Why nitric acid is added in the Lassaigne's test for halogens?

Explanation

(i) If N (or) S is present in the organic compound along with the halogen, we might obtain `Na_(2)` and NaCN in the solution which interfere with the detection of the halogen in the `AgNO_(3)` test.
(ii) Therefore we boil the Lassigne.s extract with `HNO_(3)` which decomposes NaCN and `Na_(2)S` as follows:
(iii) `NaCN+HNO_(3)overset(Delta)rarrNaNO_(3)+HCN`
`Na_(2)S+2HNO_(3)rarr2NaNO_(3)+H_(2)Suarr`
`NaCN+AgNO_(3)rarrunderset("whit ppt. confusing with AgCl")(AgCNdarr)+NaNO_(3)`
`Na_(2)S+AgNO_(3)rarrunderset("black ppt.")(Ag_(2)Sdarr)+NaNO_(3)`