Templated Tube Growth About a Reactive Gas Plume: The Pharaoh's Serpent Reaction, Thermodecomposition of Mercury Thiocyanate in Air

The pharaoh's serpent reaction, the thermodecomposition of mercury thiocyanate in air, is a classic chemical demonstration. We analyse and characterize the reaction products, composed of two different phases, graphitic carbon nitride, , and metacinnabar, HgS. We show that two growth modes are involved in forming the sinuous solid product; the more common one, jack‐up growth, forms a solid cylinder from the base up. This is accompanied by a rarer second mode, chimney growth, in which fluid mechanics plays a fundamental role. In this second growth mode, reagents are transported in gaseous form in a reactive plume to be precipitated remotely on the growing rim of a tube templated around the plume. Such templated tube growth, while common in a liquid environment, for example, in the chemical‐garden reaction, has not been reported in a gaseous medium.