The decomposition (combustion) of mercury(II) thiocyanate (Hg(SCN)2), also known as ”The Pharaoh’s Serpent” (or ”The Pharaoh’s Snake”) is one of the weirdest classical chemistry experiments.
Ignition of mercury(II) thiocyanate causes it’s decomposition and burning, releasing gases make the solid reaction products extremely fluffy. The reaction creates an illusion, that the volume of reaction products is much larger, than that of the starting substance. Several reaction formulas have been proposed:
1) Burning in oxygen from the air:
2Hg(SCN)2(s) + 9O2(g) → 2HgO(s) + 4SO2(g) + 2N2(g) + 4CO2(g)
2Hg(SCN)2 + 3O2(g) → 2HgS + 2SO2(g) + C3N4(g) + CO2(g)
2) Thermal decomposition:
2Hg(SCN)2(s) → 2HgS(s) + CS2(g) + C3N4(g)
At higher temperatures, mercury(II) sulfide (HgS) and carbon disulfide (CS2) can react with air oxygen:
HgS(s) + O2(g) → Hg(g) + SO2(g)
CS2(g,l) + 3O2(g) → CO2(g) + 2SO2(g)
Attention! Mercury and sulfur-containing gases are toxic compounds, this reaction is carried out under a fume hood.