Pharaoh’s serpents, or pharaoh’s snakes,are a type of small firework in which a lighted tablet exudes smoke and ash in a growing column which resembles a serpent. They are toxic so now this firework is only produced as a chemistry demonstration. The reaction is a result of the combustion of mercury (II) thiocyanate (Hg(SCN)2).
Igniting mercury(II) thiocyanate causes it to decompose into an insoluble brown mass that is primarily carbon nitride, C3N4. 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