Q: What is chlorine?
A: Chlorine is a chemical element, one of roughly 90 basic building blocks of matter. Naturally chemically reactive, chlorine’s tendency to combine with other elements and compounds has been used to produce thousands of essential products, ranging from drinking water disinfectants to solar energy panels to replacement knee and hip joints.
Represented by the chemical symbol “Cl,” chlorine is number 17 on the Periodic Table of the Elements, indicating each atom of chlorine contains 17 protons and 17 electrons. Due to its natural chemical reactivity, chlorine is rarely present in nature by itself as elemental chlorine, and typically exists bonded to other elements in the form of chemical compounds such as sodium chloride (table salt).
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Q: When was chlorine discovered?
A: In 1774, the Swedish chemist Carl Wilhelm Scheele observed a greenish gas when he combined hydrochloric acid with manganese dioxide. Scheele thought the gas contained oxygen. He did not realize he had generated a previously unknown chemical element. It was not until 1810 that British scientist Sir Humphrey Davey identified the gas as a distinct chemical element, and named it “chlorine” after the Greek cloros, meaning “pale green.”
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Q: What are the properties of chlorine?
A: Chlorine is a naturally reactive chemical element that bonds readily with many other substances to form a vast array of useful compounds. For example, the versatile chemistry of chlorine is employed to help produce 93 percent of life-saving pharmaceuticals and 86 percent of crop protection chemicals.
Elemental chlorine is produced industrially by applying electricity to sodium chloride (salt) brine solutions. At room temperatures and pressures, elemental chlorine is a greenish-yellow irritant gas. Chlorine gas consists of pairs of chlorine atoms bonded to one another (chemical symbol: Cl2). Chlorine gas can be detected at very low levels—it is readily identified by most people as the odor of laundry bleach at 0.2 – 0.4 parts per million (one part per million is equivalent to one drop of dye in 18 gallons of water). This excellent warning property enables rapid escape from chlorine gas. People who work with chlorine gas are trained in all aspects of its safe handling.
Q: Where is chlorine present in nature?
A: Chlorine is the 19th most abundant chemical element in the Earth. While elemental chlorine (Cl2 gas) is almost never found in nature, over 2,000 naturally occurring chlorine-containing compounds have been identified in living organisms. Chlorine is also found at some level in virtually every rock on Earth.
The weathering and erosion of the rocks of the continents cause chloride (a negatively charged form of chlorine) to be dissolved in rain and melting snow and carried to the oceans by Earth’s rivers. Over long periods of geologic time, the oceans have become great reservoirs of chloride. Ocean water has evaporated in some places over the millennia, leaving behind large salt deposits. One of the salts in these deposits, halite (sodium chloride), is used as common table salt and as the raw material from which elemental chlorine is produced.
Q: Is chlorine present in the human body?
A: Yes, human beings cannot live without chlorine. Our body's cells exist in a sea of fluid. This fluid is mostly water, along with dissolved charged atoms of sodium and chloride. Chloride plays an essential role in a delicate balancing act: providing for the electrical neutrality and the correct pressure of body fluid. One result of this balancing act is that the amount of water we retain and concentrations of salt in our bodies remain relatively constant over time. We don't dry up, nor do we bloat uncontrollably. When changes occur, the balance reasserts itself. For example, after heavy exercise and perspiration, the body requires salt; and we are usually thirsty after eating salty food.
Chloride ions are building blocks of hydrochloric acid, which is essential to our digestive system. Hydrochloric acid, made in the stomach, has two main purposes: to help destroy germs that arrive with food; and to help break down foods, ensuring that essential nutrients are made available to the body.
Chlorine is also essential to the immune system, which is charged with fighting off the daily invasion of germs. When infections take place, hypochlorite—a chlorine-containing compound which is a well-known disinfectant—forms in white blood cells. Hypochlorite either attacks germs or helps to activate other disinfecting agents.