Ethylene

Ethylene is a colorless gas. Its molecule is tiny: two carbon atoms joined by a double bond, with two hydrogen atoms on each. That double bond is the useful part. It can open up and grab onto other atoms, or onto other ethylene molecules, which is how many plastics are built.

Ethylene is made by steam cracking. Ethane from natural gas, or naphtha from crude oil, is mixed with steam and heated to around 800–850 °C for a fraction of a second inside furnace tubes. The heat breaks the larger molecules into smaller ones, and many end up as ethylene. The hot gas is quickly cooled, compressed and chilled until the different gases can be separated by distillation.

Ethylene is the world's most produced organic chemical, well over 150 million tonnes a year. Much of it becomes polyethylene, but in a TV it shows up in other forms. It becomes ethylene glycol for PET, joins benzene to make styrene, and forms the backbone of vinyl chloride and vinyl acetate. Plants make it too: it's the hormone that ripens fruit, which is why one ripe banana speeds up the others.