Sodium extract is heated with concentrated ${\mathrm{HNO}}_3$ before testing for halogens because :

${\mathrm{S}}^{2-}$ and ${\mathrm{CN}}^{-}$ if present are decomposed by cone. ${\mathrm{HNO}}_3$ and hence do not interfere in the test

Ag reacts faster with halides in acidic medium

${\mathrm{Ag}}_2\mathrm{S}$ and $\mathrm{AgCN}$ are soluble in acidic medium

silver halides are totally insoluble in nitric acid