Question

The Lassaigne’s extract is boiled with dil HNO\(_3\) before testing for halogens because

• A. AgCN is soluble in HNO\(_3\)
• B. Silver halides are soluble in HNO\(_3\)
• C. Ag\(_2\)S is soluble in HNO\(_3\)
• D. Na\(_2\)S and NaCN are decomposed by HNO\(_3\)

Answer 1

The Correct Option is D

Lassaigne's test (sodium fusion test) identifies halogens (Cl, Br, I), nitrogen, and sulfur in organic compounds. The Lassaigne's extract is boiled with dilute \(\text{HNO}_3\) before halogen testing for the following reasons:

1. The extract, prepared with metallic sodium, contains sodium halides, sodium sulfide (\(\text{Na}_2\text{S}\)), and sodium cyanide (NaCN).
2. These compounds (NaCN, Na₂S) can interfere with halogen tests by forming complexes, potentially causing false positives or complicating detection.
3. Boiling with dilute nitric acid (\(\text{HNO}_3\)) decomposes Na₂S and NaCN, removing these interfering substances via these reactions:
   • \(\text{Na}_2\text{S}+\text{2HNO}_3\rightarrow2\text{NaNO}_3+\text{H}_2\text{O}+\text{S}\)
   • \(\text{NaCN}+\text{HNO}_3\rightarrow\text{NaNO}_3+\text{HCN}\)
4. With Na₂S and NaCN decomposed, the extract is free of ions that could react with silver nitrate (AgNO₃) to form insoluble silver cyanide (AgCN) or sulfides, which could be mistaken for silver halides.
5. The purified extract can then be treated with silver nitrate (AgNO₃) to detect halides, which precipitate as characteristic silver halides (AgCl, AgBr, AgI).

Therefore, dilute \(\text{HNO}_3\) decomposes Na₂S and NaCN, ensuring accurate halogen detection by eliminating interference from other sodium compounds.