How to Make a Pharaoh's Snake Firework

When you mix chemicals in a lab, it's easy to see the reaction, but lots of chemical reactions occur in the world around you every day.
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Pharaoh's snakes or Pharaoh's serpents are a type of small firework in which a lighted tablet exudes smoke and ash in a growing column which resembles a snake. The modern version of this firework is the non-toxic black snake. Pharaoh's snakes produce a more spectacular display, but they are toxic so this firework is only produced as a chemistry demonstration. If you have the materials and a fume hood, you can make your own Pharaoh's snakes.

Safety First

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.

Making Pharaoh's Snakes

This is an extremely simple firework demonstration. All you need to do is ignite a small pile of mercury(II) thiocyanate, Hg(SCN)2. 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. All mercury compounds are toxic, so the demonstration should be performed in a fume hood. Typically the best effect is obtained by forming a depression in a shallow dish full of sand, filling it with mercury(II) thiocyanate, lightly covering the compound, and applying a flame to initiate the reaction.

Pharaoh's Snakes Chemical Reaction

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

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