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

The title of this YouTube video is "Wierdest [sic] Chemical Reaction I have Ever Freaking Seen!!" And while we've seen some pretty amazing reactions in our day, we're inclined to agree with that description. Why? Because writhing, peduncled, apparating blahrf-fest, that's why. Oh — and it's also deadly.


But let us not fear the wriggling mass. In the words of Marie Curie, "nothing in life is to be feared, it is only to be understood." So let's learn a little more about this bizarre reaction, and explore a bit of its deadly history.


As the video makes clear, the chemical you see being spread across the bottom of the tank is Hg(SCN)2, aka Mercury(II) Thiocyanate. Mercury-containing compounds can have some pretty nasty health effects, and Hg(SCN)2 is no different, but more on that in a second. For now, let's have a look at the chemistry, as broken down by's resident chemistry expert, Anne Marie Helmenstine:

Igniting mercury(II) thiocyanate causes it to decompose into an insoluble brown mass that is primarily carbon nitride, C2N4. 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

What's wild about this reaction is that pretty much every stage of the experiment has the potential to do you harm in one way or another; according to Helmenstine, "handling the mercury thiocyanate, breathing the smoke or touching the ash column," and coming into contact with the reaction products during cleanup are all risks that should be be considered when performing the reaction. Protective measures should therefore include no less than goggles, lab coat, gloves, and a fume hood — not to mention a scoopula for transferring the Hg(SCN)2 around. That's pretty crazy to think about, especially when you learn that people used to play with the stuff like it was nothing.

It was marketed in Germany for many years as a pyrotechnic product by the name of "Pharaoschlangen" (this translates to "Pharoah's snake," and is what many people call the reaction today). In a 1940 issue of the Journal of Chemical Education, MIT chemist Tenney Davis describes the unfortunate circumstances by which the compound came to be outlawed:

The sale of the mercuric thiocyanate Pharaoh's Serpents, with which many of us amused ourselves as children, is now forbidden by law in several of the states, for the vapors from these toys are poisonous, and, more serious, children have been known to eat them with fatal consequences.


Of course, we're not saying we think you would eat the reaction products in the video up top, but, in case you were considering it, don't. And don't inhale anything, either. Or touch it. And remember that mercury is toxic. Also, protective equipment is always a good idea. Just be careful, is all — that's what we're really getting at, here.

[Original video here via Carl Zimmer]


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Anekanta - spoon denier

Why did they have to do that in a terrarium full of cactii? Isn't mercury toxic to plants, too?