Chlorine is a toxic, greenish-yellow gas with the chemical formula Cl₂, consisting of two chlorine atoms bonded together. Discovered in 1774 by Swedish chemist Carl Wilhelm Scheele and later identified as an element by Humphry Davy in 1810, chlorine is one of the most reactive elements in the periodic table and rarely exists in pure form in nature.

With its distinctive sharp, suffocating odor reminiscent of bleach, chlorine revolutionized public health through water treatment and sanitation, preventing countless deaths from waterborne diseases since the early 1900s. Today, this powerful oxidizing agent is one of the most produced industrial chemicals worldwide, essential for manufacturing everything from PVC plastics and pharmaceuticals to paper and textiles, though its use as a chemical weapon in World War I serves as a stark reminder of its dangerous potential when mishandled.

Find a review of the 50 most important industrial gases here.

20 Fun Facts About Chlorine

Beyond the basics above, what else should we know about Chlorine? Check out the 20 fun facts below!

1. Chlorine is 2.5 times heavier than air, causing it to flow like a visible green river along the ground during gas leaks.
2. The element is so reactive it can spontaneously ignite steel wool, paper, and even some metals without any spark or flame.
3. Swimming pool “chlorine smell” isn’t chlorine itself but chloramines formed when chlorine reacts with sweat and urine.
4. Chlorine gas turns into a bright yellow liquid at -34°C and freezes into orange crystals at -101°C.
5. Salt deposits contain 55% chlorine by weight, with global ocean reserves estimated at 26 quadrillion tons.
6. Your stomach produces hydrochloric acid using chlorine from salt, generating about 1.5 liters of acid daily for digestion.
7. The gas was first weaponized at Ypres in 1915, releasing 168 tons that killed 5,000 soldiers in ten minutes.
8. Chlorine dioxide (ClO₂) is 10 times more potent as a disinfectant but explodes above 10% concentration in air.
9. White blood cells produce hypochlorous acid from chlorine ions to kill bacteria, acting as your body’s internal bleach.
10. The Montreal Protocol phases out chlorine-containing CFCs because one chlorine atom can destroy 100,000 ozone molecules.
11. Sucralose (Splenda) is made by replacing three hydroxyl groups in sugar with chlorine atoms, making it 600 times sweeter.
12. Chlorine trifluoride is so reactive it can burn through concrete, gravel, and asbestos while igniting them.
13. The element gives fireworks their blue and green colors when copper chloride and barium chloride burn.
14. Photographic film used silver chloride crystals that darken when exposed to light, revolutionizing image capture.
15. Venus’s atmosphere contains hydrogen chloride that forms sulfuric acid clouds, contributing to its hellish conditions.
16. Chlorine-36 is a radioactive isotope with a 301,000-year half-life used to date ancient groundwater.
17. The chemical industry produces 70 million tons of chlorine annually, consuming 1% of global electricity generation.
18. Dioxins, some of the most toxic chemicals known, form when chlorine-containing materials burn incompletely.
19. Bleaching wood pulp for white paper consumes 5 million tons of chlorine yearly, though oxygen alternatives are increasing.
20. NASA uses chlorine pentafluoride as a rocket propellant oxidizer because it’s even more powerful than liquid oxygen.

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