Chlorine was first isolated in 1774 by the Swedish-German chemist Carl Wilhelm Scheele, but it took another 36 years before anyone realized it was an element. In 1810, the English chemist Humphry Davy demonstrated that the greenish-yellow gas Scheele had produced was not a compound containing oxygen, as nearly every major chemist of the era believed, but an entirely new element. Davy coined the name “chlorine” from the Greek word chloros, meaning pale green or yellowish-green. The gap between those two dates hides one of chemistry’s more interesting identity crises.
What Scheele Actually Did in 1774
Carl Wilhelm Scheele was a brilliant but chronically unlucky chemist. He discovered oxygen before Joseph Priestley but published after him, discovered several other elements (barium, manganese, molybdenum, tungsten) yet rarely received full credit, and died at 43 from what many historians suspect was chronic exposure to the very chemicals he handled daily. In 1774, working in his modest pharmacy laboratory in Sweden, Scheele heated hydrochloric acid (then known as “spirit of salt” or “muriatic acid”) with manganese dioxide. The reaction released a pungent, greenish-yellow gas that bleached flowers and attacked metals.
Scheele described the gas carefully, noting its suffocating smell and its remarkable ability to decolorize organic matter. But he interpreted it through the lens of phlogiston theory, the dominant chemical framework of his time, which held that combustion and chemical change involved releasing or absorbing a hypothetical substance called phlogiston. He called his new gas “dephlogisticated muriatic acid,” believing it was muriatic acid that had lost its phlogiston. In other words, Scheele thought he had modified an existing substance, not isolated a new element.
Why It Took Decades to Recognize Chlorine as an Element
The confusion was not just Scheele’s. Antoine Lavoisier, the French chemist who dismantled phlogiston theory and established modern chemical nomenclature in the 1780s, replaced Scheele’s name with “oxymuriatic acid.” Lavoisier’s new system was built on the idea that all acids contained oxygen (the very name “oxygen” comes from the Greek for “acid-former”), so he assumed the green gas must be muriatic acid combined with oxygen. This was wrong, but it was wrong in an authoritative, systematically elegant way, which made it harder to challenge.
For roughly two decades after Lavoisier’s renaming, nearly every prominent chemist in Europe accepted the oxymuriatic acid label. The assumption that this gas was a compound, not an element, was embedded in the theoretical framework that had otherwise revolutionized chemistry. Researchers like Claude Louis Berthollet explored its bleaching properties and industrial applications, all while treating it as a derivative of oxygen. The caloric theory, which Lavoisier also promoted, further complicated matters by slowing Humphry Davy’s progress toward a correct understanding of chlorine’s nature.
1ScienceDirect. Late Eighteenth Century European Scientists – Section: CHAPTER 3 – ANTOINE LAVOISIER, 1743–1794Humphry Davy Settles the Question
Humphry Davy, working at the Royal Institution in London, began a series of experiments around 1807–1810 aimed at decomposing oxymuriatic acid into its supposed oxygen and muriatic acid components. He failed, repeatedly, because there was no oxygen to extract. Every attempt to break the gas down produced nothing simpler. By 1810, Davy was convinced that the gas was itself an element, not a compound. He presented his conclusion to the Royal Society, proposed the name “chlorine” based on the gas’s distinctive color, and argued that hydrochloric acid was a combination of this new element with hydrogen, containing no oxygen at all.
This was a direct challenge to Lavoisier’s oxygen theory of acids. Not everyone was persuaded immediately. Jöns Jacob Berzelius, the enormously influential Swedish chemist, resisted the idea for years before eventually coming around. But Davy’s experimental evidence was strong: no one could decompose chlorine into simpler substances, and his framework explained known reactions more cleanly than the oxygen-acid model did. Within a decade of Davy’s announcement, the chemistry community had largely accepted chlorine as element number 17.
Who Really “Discovered” Chlorine
This is genuinely ambiguous, and how you answer it depends on what you think “discovery” means. If discovery means being the first person to produce a substance and describe its properties, Scheele discovered chlorine in 1774. If it means recognizing a substance for what it actually is and correctly placing it in the catalog of elements, Davy discovered chlorine in 1810. Most chemistry historians credit Scheele with the original preparation and Davy with the identification as an element, which is a reasonable compromise but satisfies nobody who wants a single name and date.
The situation parallels other contested discoveries in chemistry. Oxygen has a similar two-name story (Priestley made it first, Lavoisier understood it first). The pattern reveals something about how science worked in the eighteenth century: isolating a new substance and understanding what it was were two separate intellectual achievements, often separated by years or decades, because the theoretical frameworks available at the time could mislead even exceptional experimenters.
The Bleaching Discovery That Made Chlorine Industrially Important
Even before anyone agreed on what chlorine was, its practical value was obvious. Scheele himself had noted the gas’s ability to strip color from flowers and cloth. In the 1780s, Claude Louis Berthollet in France developed a chlorine-based bleaching liquid, and by the 1790s, Charles Tennant in Scotland had produced bleaching powder (calcium hypochlorite) by combining chlorine gas with slaked lime. This was an enormous advance for the textile industry, which had previously relied on months of exposure to sunlight and sour milk to bleach fabrics.
Bleaching powder was cheap, effective, and could be shipped and stored. It transformed cloth production and became one of the first high-volume industrial chemical products. This commercial success also drove further research into chlorine’s chemistry, creating a feedback loop between industrial demand and scientific understanding that persisted throughout the nineteenth century. By the time the germ theory of disease emerged later in the 1800s, chlorine-based compounds were already well-established industrial chemicals, which made their adaptation to public health surprisingly quick.
Chlorine and the Transformation of Drinking Water
The single most consequential application of chlorine has been water disinfection. Jersey City, New Jersey, became the first U.S. city to employ large-scale chlorination of its water supply in 1908, using a system that was both effective and inexpensive enough for other cities to adopt rapidly.2PubMed Central. Water and Filth: Reevaluating the First Era of Sanitary Typhoid Intervention (1840–1940) – Section: INFRASTRUCTURE INVESTMENTS AND DECLINING TYPHOID MORTALITY IN US CITIES The impact on waterborne disease was dramatic. Typhoid fever, which had been a persistent killer in American and European cities, declined sharply wherever chlorination was introduced.
Chlorine disinfection undoubtedly contributed to this drop in typhoid mortality, though historians and epidemiologists point out that other sanitary improvements, including better sewage systems, filtration, and source-water protection, also played significant roles.3PubMed. Role of disinfection in suppressing the spread of pathogens with drinking water: possibilities and limitations Still, chlorination’s combination of low cost, scalability, and residual disinfecting power (chlorine continues killing pathogens as water travels through distribution pipes) made it the default water treatment method worldwide. Most tap water you drink today still contains a small amount of chlorine or chloramine for exactly this reason.
The success was so sweeping that some public health historians consider water chlorination one of the most important advances in disease prevention of the twentieth century, alongside vaccines and antibiotics. The element that Scheele had stumbled upon as a laboratory curiosity turned out to be a pivotal tool for keeping cities alive.
Chlorine Inside the Human Body
Chlorine in its elemental gas form is toxic, which makes it easy to forget that chloride, the ionic form of chlorine, is essential to human life. Your body contains roughly 100 grams of chloride ions at any given time, and they do more than just float around.
The most familiar role is in digestion. Your stomach produces hydrochloric acid, with the hydrogen ions secreted by specialized pumps in the stomach lining combining with chloride ions in the gastric space to form HCl.4PubMed Central. The Physiology of the Gastric Parietal Cell This acid is strong enough to break down food, kill most ingested bacteria, and activate digestive enzymes. Without chloride, you simply could not produce stomach acid.
Chloride also plays a critical role in the nervous system. GABA receptors, the brain’s major inhibitory signaling system, are ligand-gated chloride channels.5PubMed Central. Electrophysiology of ionotropic GABA receptors When GABA binds to these receptors, chloride ions flow into the neuron, typically making it less likely to fire. This is the basic mechanism behind neural inhibition, the process that keeps brain activity from running unchecked. Interestingly, the relationship is not always straightforward: under certain conditions, particularly during early brain development or when chloride accumulates inside neurons, GABA receptors can actually have the opposite effect, depolarizing the neuron and producing excitatory rather than inhibitory signals.6PubMed Central. GABA Receptors Can Depolarize the Neuronal Membrane Potential via Quantum Tunneling of Chloride Ions: A Quantum Mathematical Study Chloride’s role in the brain turns out to be more dynamic than a simple on-off switch.
Beyond the stomach and the brain, chloride ions help regulate fluid balance, blood pressure, and the acid-base equilibrium of blood. Table salt, sodium chloride, is the primary dietary source. The “chloride” in your blood test results at a routine checkup is measuring exactly this: how much of Scheele’s element, in ionic form, is circulating in your system.
Chlorine in the Atmosphere and the Ozone Problem
Chlorine’s reactivity, the same property that makes it useful for disinfection and dangerous as a gas, created one of the twentieth century’s most serious environmental crises. In the 1970s and 1980s, researchers showed that chlorofluorocarbons (CFCs), synthetic compounds used in refrigerators, aerosol sprays, and foam insulation, were drifting into the stratosphere and releasing chlorine atoms when broken apart by ultraviolet light. Those free chlorine atoms turned out to be devastating catalysts for ozone destruction.
The chemistry involves catalytic chain reactions in which chlorine species like Cl and ClO efficiently break down ozone molecules in the stratosphere.7PubMed Central. Stratospheric ozone depletion A single chlorine atom can destroy thousands of ozone molecules before being deactivated, which is why relatively small amounts of CFCs in the atmosphere produced outsized damage to the ozone layer. The discovery of the Antarctic ozone hole in 1985 turned this from an academic concern into a global emergency, leading to the Montreal Protocol in 1987, an international treaty that phased out CFC production.
The Montreal Protocol is widely regarded as one of the most successful international environmental agreements in history. CFC concentrations in the atmosphere have been declining since the mid-1990s, and the ozone layer is slowly recovering. But the episode illustrates a recurring theme with chlorine: the same chemical aggressiveness that makes it useful also makes it dangerous when it ends up in the wrong place. The element Scheele first bottled in 1774 has a remarkable capacity to do both tremendous good and tremendous harm, depending entirely on where it is and what it is reacting with.
Chlorine as a Chemical Weapon
The darkest chapter in chlorine’s history came on April 22, 1915, during the Second Battle of Ypres in World War I. German forces released roughly 170 tons of chlorine gas from cylinders along a four-mile front, creating a yellowish-green cloud that drifted over Allied trenches. The gas attacked the respiratory tract, causing fluid to accumulate in the lungs, and killed or incapacitated thousands of soldiers. It was the first large-scale use of a chemical weapon in modern warfare.
Fritz Haber, the German chemist who oversaw the chlorine gas program, had earlier won renown (and would later receive a Nobel Prize) for developing the Haber-Bosch process for synthesizing ammonia, which made modern fertilizers and food production possible. The juxtaposition is jarring: the same mind that helped feed the world also weaponized chlorine. After World War I, the Geneva Protocol of 1925 prohibited the use of chemical weapons in warfare, though it took until the Chemical Weapons Convention of 1993 for a comprehensive ban on their development, production, and stockpiling.
Chlorine gas has nonetheless resurfaced in recent conflicts. Reports from Syria documented chlorine barrel bombs dropped on civilian areas during the Syrian civil war, a grim reminder that the gas’s simplicity, it can be produced from commercially available chemicals, makes it difficult to eliminate entirely as a weapon. The same property that Scheele noticed in 1774, its aggressive reactivity with organic tissue, is precisely what makes it lethal when deployed against people.
How Chlorine Gets Its Distinctive Smell
Most people think they know what chlorine smells like from swimming pools. They are mostly wrong. The sharp, eye-watering odor at an indoor pool is not from chlorine itself but from chloramines, compounds formed when chlorine reacts with nitrogen-containing substances in the water, including sweat, urine, and skin oils. A well-maintained pool with adequate free chlorine and few organic contaminants has surprisingly little odor. The stronger the “chlorine smell,” the more the chlorine has been used up reacting with contaminants, meaning the pool likely needs more chlorine, not less.
Elemental chlorine gas does have a strong, distinctive odor, detectable at concentrations well below those that cause harm. Scheele described it as suffocating and pungent. At very low concentrations, some people describe it as similar to bleach, which makes sense because household bleach is a dilute solution of sodium hypochlorite, a chlorine compound. The human nose can detect chlorine gas at concentrations around 0.5 parts per million, while concentrations above about 30 ppm begin to cause chest pain and shortness of breath, and concentrations above 400 ppm can be lethal within minutes. The wide gap between the detection threshold and the danger threshold is, in a sense, a safety feature: you can smell the gas long before it can seriously hurt you, as long as you take the warning seriously and move to fresh air.
The Element’s Place on the Periodic Table
Chlorine sits at atomic number 17, in Group 17 (the halogens), between fluorine above it and bromine below. The halogens are defined by having seven electrons in their outermost energy level, which makes them hungry for one more electron to complete the set. This accounts for chlorine’s legendary reactivity: it forms compounds with almost every other element, and its tendency to grab electrons from metals, organic molecules, and other targets is what drives its behavior as a disinfectant, a bleach, a weapon, and a biological signaling ion.
Chlorine is the 21st most abundant element in Earth’s crust and overwhelmingly the most abundant dissolved anion in seawater. Sodium chloride alone makes up about 3.5% of ocean water by weight. Rock salt deposits, left behind by evaporated ancient seas, are mined on every continent. The element is so common and so widely distributed that human civilizations have been using its compounds, especially salt, for thousands of years without any awareness that they were handling a specific chemical element. Scheele’s contribution was not finding something rare but recognizing something familiar in a new and more dangerous form.