Halide ions form when halogens gain an electron, becoming negatively charged. The presence of halide ions—such as chloride $(Cl^-)$, bromide $(Br^-)$, and iodide $(I^-)$ can be identified by their reaction with aqueous silver nitrate $\ce{AgNO3}$.

This reaction forms a precipitate, and the colour of the precipitate indicates which halide ion is present:

• Chloride $(Cl^-)$: Forms a white precipitate of silver chloride $(AgCl)$.
• Bromide $(Br^-)$: Forms a cream precipitate of silver bromide $(AgBr)$.
• Iodide $(I^-)$: Forms a yellow precipitate of silver iodide $(AgI)$. The reactions follow these patterns:

$\ce{Cl^- + AgNO3 → AgCl + NO3^-}$

$\ce{Br^- + AgNO3 → AgBr + NO3^-}$

$\ce{I^- + AgNO3 → AgI + NO3^-}$