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RHYS LEWIS & MINH TAN PHAM, AHS, DECD, UNISA / SCIENCE PHOTO LIBRARY RHYS LEWIS & MINH TAN PHAM, AHS, DECD, UNISA / SCIENCE PHOTO LIBRARY
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 + KCl
HgI2 + KI --> K2[HgI4].
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