How to Make Pharaoh’s Serpent – Simple Black Snake Demo

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Pharaoh's Serpent or Black Snake
Pharaoh’s serpent is a type of black snake chemical reaction. While once used as a firework, it’s now reserved for chemistry demonstrations. (photo: Tomasz Szymborski)

Pharaoh’s serpent or Pharaoh’s snake is the original black snake firework. Igniting a chemical tablet causes a golden snake of ash to grow and branch from its origin. The reaction has been popular for over 200 years, but today it’s only seen as a chemistry demonstration. This is because there are safer chemicals that offer a comparable effect. Here is how to make Pharaoh’s serpent, a look at its chemistry, and alternatives to the reaction.

How to Make Pharaoh’s Serpent

This is an extremely simple firework demonstration. Simply ignite a small pile of mercury(II) thiocyanate [Hg(SCN)2]. While it’s not necessary, covering the pile with a thin layer of sand makes it appear that a golden snake rises from the earth.

Mercury thiocyanate is an insoluble white solid. It is available as a reagent or you can synthesize it as a precipitate from the reaction between mercury(II) chloride or mercury(II) nitrate and potassium thiocyanate.

How to Make Mercury Thiocyanate

Ideally, order mercury thiocyanate from a chemical supply house. However, it’s easy to make the chemical from precursors. Only perform this synthesis within a fume hood or outdoors. All steps of the reaction involve mercury, so dispose of waste as if it was liquid mercury.

Materials

You don’t need precise amounts of these chemicals. It’s important to use concentrated nitric acid and not dilute nitric acid in order to get the correct mercury oxidation state.

  • ~4 g potassium thiocyanate (KSCN)
  • ~0.3 mL mercury
  • ~15 mL concentrated nitric acid (HNO3)

Procedure

  1. Pour nitric acid into a small beaker. Add the mercury. This forms a green solution which bubbles and releases a dark reddish-brown vapor. Let the reaction proceed until the solution stops producing gas.

    4HNO3(aq) + Hg(l) → Hg(NO3)2(aq) + 2H2O(l) + 2NO2(g)

  2. Boil the liquid. This drives off remaining nitrogen dioxide gas and converts residual mercury(I) to mercury(II). Mercury(II) is soluble in water, while mercury(I) is not. Continue boiling until the vapor changes color from red to white and the liquid changes color from green to amber.

  3. Let the solution cool to room temperature. At this point, the liquid is clear.

  4. Pour the liquid into a beaker containing about 40 mL water. Rinse the original container with about 20 mL water. Rinse it again with another 20 mL water to collect any mercury nitrate from the glass.

  5. Add approximately 4 g potassium thiocyanate (KSCN). This reaction yield mercury thiocyanate and potassium nitrate. Mercury thiocyanate is an insoluble white precipitate, while any remaining potassium thiocyanate is solution and remains in solution.

  6. Let the mercury thiocyanate sink to the bottom of the container. Decant off the liquid and dispose of it as mercury waste. Transfer the white solid to a filter paper. Rinse the beaker with water to collect any remaining solid.

  7. Let the powder dry enough that it resembles a thick paste. Use a spatula or other implement (not your hands) to shape the white paste into one or more pellets (Pharaoh’s serpents). Let these pellets dry overnight and ignite them for the reaction!

Pharaoh’s Serpent Chemical Reaction

Igniting mercury(II) thiocyanate causes it to decompose into an insoluble brown mass that is primarily carbon nitride, C3N4. Other products are mercury(II) sulfide and carbon disulfide.

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, releasing mercury vapor and sulfur dioxide. If you perform the reaction inside a container, observe a gray mercury film coating its interior surface.

HgS + O2 → Hg + SO2

Potential Practical Application

The Pharaoh’s serpent reaction isn’t just visually appealing. It may hold promise for metal electrocatalyst synthesis. Specifically, the reaction works in making Fe-/N-doped carbon (Fe/N-C) nanosheet or nanotube electrocatalysts. This type of catalyst finds use in rechargeable Zn-air batteries.

Safety

While Pharaoh’s serpents don’t explode or release sparks, the reaction is hazardous because it involves a mercury compound. Only perform the demonstration within a fume hood. Don’t inhale the smoke or touch the ash. Dispose of the remains with other mercury waste. As always, wear proper safety attire, including a lab coat, gloves, and safety goggles.

Pharaoh’s Serpent Alternatives

Fortunately, there are safer alternatives to the Pharaoh’s serpent demonstration:

  • Sugar and Baking Soda Black Snakes: Pharaoh’s serpents are golden yellow, while the sugar and baking soda reaction produces black snakes. This reaction uses non-toxic chemicals.
  • Calcium Supplement Black Snakes: Burning certain calcium supplements produces a black snake. The reaction is not as dramatic as the Pharaoh’s serpent, but the materials are readily available and non-toxic.
  • Sulfuric Acid and Sugar Black Snake: The dehydration of sugar using sulfuric acid is as dramatic as Pharaoh’s serpent. Because the reaction involves a strong acid, it’s best reserved for the chemistry lab.

References

  • Davis, T. L. (1940). “Pyrotechnic Snakes”. J. Chem. Educ. 17(6):268–270. doi:10.1021/ed017p268
  • Ren, G.; et al. (2018). “Ancient Chemistry “Pharaoh’s Snakes” for Efficient Fe-/N-Doped Carbon Electrocatalysts”. ACS Appl. Mater. Interfaces. 10(13):10778-10785. doi:10.1021/acsami.7b16936