Nitric acid is added to sodium extract and boiled before adding silver nitrate to test halogens because

To convert sodium cyanide and sodium sulphide in to HCN and ${\mathrm{H}}_2\mathrm{S}$ which are volatile

To convert sodium cyanide and sodium sulphide in to sodium thiocyanate

To neutralize alkaline solution of sodium fusion extract

To get white precipitate of AgCN and black ppt of ${\mathrm{Ag}}_2\mathrm{S}$