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

Answer: (D)

decomposes $Na_2S$ and $NaCN$, if formed

Answer 2

Na$_2$S and NaCN, if present in the extract, will be
decomposed to H$_2$S and HCN by HNO$_3$ .
NaCN + HN$O_3$ $\rightarrow$ NaN$O_3$ + HCN
Na$_2$S + 2HNO$_3$ $\rightarrow$ 2NaN$O_3$ + H$_2$S
These will escape from the solution and will not
interfere with the test for halogens.