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Making Mercury Thiocyanate (The Pharaoh's Serpent)

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 titanic1    244



Pharaoh's snakes or Pharaoh's serpents are a type of small firework in which a lighted tablet exudes smoke and ash in a growing column which resembles a snake. The modern version of this firework is the non-toxic black snake. Pharaoh's snakes produce a more spectacular display, but they are toxic so now this firework is only produced as a chemistry demonstration. If you have the materials and a fume hood, you may wish to make your own Pharaoh's snakes.
Making Pharaoh's Snakes


This is an extremely simple firework demonstration. All you need to do is ignite a small pile of mercury(II) thiocyanate, Hg(SCN)2. Mercuy thiocyanate is an insoluble white solid which can be purchased as a reagent or can be obtained as a precipitate by reacting mercury(II) chloride or mercury(II) nitrate with potassium thiocyanate. All mercury compounds are toxic, so the demonstration should be performed in a fume hood. Typically the best effect is obtained by forming a depression in a shallow dish full of sand, filling it with mercury(II) thiocyanate, lightly covering the compound, and applying a flame to initiate the reaction.
Pharaoh's Snakes Chemical Reaction

Igniting mercury(II) thiocyanate causes it to decompose into an insoluble brown mass that is primarily carbon nitride, C3N4.


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 about.com'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.


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