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Pharaoh's serpent or Pharaoh's snake is a very interesting but also very dangerous pyrotechnic demonstration, made from the thermal decomposition of mercury(II) thiocyanate, which produces a large mass of coiling serpent-like lightweight solid. A small blueish or slight orange flame which accompanies the combustion. The resulting "snake" can range from dark graphite or bluish grey to light tan in color with the inside generally much darker than the outside.
The reaction occurs in several stages:
- 2 Hg(SCN)2 → 2 HgS + CS2 + C3N4
- CS2 + 3 O2 → CO2 + 2 SO2
- 2 C3N4 → 3 (CN)2 + N2
Mercury sulfide reacts with oxygen to form mercury vapor and sulfur dioxide. If the reaction is performed inside a container, a grey film of mercury coating on its inner surface can be observed.
- HgS + O2 → Hg + SO2
The resulting solid is extremely light and can be broken easily.
NileRed made a few videos on how to do this demonstration.
Thermal decomposition of mercury(II) thiocyanate produces lots of mercury vapors, which are extremely toxic. This demonstration must only be performed in a well-ventilated area or in a closed space. If done in a closed space, mercury dust will deposit inside the chamber walls.