Chlorine

[DrLeftover]

Sodium chloride is chemically very stable - but split it into its constituent elements and you release the chemical equivalent of demons.

The process is brutal. Vast amounts of electricity are used to tear apart the sodium and chlorine atoms in salt molecules through the process of electrolysis. It happens at vast industrial sites known as chlor-alkali plants, the biggest of which can use as much electricity as a small country.

Which is why the price of both chlorine and sodium tend to track the price of electricity very closely.

It also explains why Industrial Chemicals Ltd's chlor-alkali plant in Thurrock, Essex, is right next to an electricity substation.
...

Inside ICL's laboratory, Andrea Sella, professor of chemistry at University College London, hands me a fragile-looking glass balloon. It is an evil-looking greenish-yellow colour.

"That's chlorine," says Professor Sella, with a wicked grin, "one of the most ferociously aggressive materials out there."

...

Something like 15,000 different chlorine compounds are used in industry, including the vast majority of pharmaceuticals and agricultural chemicals.

http://www.bbc.com/news/magazine-27057547