MCQ
The Lassaigne’s extract is boiled with dil. $HNO_3$ before testing for halogens because
- ASilver halides are soluble in $HNO_3$
- ✓$Na_2S$ and $NaCN$ are decomposed by $HNO_3$
- C$Ag_2S$ is soluble in $HNO_3$
- D$AgCN$ is soluble in $HNO_3$
✓
Answer
Correct option: B.
$Na_2S$ and $NaCN$ are decomposed by $HNO_3$
b
$Na_2S$ and $NaCN$, formed during fusion with metallic sodium, must be removed before adding $AgNO_3$, otherwise black ppt. due to $Na_2S$ or white precipitate due to $AgCN$ will be formed and thus white precipitate of $AgCl$ will not be identified easily.
$Na_2S$ and $NaCN$, formed during fusion with metallic sodium, must be removed before adding $AgNO_3$, otherwise black ppt. due to $Na_2S$ or white precipitate due to $AgCN$ will be formed and thus white precipitate of $AgCl$ will not be identified easily.
$N{a_2}S + 2AgN{O_3} \to 2NaN{O_3} + \mathop {A{g_2}S}\limits_{Black} \downarrow $
$NaCN + AgN{O_3} \to NaN{O_3} + \mathop {AgCN}\limits_{White} \downarrow $
$NaCl + AgN{O_3} \to NaN{O_3} + \mathop {AgCl}\limits_{white} \downarrow $
$N{a_2}S + 2HN{O_3}\xrightarrow{{boil}}2NaN{O_3} + {H_2}S \uparrow $
$NaCN + HN{O_3}\xrightarrow{{boil}}NaN{O_3} + HCN \uparrow $
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