I found this reaction of $\ce{NH4Cr2O7 + Hg(SCN)2 ->[\Delta]\ ?}$ on youtube, and I'm curious as to how it works. I tried searching online, but didn't find much on it.



1 Answer 1


This is a combination of two reactions. The first is the ammonium dichromate vocano and the second is called Pharaoh's snakes.

The volcano

The reaction occurring here is: $$\ce{(NH4)2Cr2O7(s) -> Cr2O3(s) + N2(g) + 4 H2O(g)}$$ The reference above provides several pictures and a description of the chemistry.

Pharaoh's snakes

Again, the reference above describes what is happening, so only a summary of the chemical reactions is given here: $$\ce{2 Hg(SCN)2->2 HgS + CS2 + C3N4}$$ $$\ce{CS2 + 3O2 -> CO2 + 2SO2}$$ $$\ce{2C3N4->3(CN)2 + N2}$$ $$\ce{HgS + O2->Hg + SO2}$$

Pharaoh's snakes is a rather hazardous demonstration and should not be performed without the proper ventilation, protective gear and waste disposal equipment.

  • $\begingroup$ Which $C_3N_4$ isomer forms preferentially? This list of carbon nitrides provides such options as beta carbon nitride, graphitic carbon nitride, dicyanodiazomethane, nitrogen tricyanide and carbon bis(cyanamide). $\endgroup$
    – J.G.
    Aug 18, 2019 at 11:18

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