Lassaigne's Test for Halogens

Formula / Core Reaction:

$$\text{Na} + \text{X} \xrightarrow{\Delta} \text{NaX} \quad (\text{X} = \text{Cl}, \text{Br}, \text{I})$$

$$\text{NaX} + \text{AgNO}_3 \xrightarrow{\text{HNO}_3} \text{AgX} \downarrow + \text{NaNO}_3$$

Concept: Halogens are converted to sodium halides ($$\text{NaX}$$) by fusion. Acidification with dilute $$\text{HNO}_3$$ (to destroy $$\text{NaCN}$$ or $$\text{Na}_2\text{S}$$) followed by addition of $$\text{AgNO}_3$$ precipitates silver halides with distinct solubilities in aqueous ammonia.

Notation / Key Observation:

• $$\text{AgCl}$$: White precipitate (Soluble in dilute $$\text{NH}_4\text{OH}$$);
• $$\text{AgBr}$$: Pale yellow precipitate (Sparingly soluble in concentrated $$\text{NH}_4\text{OH}$$);
• $$\text{AgI}$$: Yellow precipitate (Insoluble in $$\text{NH}_4\text{OH}$$).