Hydrogen chloride

Hydrogen chloride is a molecule of one hydrogen atom and one chlorine atom. At room temperature it's a colorless gas with a stinging smell. It dissolves eagerly in water, and the solution is hydrochloric acid, the same acid your stomach uses to digest food.

One way to make it is simply to burn hydrogen gas in chlorine gas. The two react in a pale flame, giving off a lot of heat, and the product is pure HCl. Both gases often come from the same plant: passing electricity through salt water to make chlorine also releases hydrogen. Even more HCl is made as a by-product whenever chlorine is used to make chemicals such as vinyl chloride. In many of those reactions, for every chlorine atom that ends up in the product, another leaves as HCl, which is captured and reused.

In this build, dry hydrogen chloride gas is blown through powdered metallurgical-grade silicon at about 300 °C. It turns the silicon into trichlorosilane, a liquid that boils just above room temperature. A liquid can be distilled to extreme purity in a way a solid can't, which makes this the key step on the road to chip-grade silicon.