Over the weekend, the above animated gif did the rounds on Twitter billed as being akin to “opening a portal to hell”.

But what exactly is going on?

Enter io9 with the science bit (and a nice long video):

http://www.youtube.com/watch?v=ritaljhhk7s

Igniting mercury(II) thiocyanate causes it to decompose into an insoluble brown mass that is primarily carbon nitride, C3N4. Mercury(II) sulfide and carbon disulfide are also produced.
2Hg(SCN)2 → 2HgS + CS2 + C3N4

Flammable carbon disulfide combusts to carbon(IV) oxide and sulfur(IV) oxide:

CS2 + 3O2 → CO2 + 2SO2

The heated C3N4 partially breaks down to form nitrogen gas and dicyan:

2C3N4 → 3(CN)2 + N2

Mercury(II) sulfide reacts with oxygen to form mercury vapor and sulfur dioxide. If the reaction is performed inside a container, you will be able to observe a gray mercury film coating its interior surface.

HgS + O2 → Hg + SO2

Well that explains that.

Being such a spectacular reaction, it was marketed in Germany as Pharaoschlangen (“Pharaoh’s snakes”). In the tradition of all great fireworks of bygone times, it was incredibly dangerous:

All aspects of the reaction can be hazardous, including handling the mercury thiocyanate, breathing the smoke or touching the ash column, and contact with the remains of the reaction during clean-up.

What in the name of all that is holy is going on in this video? (io9)