's test since AgF is soluble.
If N or S is present, they must be removed first to prevent interference (AgCN or Ag₂S may form).

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