Question

The Lassaigne's extract is boiled with dil. $HN{O}_3$ before testing for halogens because :

• A. silver halides are soluble in $HN{O}_3$
• B. $N{a}_{2}S$ and $NaCN$ are decomposed by $HN{O}_3$
• C. $A{g}_{2}S$ is soluble in $HN{O}_3$
• D. $AgCN$ is soluble in $HN{O}_3$

Answer 1

The correct option is B $N{a}_{2}S$ and $NaCN$ are decomposed by $HN{O}_3$

Dil. $HN{O}_3$ decomposes, $N{a}_{2}S$ and $NaCN$ formed during Lassaigne's test.

While testing the Lassaigne's extract for the presence of halogens, it is first boiled with dilute nitric acid. This is done to decompose $NaCN$ to $HCN$ and $N{a}_{2}S$ to $H_{2}S$ and to expel these gases. That is, if any nitrogen and sulphur are present in the form of $NaCN$ and $N{a}_{2}S$, then they are removed.

The chemical equations involved in the reaction are represented as

$NaCN+HN{O}_3\to NaN{O}_3+HCN$

$N{a}_{2}S+2HN{O}_3\to 2NaN{O}_3+H_{2}S$

Option B is correct.