Here’s element number 17, chlorine, in our International Year of the Periodic Table series with the Royal Society of Chemistry. Chlorine is essential for safe drinking water, but also has a history of use in chemical warfare.

Chlorine is a green-yellow gas at room temperature. It’s a toxic gas and was used as a chemical weapon during World War I. Denser than air, its noxious cloud moved across the ground driven by the wind, and formed hydrochloric acid in the lungs of anyone unfortunate enough to breathe it in. In more recent years, it’s been concluded that chlorine gas was likely used in attacks in Syria.

Despite its toxic nature, chlorine also has applications that are of great benefit to us. Its use at low concentrations makes water safe to drink by killing bacteria and other microbes. It’s helped essentially wipe out water-borne diseases like cholera in countries where this is practised. It’s also used to treat the water in swimming pools.

Chlorine also finds use in plastic production. It’s used to produce polyvinyl chloride, more commonly known as PVC, the plastic used for window frames, drainpipes, flooring and more. Its reactions don’t just find use in plastics; the Royal Society of Chemistry state that 85% of pharmaceuticals use chlorine or its compounds at some stage during their manufacture.

Remember, you can keep track of all of the previous entries in this series on the site here, or on the Royal Society of Chemistry’s dedicated page.