Halogens are detected by AgNO3 due to the formation of AgX ppt. But, if the Lassaigne’s extract contains S2– or CN– ions due to the compound, then they may interfere in the test for halogens. CN– and S2– form AgCN and Ag2S with AgNO3 and may produce erroneous results. Therefore, the Lassaigne’s extract is treated with HNO3 to decompose HCN and H2S (vapours) so as to prevent interference.