It’s in the ocean, it’s in your pool, and you need it in order to survive - and no, it’s not water! Chlorine (Elemingle answer #46) is an extremely reactive halogen element, and is the third most abundant element in the ocean. It’s essential for living organisms, and we consume most of ours through salt (NaCl), allowing chloride ions to regulate osmotic pressure and pH and aid digestion in the stomach. When added to pool water, chlorine forms a weak acid called hypochlorous acid that kills bacteria like salmonella and E. coli, and germs that cause viruses such as diarrhea and swimmer’s ear. In addition to acting as a disinfectant, chlorine prevents algae growth and helps break down dirt. 

However, chlorine also has its dark side. Chlorine gas is toxic, and heavier than air, which enabled Germany’s use of it as a chemical weapon in World War I. The gas irritates the eyes, skin, and respiratory system, and a high enough exposure can be fatal. Soldiers would seek protection from it by ascending to higher ground, and breathing through damp cloth, as the gas dissolved in water.  

Pure chlorine is obtained through electrolysis of saltwater. Chlorine is also used in the manufacturing of many compounds such as synthetic rubber, and chlorine compounds are used in the production of medicines, plastic, food, paint, solvents, and pesticides. If you’ve ever bleached your white clothes, you used sodium hypochlorite (NaOCl), which is manufactured by the reaction of molecular chlorine with sodium hydroxide and water. You can thank French chemist Claude Louis Berthollet for discovering that in 1789!

Learn more: Chemical Safety Facts, Britannica, ScienceDirect

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