Aniline is a weak base and added to dil. HCl to form aniline hydrochloride (black precipitate) While benzene does not react with dil. HCl. Thus these can be separated by using dil. HCl $ {{C}_{6}}{{H}_{5}}N{{H}_{2}}\xrightarrow{dil.\,HCl}\underset{\begin{smallmatrix} Aniline\,\,hydrochloride \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(black) \end{smallmatrix}}{\mathop{{{C}_{6}}{{H}_{5}}N{{H}_{2}}\cdot HCl\downarrow }}\, $