Hydrogen chloride is a colorless gas with the chemical formula HCl, consisting of one hydrogen atom covalently bonded to one chlorine atom. First prepared by alchemists in the medieval period by heating salt with sulfuric acid, this pungent compound forms white fumes in moist air and dissolves eagerly in water to create hydrochloric acid – the same acid found in our stomachs. As one of the most important industrial chemicals, with production exceeding 40 million tons annually, hydrogen chloride serves crucial roles in steel pickling, food processing, and manufacturing everything from PVC plastics to pharmaceuticals. Despite its corrosive nature and ability to cause severe respiratory damage, HCl’s unique properties as a strong acid and chlorinating agent make it indispensable to modern industry, where it’s often produced as a valuable byproduct of organic chemical reactions.

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

20 Fun Facts About Hydrogen Chloride

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

1. Your stomach produces about 1.5 liters of hydrochloric acid daily at a concentration strong enough to dissolve zinc metal.
2. HCl molecules vibrate 2,886 times per centimeter in infrared light, creating a spectroscopic “fingerprint” visible from space.
3. The gas forms perfect cubic crystals when frozen at -114°C, with each molecule surrounded by 12 neighbors.
4. Hydrogen chloride fountains dramatically when bubbled into water, creating a vacuum that sucks water up into the container.
5. Ancient Romans accidentally produced HCl fumes at salt works, calling the irritating gas “spiritus salis” or spirit of salt.
6. The compound is exactly 1.05% denser than air at STP, making it sink slowly but not pool as dramatically as heavier gases.
7. Venus’s atmosphere contains HCl at 0.1-0.6 ppm, likely from volcanic activity reacting with salt deposits.
8. Hydrogen chloride dissolves in water so exothermically it can boil the solution, releasing up to 74.8 kJ/mol of heat.
9. The bond in HCl is 17% ionic and 83% covalent, making it a textbook example of polar covalent bonding.
10. Semiconductor factories use ultra-pure HCl at 1,000°C to etch atomic-scale features in silicon chips.
11. Blue whales produce HCl strong enough (pH 1.5) to dissolve small fish bones within hours of swallowing.
12. The gas liquefies at -85°C under atmospheric pressure, forming a clear liquid that conducts electricity poorly.
13. HCl was the first gas liquefied by pressure alone (at 40 atmospheres) by Michael Faraday in 1823.
14. Cocaine hydrochloride forms when cocaine base reacts with HCl, creating the water-soluble salt form.
15. The molecule rotates exactly 10.59 × 10¹¹ times per second at room temperature, measured by microwave spectroscopy.
16. PVC plastic production consumes about 35% of all HCl, using it to make vinyl chloride monomer from ethylene.
17. Hydrogen chloride lasers emit at 3.8 micrometers wavelength, used for atmospheric research and military applications.
18. The compound forms constant-boiling azeotropes with water at 20.2% HCl, distilling unchanged at 108.6°C.
19. Medieval leather tanners used HCl fumes from salt and vitriol to remove hair, not knowing the chemistry involved.
20. Interstellar space contains HCl molecules detected by their characteristic radio emissions at 625.9 GHz frequency.

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