Mercury Thiocyanate Serpent

How to make the Pharoah’s Serpent (Mercury (II) Thiocyanate). Mercury (II) thiocyanate is made using potassium thiocyanate, mercury and concentrated nitric acid.

How to perform the Pharaoh’s Serpent

This is a simple firework demonstration. All you need to do is ignite a small pile of mercury(II) thiocyanate. Mercury thiocyanate is an insoluble white solid which can be purchased as a reagent or can be obtained as a precipitate by reacting mercury(II) chloride or mercury(II) nitrate with potassium thiocyanate. Please make sure to provide proper safety! 

What happened?

“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

“Helmenstine, Anne Marie, Ph.D

Safety & Disposal

Although Pharaoh’s snakes are considered a type of firework, they do not explode or emit sparks. They burn on the ground and release smoky vapors. All aspects of the reaction can be hazardous, including handling the mercury thiocyanate, breathing the smoke or touching the ash column, and contact with the remains of the reaction during clean-up. If you perform this reaction, use appropriate safety precautions for dealing with mercury.

The use of this instruction and the information provided therein takes place at the user’s own risk.

Experiment Video