Discussion

This sequence of images shows 0.1 M aqueous potassium iodide, KI, solution being added from a buret to 0.1 M aqueous mercury(II) nitrate, Hg(NO₃)₂, solution in a crystallizing dish. The precipitate is mercury(II) iodide, HgI₂(s). When HgI₂ precipitates, the yellow form, β-HgI₂, forms first. However β-HgI₂ is less stable than the red α-HgI₂, which forms after a few seconds. The α and β forms have different crystal structures. In the α form, each Hg²⁺ ion is surrounded tetrahedrally by four I^- ions. In the β form, two Hg—I distances are shorter than all the others and more I^- ions are next-nearest neighbors, forming a distorted octahedron around the Hg²⁺ ion.