A small pile of powder on a heat-resistant surface and a flame, that’s all it takes to create this experiment. Slowly but surely, the powder starts to smolder. At first, there’s only a wisp of smoke, but then something almost magical happens: from the powder, a strange, snake-like structure begins to grow. It twists and turns as it extends upward, slithering in slow-motion coils. As it continues to grow, the ash becomes long and snake-like, giving the illusion of a serpent rising from the ground.

Of course, there’s no magic involved — just chemistry. What you’re seeing is basically a very expanded product.

At the heart of the Pharaoh’s Snake is a chemical called mercury(II) thiocyanate. When heated, mercury(II) thiocyanate undergoes thermal decomposition, breaking down into simpler substances and releasing gases like carbon disulfide and nitrogen. The solid remnants left behind are what we see as the “snake.”

This chemical reaction was discovered by Friedrich Wöhler in 1821, soon after the first synthesis of mercury thiocyanate. Wöhler described it as “winding out from itself at the same time worm-like processes, to many times its former bulk, of a very light material of the color of graphite.” For some time, the product was sold as a firework or party trick, but it’s extremely toxic if ingested, so it was shelved not long afterward.

Nowadays, there’s a similar and safer experiment that’s typically carried out.

The carbon snake

Instead of using hazardous chemicals like mercury(II) thiocyanate, the carbon snake relies on everyday, non-toxic ingredients such as sugar and baking soda. The sugar and baking soda are mixed in a ratio of about 4 to 1. The ratio of sugar to baking soda can be adjusted slightly, but this mix generally works well.

The result is a long, black, ashy “snake” that twists and curves, growing far larger than the small mound of sugar and baking soda you started with. It’s the same physical principle, just with safer substances (though not quite as spectacular).

What happens on a molecular level

1. Mercury(II) thiocyanate reacts to the heat and breaks apart.
2. The chemical reaction releases carbon disulfide (CS₂) and nitrogen gas (N₂), which causes the mass of the powder to expand as these gases push out.
3. The expanding solid forms the long, snake-like structure as the gases escape and leave behind the solid carbon and mercury sulfide (HgS).

This reaction is exothermic, meaning it releases heat, which helps sustain the continuous breakdown of the compound, creating the appearance of the snake steadily growing as it “crawls” out of the fire. What we see is mostly carbon-based ash that curls into intricate shapes due to the uneven expansion of the reaction’s by-products.

A spectacular chemistry experiment

The Pharaoh’s Snake is one of many chemistry demonstrations designed to wow an audience. Such experiments are often used in educational settings to spark curiosity about science. But they also offer a glimpse into the amazing ways chemistry impacts our lives.

The materials we use, the food we eat, and even the air we breathe are all governed by chemical principles. Understanding the basics of how substances interact with each other not only deepens our appreciation of the world around us but can also lead to advancements in technology, medicine, and environmental protection.

For example, thermal decomposition—the process that drives the Pharaoh’s Snake—is also at work in industrial processes like smelting metals or producing cement. On a smaller scale, it’s what happens when you burn fuel in your car or light a candle at home. The study of chemical reactions is at the heart of some of the most significant innovations in history, from the discovery of new medicines to the development of clean energy solutions.

Unless you really know what you’re doing, you shouldn’t attempt to recreate the pharaoh’s snake experiment — try the carbon one. While the carbon snake experiment is generally safe and uses common household items like sugar and baking soda, it still involves open flames and chemical reactions that require caution.

Always perform the experiment in a well-ventilated area, preferably outdoors, to avoid inhaling any gases released during the reaction. Keep flammable materials away from the setup, and ensure that a heat-resistant surface like sand or a ceramic dish is used to prevent fires. Never leave the experiment unattended, and have water or a fire extinguisher nearby for emergencies.