Lassaigne's extract of p-nitrochlorobenzene is acidified with dil.HNO3 and then treated with silver nitrate solution, the white precipitate formed is due to

Lassaigne's extract for the compound p-nitrochlorobenzene contains both NaCN as well as NaCl, so on treatment with AgNO3, the extract gives precipitate of AgCN as well as AgCl unless it is boiled with HNO3 which will remove NaCN as gaseous HCN.