Discussion

Initially, a small volume (35ml) of 0.1M mercury(II) nitrate Hg(NO₃)₂ is in solution with water (3500ml) in a large beaker on a magnetic stirrer. At first, a few 0.5ml drops of 1.0M potassium iodide KI are added to the vortex of the solution and a very faint tornado appears in the bottom center of the beaker. After about 3.5ml is added, the vortex becomes silvery-orange. Eventually the orange color disperses through the beaker and the solution becomes translucent.

The addition of potassium iodide to the solution of mercury(II) nitrate results in the formation of solid mercury(II) iodide, which is orange in color.

Hg(NO₃)₂(aq) + 2 KI(aq) → HgI₂(s) + 2 KNO₃(aq)

By adding very small amounts of KI solution to the vortex, a concentration gradient is introduced which confines the HgI₂ precipitate to the center of the beaker, causing the appearance of a tornado. As more KI solution is added, more precipitate is formed, resulting in better-defined color and eventual mixing of solid throughout the beaker.

Viewing note

There are several cuts of a few seconds each from the movie to eliminate some redundancy.