Mercury(II) thiocyanate, Hg(SCN)2, is a very interesting compound. At room temperature, it looks pretty boring: a white or gray powdery solid that you probably wouldn’t give a second glance if you saw it.
But as soon as you put a flame to it, something truly amazing happens. The flame sparks mercury thiocyanate’s decomposition process, and as the compound is converted into pure mercury, it releases plumes of ash and smoke in winding snake-like columns. Check it out in the video below.
Combining mercury thiocyanate with some ammonium dichromate powder adds some extra flare to the reaction:
Because of its strange behavior, mercury thiocyanate was given the nickname Pharoah’s serpents, and is one of the world’s oldest forms of fireworks.
However, the entire process is highly toxic, from the handling of the solid mercury thiocyanate to the toxic gases released as the substance decomposes.
For more on the specifics of the chemistry involved in the reaction, check out this post from Ph.D chemist Anne Marie Helmenstine on About.com.