When potassium iodide (KI) solution is added to certain cations, it forms insoluble precipitates. Mercury(II) ions (Hg²⁺) react with KI to form mercury(II) iodide (HgI₂), which is initially red or yellow but turns black upon standing or with excess KI due to the formation of a complex. The reaction is:

$\mathrm{Hg}{}^{2+}+2I^{-}\to \mathrm{Hg}{I}_2(s)$ (black)

Other cations like Cd²⁺ (yellow), Cu²⁺ (brown), and Bi³⁺ (black or orange) may form precipitates, but only Hg²⁺ is specifically known for its characteristic black precipitate with KI.

Final Answer: Hg⁺²