Chlorine

Chlorine in the form of common table salt (sodium chloride, NaCl) has been known and used for many thousands of years. In Medieval times, Arabic alchemists experimented with various chloride salts and produced hydrogen chloride gas. When dissolved in water, the salt becomes hydrochloric acid, a strong and useful acid.

In 1774, Carl Wilhelm Scheele, a Swedish chemist, heated hydrochloric acid with a mineral now known to be manganese dioxide. One of the products of the reaction was chlorine gas (Cl₂). Sheele observed the dense gas and noted its yellow-green color, its choking smell, and its bleaching effect on litmus, leaves, and flowers. He saw that it killed insects.

In 1809, French chemists Joseph Louis Gay-Lussac and Louis-Jacques Thénard tried to make chlorine gas react with charcoal. If the gas were an oxide of something, as Scheele had thought, the charcoal should peel the oxygen away to make carbon dioxide and some other material would remain. The following year, English chemist Sir Humphry Davy tried the same experiment and confirmed what the French researchers had found: the technique didn’t work. Chlorine gas, he correctly concluded, was not an oxide—it was its own element. He named it chlorine after the Greek word chloros, meaning yellow green.

Chlorine is a member of the halogen (salt-forming) column, or group, on the periodic table. It is extremely reactive, with the third highest electronegativity among all the elements, after fluorine and oxygen, and the highest electron affinity. This means it has an extremely strong tendency to pull electrons from another element when bonding and releases the most energy when doing so. It readily and directly combines with nearly all elements.

Most chlorine is used to manufacture chlorinated compounds for sanitation, paper-pulp bleaching, disinfectants, and textile processing. It is also extensively used in the production of petroleum products, medicines, antiseptics, bleach cleaners, insecticides, food, solvents, plastics, paints, and many other consumer products.

About 20 percent goes into polyvinyl chloride (PVC), the third most widely used synthetic plastic. By weight, PVC is more than 50 percent chlorine. It is used for flooring, pipes, bottles and packaging, inflatable products, car interiors, wiring insulation, synthetic rubber, and many, many other things.

As a disinfectant, chlorine kills bacteria. It is used to treat drinking water the world over. It is likewise used to make swimming pools safe to swim in.

While chlorine is a daily part of our lives, its gas form is extremely poisonous. At a modest concentration of 1000 parts per million in air, it is fatal after just a few deep breaths. It was first used as a chemical weapon in World War I.

In nature, chlorine is found in the combined state only, chiefly with sodium as table salt. Salt dissolves readily in water and therefore finds its way into the ocean over geologic time.

Chlorine is the 21st most abundant element in the earth’s crust, often found as salts in evaporated seabeds. But it is the third most abundant element in the oceans after oxygen and hydrogen! In fact, it comprises nearly 2 percent of the mass of the oceans. Some inland seas, such as the Dead Sea and the Great Salt Lake, have even higher concentrations.

Chlorine gas, hydrogen gas, and sodium hydroxide are produced by passing a current through salt water. (This manner of using electricity to separate chemicals is called “electrolysis.”). All three outputs are commercially valuable.