Halogen detection requires removing N and S interference first.

Step 1: Acidify Lassaigne's extract with dilute HNO3 and BOIL. This decomposes CN- → HCN (gas escapes) and S2- → H2S (gas escapes). If not removed: AgCN (white) and Ag2S (black) precipitate would mask AgX results.

Step 2: Add AgNO3 to the boiled, acidified extract.

• NaCl + AgNO3 → AgCl (white precipitate, dissolves in dilute NH3 forming [Ag(NH3)2]+Cl-)
• NaBr + AgNO3 → AgBr (pale yellow, partially soluble in concentrated NH3)
• NaI + AgNO3 → AgI (yellow, insoluble in NH3 — even concentrated)

This solubility progression in NH3 is the definitive identification.

Beilstein test (quick qualitative): Heat copper wire → dip in organic compound → return to Bunsen flame. Green/blue-green flame indicates halogen (volatile CuX2 formed). Does NOT distinguish Cl/Br/I. Not reliable for fluorine (CuF2 is non-volatile). Also gives false positive with some N-containing compounds.