What is chlorine?
Chlorine (symbol Cl) is element 17 on the periodic table, a halogen in period 3, group 17. A pale green gas that kills bacteria, bleaches fabric — and, bound to sodium, sits on your dinner table. At room temperature it is a gas.
The story
What chlorine is, and how we found it
Chlorine is the Jekyll and Hyde of elements - a deadly poison that keeps you healthy. As a gas, it's a greenish-yellow chemical weapon so toxic it was used in World War I trenches. But combined with sodium, it becomes table salt, essential for life. This schizophrenic element disinfects swimming pools and drinking water, saving millions from waterborne diseases, yet in its pure form, it attacks your lungs and can kill in minutes. Chlorine bleach whitens your clothes and kills 99.9% of germs, making it one of humanity's greatest public health tools. Your stomach acid contains chlorine as hydrochloric acid, powerful enough to dissolve metal. From PVC pipes to pesticides, from medicines to microchips, chlorine is everywhere in modern life - just never in its pure, terrifying form.
The discovery of chlorine
The Discovery and History of Chlorine
Early Chemical Investigations
The story of chlorine begins with Carl Wilhelm Scheele (1742-1786), a Swedish pharmacist working in Uppsala, Sweden. On a winter day in 1774, Scheele heated pyrolusite (manganese dioxide) with muriatic acid (hydrochloric acid) and observed a pungent, greenish-yellow gas that bleached flowers and burned his lungs. He called this mysterious substance "dephlogisticated muriatic acid," believing it was muriatic acid with its phlogiston removed.
Scheele recorded the gas's yellow-green colour, a smell like that of aqua regia, its bleaching of litmus and its deadly effect on insects. His reaction (MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂) had created the first recorded sample of pure chlorine gas.
The Elemental Controversy
Claude Louis Berthollet (1748-1822), the renowned French chemist, studied Scheele's gas extensively from 1785-1789. Initially supporting Scheele's compound theory, Berthollet first used the gas to bleach textiles (1785) and developed Javel water, a sodium hypochlorite bleaching liquid (1789). However, he remained convinced the gas contained oxygen and called it "oxygenated muriatic acid."
The controversy intensified when Antoine Lavoisier (1743-1794) insisted all acids must contain oxygen (from Greek "oxys genes" - acid producer). This created a 30-year scientific debate about chlorine's true nature.
Humphry Davy's Definitive Proof
The mystery was solved by Sir Humphry Davy (1778-1829) at the Royal Institution in London. Using his newly invented electrolysis apparatus, Davy attempted to decompose the green gas and obtain its supposed oxygen component. Despite months of experiments in 1807-1810, he failed to extract any oxygen.
On 15 November 1810, Davy presented his conclusion to the Royal Society: the gas was not a compound but an element. He named it "chlorine" from the Greek "chloros" (pale green), finally recognizing it as a fundamental element.
Davy's colleague Michael Faraday (1791-1867) provided crucial support by successfully liquefying chlorine in 1823, demonstrating its distinct physical properties and confirming its elemental nature.
Industrial Revolution and Bleaching Powder
Charles Tennant (1768-1838), a Scottish chemist and businessman, transformed the textile industry by developing bleaching powder in 1799. His factory in St. Rollox, Glasgow, became the world's largest chemical works, producing calcium hypochlorite [Ca(ClO)₂] by passing chlorine gas over slaked lime.
Tennant's innovation transformed textile manufacturing - traditional bleaching with buttermilk and sunlight required 6-8 months, while chlorine bleaching took just hours. By 1815, his company produced 9,000 tons annually, employing over 1,000 workers and generating enormous profits.
The Chloralkali Industry Birth
The modern chlorine industry began with Hamilton Young Castner (1858-1899) and Karl Kellner (1851-1905), who independently developed electrolytic processes for producing chlorine and sodium hydroxide from salt water. Castner and Kellner developed similar mercury cells independently in the early 1890s and joined forces in 1895.
Herbert Henry Dow (1866-1930) transformed American chlorine production by founding Dow Chemical Company in 1897. His innovative electrolytic cells used Midland, Michigan's underground brine deposits, making Dow the world's largest chlorine producer by 1920.
World War I and Chemical Warfare
Chlorine's dark chapter began on April 22, 1915, at the Second Battle of Ypres when German forces released 168 tons of chlorine gas, creating the first large-scale chemical weapon attack. Fritz Haber (1868-1934), later Nobel laureate for ammonia synthesis, directed this program despite his wife Clara's opposition and eventual suicide in protest.
The attack killed 5,000 Allied soldiers and wounded 10,000 more, opening a new era of chemical warfare. Both sides subsequently developed more sophisticated chemical weapons, though chlorine's visibility and crude delivery methods limited its effectiveness.
Public Health
Chlorine's greatest legacy emerged through water disinfection. John L. Leal (1858-1914) conducted the first continuous municipal water chlorination in Jersey City, New Jersey, in 1908. Despite legal challenges from residents fearing "chemical poisoning," Leal's system eliminated typhoid fever deaths within two years.
Abel Wolman (1892-1989) and Linn Enslow refined chlorination science, establishing dosage calculations and residual monitoring that became global standards. Their work enabled safe drinking water for billions and ranks among the greatest public health achievements of the 20th century.