Lunar caustic $\left({AgNO}_{3}\right)$ is used as reagent hare to distinguish ${Cl}^{-}, {Br}$ and ${I}^{-}$respectively as follows.

${Cl}^{-}({aq}) \stackrel{{AgNO}_{3}}{\longrightarrow} {AgCl} \downarrow_{{ppt}}$ white

${Br}^{-}({aq})\stackrel{{AgNO}_{3}}{\longrightarrow} {AgBr} \downarrow_{{ppt}} {pale}$ yellow

${I}^{-}({aq}) \stackrel{{AgNO}_{3}}{\longrightarrow} {AgNO}_{3} {AgI} \downarrow_{{ppt}}$ Dark yellow