Question

The Lassaigne's extract is boiled with conc. $HNO_3$ while testing for halogens. By doing so it

• A. helps in the precipitation of $AgCl$
• B. increases the solubility product of $AgCl$
• C. increases the concentration of $NO^-_3$ ions
• D. decomposes $Na_2S$ and $NaCN$, if formed

Answer 1

The Correct Option is D

To test for halogens in an organic compound, Lassaigne's extract, which is obtained by fusing the compound with sodium metal, is used. This extract may contain sodium halides, sodium cyanide (NaCN), and sodium sulfide (Na_2S) as impurities.

While testing for halogens using Lassaigne's test, the extract is boiled with concentrated nitric acid (HNO_3) for a specific purpose:

1. Concentrated HNO_3 decomposes the sodium cyanide (NaCN) and sodium sulfide (Na_2S) that might be present in the extract. This is necessary because if these impurities are not decomposed, they can interfere with the silver nitrate test for halogens by forming complexes and giving a false positive test.
2. The chemical reactions involved are as follows:
   • NaCN + HNO_3 \rightarrow HCN (gas) + NaNO_3
   • Na_2S + 2 HNO_3 \rightarrow 2 NaNO_3 + H_2O + S

Once NaCN and Na_2S are decomposed, any halogen present in the form of sodium halide is free from interferences and can react with silver nitrate to form a precipitate of silver halide (e.g., AgCl, AgBr, AgI), which is tested for the presence of halogens.

Therefore, the correct answer to the question is: decomposes Na_2S and NaCN, if formed.