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

Explanation

Sodium extract is boiled with nitric acid to decompose NaCN and NaS if present.
`Na CN + HNO_3 to Na NO_3 + HCN uarr`
`Na_2 S + HNO_3 to 2 Na NO_2 + H_2 S uarr`
If cyanide and sulphide ions are not decomposed, they will interfere with the test by forming precipitate of `Ag CN and Ag_2 S`
`Na CN + Ag NO_3 to underset("White ppt.") (Ag CN + Na NO_3)`
`Na_2 S + 2 Ag NO_3 to underset(underset("(Black ppt.)")("Silver sulphide")) (Ag_2 S + 2 Na NO_3)`