JEE Chemistry 2024 Question with Solution

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

Find: The reason for this boiling step.

The Lassaigne's extract may contain sodium halides along with interfering species such as $\text{Na}_2\text{S}$ and $\text{NaCN}$ formed during sodium fusion.

These ions interfere with the halogen test because they can react with $\text{AgNO}_3$ and give precipitates that may be confused with silver halides.

Boiling with dilute nitric acid decomposes these interfering compounds:

After removal of $\text{S}^{2-}$ and $\text{CN}^-$ ions, the extract can be safely treated with $\text{AgNO}_3$ to test for halides.

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

Given: Lassaigne's test is used to detect halogens, nitrogen, and sulfur in an organic compound.

Find: Why the extract is boiled with dilute $\text{HNO}_3$ before adding $\text{AgNO}_3$.

During sodium fusion, the extract can contain sodium halides, sodium sulfide, and sodium cyanide.

If $\text{Na}_2\text{S}$ and $\text{NaCN}$ are not removed, they interfere in the silver nitrate test by producing species such as silver sulfide or silver cyanide, which may lead to false identification.

Dilute $\text{HNO}_3$ removes these interfering ions by decomposition before the actual halogen test is performed.

Thus, the boiling step is not to dissolve silver halides, but to destroy interfering $\text{Na}_2\text{S}$ and $\text{NaCN}$.

Hence, the correct option is D.