Q2 / 125 · BITSAT 2020 · Chemistry
The Lassaigne's extract is boiled with dil. $\mathrm{HNO}_{3}$ before testing for halogens because
- Asilver halides are soluble in $\mathrm{HNO}_{3}$
- Correct answerB$\mathrm{Na}_{2} \mathrm{~S}$ and $\mathrm{NaCN}$ are decomposed by $\mathrm{HNO}_{3}$
- C$\mathrm{Ag}_{2} \mathrm{~S}$ is soluble in $\mathrm{HNO}_{3}$
- D$\mathrm{AgCN}$ 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.
$\mathrm{Na}_{2} \mathrm{~S}+2 \mathrm{AgNO}_{3} \longrightarrow 2 \mathrm{NaNO}_{3}+\underset{\text { Black }}{\mathrm{Ag}_{2} \mathrm{~S}} \downarrow$
$\mathrm{NaCN}+\mathrm{AgNO}_{3} \longrightarrow \mathrm{NaNO}_{3} + \underset{\text { White }}{\mathrm{Ag} \mathrm{CN}} \downarrow$
$\mathrm{NaCl}+\mathrm{AgNO}_{3} \longrightarrow \mathrm{NaNO}_{3}+\underset{\text { White }}{\mathrm{Ag} \mathrm{Cl}} \downarrow$
$\mathrm{Na}_{2} \mathrm{~S}+2 \mathrm{HNO}_{3} \stackrel{\text { boil }}{\longrightarrow} 2 \mathrm{NaNO}_{3}+\mathrm{H}_{2} \mathrm{~S} \uparrow$
$\mathrm{NaCN}+\mathrm{HNO}_{3} \stackrel{\text { boil }}{\longrightarrow} \mathrm{NaNO}_{3}+\mathrm{HCN} \uparrow$
Hence, the correct answer is option: 'B'.