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, also known as the sodium fusion test, is used to detect the presence of halogens (Cl, Br, I), nitrogen, and sulfur in organic compounds. Here's a step-by-step explanation of why the Lassaigne's extract is boiled with dilute \(\text{HNO}_3\) before testing for halogens:

1. The Lassaigne's extract contains a variety of compounds, including sodium halides, sodium sulfide (\(\text{Na}_2\text{S}\)), and sodium cyanide (NaCN) due to its preparation with metallic sodium.
2. In the presence of sodium, compounds like NaCN and Na₂S can interfere with the test for halogens by potentially forming complex compounds that can lead to false positives or complicate the detection.
3. Boiling the extract with dilute nitric acid (\(\text{HNO}_3\)) serves the purpose of decomposing Na₂S and NaCN. This reaction is crucial because it removes these potential interfering species:
   • \(\text{Na}_2\text{S}+\text{2HNO}_3\rightarrow2\text{NaNO}_3+\text{H}_2\text{O}+\text{S}\)
   • \(\text{NaCN}+\text{HNO}_3\rightarrow\text{NaNO}_3+\text{HCN}\)
4. Once Na₂S and NaCN are decomposed, the extract is free from these ions, which could otherwise react with silver nitrate (AgNO₃) to form insoluble silver cyanide (AgCN) or precipitate sulfides that might be confused with silver halides.
5. Finally, the cleaned extract can then be treated with silver nitrate (AgNO₃) to test for the presence of halides, which will precipitate as silver halides (AgCl, AgBr, AgI), which are characteristic and identifiable.

In conclusion, the correct option is that Na₂S and NaCN are decomposed by dilute \(\text{HNO}_3\), ensuring accurate detection of halogens by preventing interference from other sodium compounds.