Chlorine smells like a swimming pool. That's why sharp. Clean. A little aggressive. Most of us know that smell before we know the name.
But here's the thing — chlorine wasn't discovered in a pool. In practice, it wasn't even discovered on purpose. And the man who finally figured out what it actually was? Here's the thing — the guy who first made it thought he'd created something else entirely. He nearly killed himself proving it.
Let's back up.
What Is Chlorine
Chlorine is a halogen. Group 17 on the periodic table. Atomic number 17. At room temperature it's a yellow-green gas, about 2.5 times heavier than air. It reacts with almost everything — metals, nonmetals, organic compounds, you name it. That reactivity is exactly why it's useful and exactly why it's dangerous.
It's not "chlorine" in your tap water
Technically, no. What you smell at the pool or taste in municipal water isn't elemental chlorine. It's hypochlorous acid and chloramines — compounds formed when chlorine reacts with organic matter. The gas itself? You'd know. It burns. It chokes. In high concentrations, it kills.
Where it sits in nature
Chlorine doesn't exist free in nature. The Earth's crust holds massive deposits of rock salt. That's why you'll find it locked up in salts — sodium chloride (table salt), potassium chloride, magnesium chloride. 9% chloride by weight. Plus, seawater is roughly 1. Too reactive. But the element itself? Had to be pried loose.
Why It Matters / Why People Care
Without chlorine, the modern world looks different. Drastically different.
Clean water is the big one. Chlorination dropped typhoid, cholera, and dysentery rates off a cliff in the early 1900s. Life expectancy jumped. Worth adding: cities became livable at scale. That's not hyperbole — it's public health history.
Then there's PVC. Polyvinyl chloride. Pipes, window frames, vinyl flooring, medical tubing, insulation on every wire in your walls. None of it exists without chlorine chemistry.
Bleach. Still, disinfectants. Solvents. Still, pharmaceuticals. Crop protection. Polyurethanes in your mattress and car seats. The list runs long.
But it's not all clean water and convenient plastics. Chlorine gas was the first modern chemical weapon — deployed at Ypres in 1915. On top of that, it burns lungs, blinds eyes, drowns you in your own fluids. The same reactivity that purifies water destroys tissue.
And then there's the ozone story. But cFCs — chlorofluorocarbons — ate a hole in the stratosphere. We banned them. The ozone layer is healing. But it took a global treaty and decades to fix what chlorine chemistry broke.
So yeah. It matters.
How It Was Discovered
The story starts in a pharmacy in Sweden. 1774. A man named Carl Wilhelm Scheele is heating things up.
Scheele's kitchen chemistry
Scheele wasn't a university professor. On top of that, died at 43, likely from cumulative heavy metal exposure. Worth adding: he was an apothecary — a pharmacist, basically — with a side obsession for gases. He had a small lab, a blowpipe, and a habit of tasting his experiments. In practice, (He survived longer than you'd expect. Not chlorine.
One day he drops powdered manganese dioxide — pyrolusite, they called it — into a flask with spirit of salt. That's hydrochloric acid to us. In real terms, concentrated. The reaction is violent. A dense, yellow-green gas pours out.
Scheele collects it over water. Notes the color. On the flip side, the way it bleaches litmus paper, flowers, green leaves. The smell — "suffocating," he writes. He dissolves it in water and gets an acid solution that bleaches too.
He calls it "dephlogisticated muriatic acid air."
Wait — phlogiston?
Right. Metals were "calxes" (oxides) plus phlogiston. The theory of the day. Even so, combustible materials contained "phlogiston," a fire-like substance released during burning. That said, acids were... complicated.
Scheele thought he'd stripped phlogiston from muriatic acid (HCl). The gas was the acid minus its phlogiston. Still, in his framework, that made sense. He didn't think he'd found an element. He thought he'd modified a compound.
If you found this helpful, you might also enjoy what is in fix a flat or periodic table with the mass number.
And honestly? Given what he knew, it was a reasonable conclusion.
What he actually observed
Scheele was meticulous. He documented:
- The gas is heavier than air (he poured it between vessels)
- It doesn't support combustion — candles go out
- It reacts with metals: tin, copper, mercury all form salts
- It destroys vegetable colors — early bleaching discovery
- It combines with "fixed air" (CO2) to form... something
- Water absorbs it, creating an acidic, bleaching solution
He even reacted it with ammonia. Got a white smoke — ammonium chloride. Didn't pursue it.
The paper publishes in 1777. Consider this: chemical Treatise on Air and Fire*. But by then, Priestley has published on oxygen. Lavoisier is building the new chemistry. Scheele's chlorine work gets noticed but not fully understood.
The missing piece: nobody thinks it's an element
Here's what's wild. He characterized* it. Also, scheele isolated* chlorine. He used* it. But he died in 1786 still believing it was a compound — muriatic acid minus phlogiston.
Lavoisier reads Scheele. (Spoiler: they don't. So he repeats the experiment. In practice, he agrees: it's oxidized muriatic acid. Also, the "oxygen" in "oxidized" is literal here — Lavoisier thinks all acids contain oxygen. HCl has zero oxygen.
So the father of modern chemistry gets it wrong too.
Enter Humphry Davy
Fast forward to 1810. On top of that, davy is 31. Already famous for isolating potassium, sodium, calcium, magnesium, strontium, barium — using the new voltaic pile. Electricity splits compounds nobody could crack with heat alone.
He turns his attention to "oxymuriatic acid" — the French name for Scheele's gas.
Davy tries electrolysis. Consider this: just the dry gas. On the flip side, no water. On top of that, nothing happens. Nowhere. So the chlorine? In practice, he tries dissolving it in water and electrolyzing that*. Gets hydrogen at the cathode, oxygen at the anode. It's reacting with the water.
So he goes dry. Heats the gas with charcoal. And no reaction. With phosphorus.
in a cloud of white smoke. On top of that, the reaction is exothermic, releasing phosphorus trichloride (PCl₃). Davy realizes the gas is not merely "acid without phlogiston" but a distinct substance capable of forming new compounds. He names it chlorine* (from the Greek chloros*, meaning "greenish-yellow") and identifies it as an element—one of the first to be discovered through chemical decomposition rather than isolation from natural sources.
The Element Emerges
Davy’s work solidifies chlorine’s status as an element. He publishes his findings in 1810, coinciding with the rise of atomic theory. Unlike Scheele, who framed the gas as a modified compound, Davy recognizes its fundamental nature. He also clarifies that chlorine’s reactivity stems from its ability to gain electrons, a concept later refined by Michael Faraday’s electrolysis experiments. The gas’s bleaching power, once a mystery, is now understood as its capacity to oxidize organic molecules—though the mechanism remains elusive until the 19th century.
Legacy and Modern Significance
Chlorine’s discovery reshaped chemistry. It became a cornerstone of bleaching agents, disinfectants, and industrial processes. Scheele’s early observations—its density, lack of combustion support, and reactivity with metals—were reinterpreted through the lens of modern atomic theory. Today, chlorine’s role in environmental chemistry (e.g., ozone depletion) and pharmaceuticals underscores its dual nature: a life-saving antiseptic and a pollutant when mismanaged.
Scheele’s story reminds us that scientific progress is a relay race. Plus, he saw the baton but couldn’t pass it fully; Davy sprinted past the finish line. So yet both men’s contributions—rooted in meticulous observation and bold experimentation—illustrate how even flawed frameworks can pave the way for breakthroughs. In the end, chlorine’s journey from "dephlogisticated muriatic acid air" to a vital element mirrors the evolution of chemistry itself: a field forever chasing the invisible, one reaction at a time.
