The revelation of a surprise element

Greenish. That is the meaning of the Greek word khlorós, which gave its name to the natural chemical element with the symbol CI 17, one of the 100 that make up the structure of our planet. Despite being the 21st most abundant chemical element in the Earth's crust, chlorine is rare in its free form and is found in nature combined with other elements, mainly as sodium chloride. Yes, you read that correctly - its most common compound is simple table salt, known since ancient times, with archaeological evidence that it has been used for more than eight thousand years.

Chlorine itself, and its fundamental properties for human civilisation, was only discovered much more recently. It was officially discovered in 1774, although, in 1630, chlorine gas had already been synthesised for the first time in a chemical reaction in the alchemist experiments of the Belgian chemist Jan Baptist van Helmont, who did not realise its importance.

It took more than 140 years for the Swedish pharmacist Carl Wilhem Scheele to pour a few drops of hydrochloric acid into a piece of manganese dioxide in a small experiment lab and, in seconds, the yellow-green gas appeared.

As a result of this reaction: MnO₂ + 4 HCl → MnCl₂ + Cl₂ + 2 H₂O, he called it "dephlogisticated muriatic acid air”, since it was a gas that came from hydrochloric acid (until then known as "muriatic acid") and because it had been released according to the phlogiston theory. He did not, however, identify chlorine as an element, incorrectly believing that it was an oxide obtained from hydrochloric acid, which he called muriaticum.

Even though he had no idea that he had discovered chlorine, because he was the first to study it and to observe several of its properties, such as the whitening effect on the Litmus test, the fatal effect on insects, and the odour similar to aqua regia (an acidic mixture capable of rendering metals soluble), Scheele is now considered responsible for its discovery.

The discovery that that yellowish-green gas was not just a compound of oxygen, but a natural chemical element that usually results from direct or indirect oxidation by oxygen, only emerged several decades later, through the work of British chemist Sir Humphrey Davy. It was he who, in 1810, proposed the name chlorine, inspired by the Greek word.

Because it is a very reactive element, chlorine in nature is usually linked to compounds such as sodium, potassium and magnesium and when isolated it becomes a gas that is 2.5 times heavier than air, although it is (after fluorine) the second lightest of the halogen elements - the category to which it belongs. It takes on a liquid state at -34ºC and becomes a yellowish crystalline solid at minus 103ºC.

The fact of having the highest electron affinity and the fourth highest electronegativity of all reactive elements also makes chlorine a strong oxidising and disinfectant agent, since it has the capacity to bind and destroy the outer surface of bacteria and viruses. In fact, its first use as a germicide was as early as 1847, in the maternity ward of Vienna Hospital, Austria, to prevent the puerperal fever that killed many women after childbirth.

Since then, over decades of research, scientists have learned to use its powerful properties to make the planet safer for humanity - from the quality of the drinking water we drink and bathe in, to the elimination of dangerous germs, as well as the production of medicines and medical equipment that save lives, the protection of police and firefighters on their missions and even our abundant food supply. And we must not forget that, without sodium chloride, there would not even be life, as it is this compound that prevents our bodies from drying out and allows us to move our muscles. Besides giving a lot of flavour to our lives.