JEE Chemistry 2024 Question with Solution

Given: The Lassaigne's extract is boiled with dilute $\text{HNO}_3$ before testing for halogens.

Find: Why this step is necessary.

Lassaigne's test is used to detect halogens, nitrogen, and sulfur in an organic compound. The extract may contain sodium halides along with interfering species such as $\text{Na}_2\text{S}$ and NaCN.

These interfering ions must be removed before adding $\text{AgNO}_3$ because they can also form precipitates and disturb halogen detection.

On boiling with dilute $\text{HNO}_3$, they are decomposed:

After removal of these ions, the extract can be tested with $\text{AgNO}_3$ for halides, which then give characteristic silver halide precipitates.

Therefore, the correct option is D: $\text{Na}_2\text{S}$ and NaCN are decomposed by dilute $\text{HNO}_3$.

Given: The halogen test is performed on Lassaigne's extract after boiling it with dilute $\text{HNO}_3$.

Find: The chemical reason for this treatment.

The extract prepared with sodium can contain sodium halides, $\text{Na}_2\text{S}$, and NaCN. If $\text{Na}_2\text{S}$ and NaCN are not removed, they may react during the silver nitrate test and produce confusing results such as silver cyanide or sulfide-related interference.

Dilute $\text{HNO}_3$ removes these interfering species by decomposition before the addition of silver nitrate. This ensures that any precipitate formed afterward is due only to halide ions such as $\text{Cl}^-$, $\text{Br}^-$, or $\text{I}^-$.

Hence, boiling with dilute $\text{HNO}_3$ is done to decompose $\text{Na}_2\text{S}$ and NaCN, so the correct option is D.