Mercury is one of five liquid metals near room temperature and standard pressure, the only liquid one at standard conditions. Elementary mercury can be described as heavy and silver in color, its salts are highly toxic.
In this experiment a solution of mercury(II) chloride, HgCl2, is added dropwise to a solution of potassium iodide, KI, stirred on a magnetic stirrer. This experiment involves beautifully colored compounds, but due to toxicity of mercury(II) salts it is hazardous.
When two colorless solutions are mixed, yellow orange precipitate of mercury(II) iodide is initially formed. HgCl2 as well as KI have previously been dissolved in distilled water. Solution of mercury(II) chloride is added to the center of the vortex. An orange precipitate of mercury(II) iodide is instantly formed in the center of the vortex, and with being dispersed it dissolves in excess of potassium iodide. Concentration of iodide ions is higher from vortex toward the periphery of the beaker, so due to complexing when the mercury salt is dispersed it passes into solution fast.
Mercury(II) chloride reacts with potassium iodide according to the following equation:
This precipitate dissolves in the reaction with excess of iodide ions present in the beaker. Due to the complexation it is easily removed, and colorless potassium tetraiodomercurate(II) is formed:
Orange HgI2 precipitate is at equilibrium with [HgI4]2- (colorless solution).
The macroscopic net effect of these two reactions resembles a tornado. If you take a careful look at the slow motion part of the video, you can observe that mercury(II) iodide is initially formed as a yellow precipitate. Second or two later, the yellow precipitate changes its color to orange, and finally dissolves yielding a colorless solution.
The same compound (HgI2 in this case) exists in two crystalline forms: yellow rhombic HgI2 and orange tetragonal modification. These compounds are known as allotropic modifications. This is the same as in case of elemental carbon, where we have three allotropic modifications: graphite, diamond and fullerene. Yellow rhombic mercury(II) iodide is formed first, which means this is the kinetic product of the reaction. However, this crystalline form is rapidly and spontaneously converted to the more stable orange tetragonal form. This is so called thermodynamic product of the reaction: