Q: What is growing from that white powder?

OK, I’m gonna say it- this demo grosses me out a little. I don’t have any real reason for this reaction. I’m fine with snakes, and while this demo could arguably be labeled as “Devil’s Snare” in a Harry Potter-themed demonstration show, it’s just a little unsettling. Yes, I realize that this is a silly reaction. Moving on.
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Pharaoh’s Snake or (Pharaoh’s Serpent) is a demonstration that involves basically two ingredients: mercury (II) thiocyanate and fire.






Mercury (II) thiocyanate (Hg(SCN)2) is an inorganic, odorless, white crystalline powder. The “mercury (II),” by the way, refers to the +2 charge of the mercury cation (Hg2+). Old school chemists may write mercury (II) thiocyanate as mercuric thiocyanate. When mercury (II) thiocyanate is lit on fire, it decomposes or breaks up, forming carbon nitride (C3N4), mercury (II) sulfide (HgS), and carbon disulfide (CS2).

2 Hg(SCN) 2 (s) → C3N4 (s) + 2 HgS (s) + CS2 (l)

(Abbreviations (s), (l), and (g) refer to solid, liquid, and gas respectively.)

Some of the carbon nitride further breaks down, forming cyanogen ((CN) 2) and nitrogen gas (N2).

2 C3N4 (s) → 3 (CN)2 (g) + N2 (g)

The mercury (II) sulfide can further react with oxygen in the air to create mercury gas and sulfur dioxide (SO2).

HgS (s) + O2 (g) → Hg (g) + SO2 (g)

Carbon disulfide is flammable and easily combusts to make carbon dioxide (CO2) and sulfur dioxide.

CS2 (l) + 3 O2 (g) → CO2 (g) + 2 SO2 (g)

The resulting “Pharaoh’s snake” is made of mainly carbon nitride foam and any remaining mercury (II) sulfide that has not broken down. The gas fluffs up the solid foam, creating a very light, solid “snake.”

Mercury (II) thiocyanate is toxic if inhaled or ingested. Mercury (II) thiocyanate used to be sold as a pyrotechnic in Germany, but it was banned after several kids ate the stuff and died. That’s why this demo should only be performed in a fume hood by a trained professional wearing protective gear.

Don’t do this at home. Instead, become a chemist.

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