The Lassaigne's extract is boiled with dil. $\mathrm{HNO}_{3}$ before testing for halogens because
The Lassaigne's extract is boiled with dil. $\mathrm{HNO}_{3}$ before testing for halogens because
silver halides are soluble in $\mathrm{HNO}_{3}$
$\mathrm{Na}_{2} \mathrm{~S}$ and $\mathrm{NaCN}$ are decomposed by $\mathrm{HNO}_{3}$
$\mathrm{Ag}_{2} \mathrm{~S}$ is soluble in $\mathrm{HNO}_{3}$
$\mathrm{AgCN}$ is soluble in $\mathrm{HNO}_{3}$
Solution
$\mathrm{Na}_{2} \mathrm{~S}$ and $\mathrm{NaCN}$, formed during fusion with metallic sodium, must be removed before adding $\mathrm{AgNO}_{3}$, otherwise black ppt. due to $\mathrm{Na}_{2} \mathrm{~S}$ or white precipitate due to $\mathrm{AgCN}$ will be formed and thus white precipitate of $\mathrm{AgCl}$ will not be identified easily.
