High-speed footage of the mercury iodide tornado demonstration. The beaker contains a solution of potassium iodide, with a vortex created by a magnetic stirrer. Mercury (II) chloride is added to the centre of this vortex. This forms an insoluble precipitate of yellow rhombic mercury (II) iodide, which rapidly undergoes a modification to orange tetragonal mercury (II) iodide. As the mercury iodide is spun away from the vortex it encounters a greater concentration of potassium iodide, and it forms colourless potassium tetraiodomercurate, which dissolves again. The equations for the reactions are:HgCl2 + KI --> HgI2 + KClHgI2 + KI --> K2[HgI4]