Halogens are detected by $AgNO_3$ due to the formation of AgX ppt. But, if the Lassaigne’s extract contains $S^{2–}$ or $CN^{–}$ ions due to the compound, then they may interfere in the test for halogens. $CN^{–}$ and $S^{2–}$ form AgCN and $Ag_2S$ with $AgNO_3$ and may produce erroneous results. Therefore, the Lassaigne’s extract is treated with $HNO_3$ to decompose HCN and $H_2S$ (vapours) so as to prevent interference.