Lassaigne ‘s solution is boiled with dilute ${\text{HNO}}_{\text{3}}$ before testing for halogens because

Lassangne’s solution is boiled with dil  ${\text{HNO}}_{\text{3}}$ before testing for halogens to decompose $\text{NaCN\hspace{0.17em}}\left(\text{or}\right){\text{\hspace{0.17em}Na}}_{\text{2}}\text{S}$ present in the lassaigne’s extract other wise these will Produce white precipitate with ${\text{AgNO}}_3$