Chlorine
[CAS# 7782-50-5]

Identification

• Classification: Inorganic chemical industry >> Industrial gases such as hydrogen, nitrogen and oxygen
• Name: Chlorine
• Molecular Formula: Cl₂
• Molecular Weight: 70.91
• CAS Registry Number: 7782-50-5
• EC Number: 231-959-5
• SMILES: ClCl

Properties

• Density: 1.4±0.1 g/cm³ Calc.^*
• Melting point: -101 °C (Expl.)
• Boiling point: -34.6±9.0 °C 760 mmHg (Calc.)^*, -34 °C (Expl.)
• Solubility: 0.7% in NIOSH (Expl.)
• Index of refraction: 1.375 (Calc.)^*

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS03;GHS04;GHS06;GHS09 Danger
• Risk Statements: H270-H331-H335-H315-H319-H400
• Safety Statements: S9;S45;S61

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400
Skin irritation	Skin Irrit.	2	H315
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	3	H335
Oxidising gases	Ox. Gas	1	H270
Acute toxicity	Acute Tox.	2	H330
Gases under pressure (compressed)	Press. Gas (Comp.)		H280
Acute toxicity	Acute Tox.	3	H331
Gases under pressure (liquid)	Press. Gas (Liq.)		H280
Chronic hazardous to the aquatic environment	Aquatic Chronic	1	H410
Acute toxicity	Acute Tox.	4	H302
Specific target organ toxicity - repeated exposure	STOT RE	2	H373

• Transport Information: UN 1017

Discovery and Applications

Chlorine is a chemical element with atomic number 17 and the symbol Cl. It was first prepared in 1774 by Carl Wilhelm Scheele, who generated the gas by treating manganese dioxide with hydrochloric acid. Scheele recognized the distinct greenish-yellow color and pungent odor of the gas but believed it to be a compound containing oxygen. In 1810, Humphry Davy demonstrated that the substance was an element and named it chlorine, derived from the Greek word chloros, meaning pale green. This clarification established chlorine as a member of the halogen family of elements.

During the nineteenth century, chlorine became important in the textile and paper industries because of its bleaching properties. Early bleaching processes relied on elemental chlorine, which was capable of destroying colored organic compounds through oxidation and chlorination reactions. The development of bleaching powder, produced by the reaction of chlorine with slaked lime, provided a more practical and transportable disinfecting and bleaching agent. These innovations significantly improved large-scale textile processing and paper production.

Chlorine also played a decisive role in public health. In the early twentieth century, controlled chlorination of municipal water supplies was introduced to reduce the spread of waterborne diseases such as typhoid fever and cholera. The addition of small, carefully measured quantities of chlorine or chlorine-releasing compounds to drinking water proved highly effective at inactivating pathogenic microorganisms. This practice became standard in many countries and contributed substantially to improvements in urban sanitation and life expectancy.

In the chemical industry, chlorine is a fundamental raw material. The large-scale production of chlorine is typically achieved by the electrolysis of aqueous sodium chloride solutions, a process that also yields hydrogen and sodium hydroxide. Chlorine is used in the manufacture of a wide range of inorganic and organic compounds. It is an essential reagent in the production of polyvinyl chloride, one of the most widely used plastics. Chlorinated intermediates are also involved in the synthesis of solvents, agrochemicals, and pharmaceuticals. Reactions involving chlorine enable the introduction of chlorine atoms into organic molecules, often modifying their physical and biological properties.

Chlorine has also been used in the production of disinfectants for surfaces and equipment. Compounds such as sodium hypochlorite release active chlorine species that are effective against bacteria, viruses, and fungi. These applications have been important in medical facilities, food processing, and household sanitation. At the same time, the reactivity and toxicity of chlorine gas require careful handling and strict safety controls in industrial settings.

The element has had both beneficial and harmful historical associations. While chlorine-based disinfectants have improved hygiene and disease control, chlorine gas was also used as a chemical weapon during the First World War, demonstrating its hazardous potential. Modern regulations strictly govern its production, transport, and use to minimize risks to workers and the environment.

Today, chlorine remains a cornerstone of the chemical industry and public health infrastructure. Its discovery clarified the nature of halogen chemistry, and its applications in bleaching, water treatment, materials production, and synthesis have made it one of the most industrially significant elements.

References

2025. Combined impacts of chlorine and pharmaceutical discharge on river periphyton and zoobenthos. Journal of Environmental Sciences (China).
DOI: 10.1016/j.jes.2024.10.020

2025. Impact of marine chlorine emissions on secondary organic aerosols in North China Plain. Environmental Pollution.
DOI: 10.1016/j.envpol.2025.126524

2025. Genomic insights into chlorine resistance of a Mycobacterium sp. strain isolated from treated wastewater effluent. Water Research.
DOI: 10.1016/j.watres.2025.123807