The Lassaigne's extract is boiled with conc $\mathrm{HNO}_3$ while testing for halogens. By doing so it
The Lassaigne's extract is boiled with conc $\mathrm{HNO}_3$ while testing for halogens. By doing so it
helps in the precipitation of $\mathrm{AgCl}$
increases the solubility product of $\mathrm{AgCl}$
increases the concentration of $\mathrm{NO}_3^{-}$ions
decomposes $\mathrm{Na}_2 \mathrm{~S}$ and $\mathrm{NaCN}$, if formed
Solution
$\mathrm{Na}_2 \mathrm{~S}$ and NaCN, if present in the extract, will be decomposed to $\mathrm{H}_2 \mathrm{~S}$ and $\mathrm{HCN}$ by $\mathrm{HNO}_3$.
$\begin{aligned}
& \mathrm{NaCN}+\mathrm{HNO}_3 \longrightarrow \mathrm{NaNO}_3+\mathrm{HCN} \\
& \mathrm{Na}_2 \mathrm{~S}+2 \mathrm{HNO}_3 \longrightarrow 2 \mathrm{NaNO}_3+\mathrm{H}_2 \mathrm{~S}
\end{aligned}$
These will escape from the solution and will not interfere with the test for halogens.