Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens?

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Solution

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 Na2S, then they are removed. The chemical equations involved in the reaction are represented as
$N a C N + H N O_{3} \to N a N O_{3} + H C N$
$N a_{2} S + 2 H N O_{3} \to 2 N a N O_{3} + H_{2} S$

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