Chlorine

Chlorine is a greenish-yellow, choking gas. It is made by passing electricity through salt water, which also yields sodium hydroxide and hydrogen. Carl Wilhelm Scheele first made it in 1774, and Humphry Davy later showed it was an element. Chlorine kills germs, which is why it is added to drinking water and swimming pools.

Much of it goes into plastics and chemicals. In the television it is added to ethylene to make vinyl chloride for PVC, and gives epichlorohydrin the reactivity that makes epoxy. Chlorine atoms shift copper phthalocyanine from blue to the green pigment and deepen the red pigment. It also forms chloromethane and hydrogen chloride, key steps toward silicone and chip-grade silicon.