Pharaoh's serpent
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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.
Contents
Overview
The reaction occurs in several stages:[1]
Igniting mercury thiocyanate causes it to form an insoluble brown mass that is primarily carbon nitride (C3N4), with mercury sulfide and carbon disulfide also being produced.
- 2 Hg(SCN)2 → 2 HgS + CS2 + C3N4
Carbon disulfide burns releasing carbon dioxide and sulfur dioxide:
- CS2 + 3 O2 → CO2 + 2 SO2
The heated C3N4 partially breaks down to form nitrogen gas and cyanogen:
- 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.
Preparation
NileRed made a few videos on how to do this demonstration.
Safety
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.
