# Chlorine Gas

Laplace’s Mini World · Plate 87 · Matter, Light & Sound · Matter

> A jar of yellow-⁠green gas: a damp flower fades in it, and turned over burning sodium it makes the yellow flame flare and fills with white smoke of salt.

- Interactive model: https://world.lapla.si/en/chlorine/
- Embed: https://world.lapla.si/embed/en/chlorine/
- 中文: https://world.lapla.si/zh/chlorine/
- Updated: 2026-11-03

## 01 A Yellow-⁠Green Gas

A molecule of chlorine is two chlorine atoms joined together, Cl₂. At room temperature it is a yellow-⁠green gas, often described as pale yellow-⁠green.

The colour comes from the light it takes out: chlorine absorbs violet and blue and lets nearly everything else through. Worked out from its measured absorption cross sections, white light through 10 cm of pure chlorine keeps only about a quarter of its blue at 440 nm and 93% of its light at 500 nm. So the jar does not darken; it is tinted yellow-⁠green, and the longer the path, the deeper the colour.

These jars are 6 cm across inside: seen from the side, the light crosses about 6 cm of chlorine; looking down a jar it crosses nearly 30 cm, and the colour is much deeper. Chlorine boils at −34 °C; bromine, the next halogen down, is already a deep red-⁠brown liquid at room temperature, on the page “Bromine from Seawater”.

*The colour worked out from the absorption cross sections: white light through 2 · 5 · 10 · 20 · 40 cm of pure chlorine*

## 02 Heavier Than Air

Chlorine’s molar mass is 70.90 g⁠/⁠mol; air’s is about 28.97 g⁠/⁠mol on average. At the same temperature and pressure a gas’s density goes with its molar mass, so chlorine is about 2.45 times as heavy as air. Measured, it is a little heavier still: 3.213 g a litre at 0 °C, about 2.5 times.

So it sinks. A teacher makes it in a fume cupboard and runs the delivery tube to the bottom of an upright gas jar under a card: the chlorine collects from the bottom and pushes the air out at the top. This is downward delivery.

When the jar looks distinctly yellow-⁠green, it is full enough. The full jar is closed with a ground-⁠glass cover plate sealed with a little petroleum jelly, and before the demonstration it stands upside down on its own cover.

*Molar mass: chlorine 70.90 · air 28.97 g⁠/⁠mol · measured, about 2.5 times*

## 03 Inside the Fume Cupboard

Chlorine is toxic and irritates the airways. NIOSH in the United States recommends no more than 0.5 ppm in any 15 minutes at work; 10 ppm is immediately dangerous to life or health. People can smell it at about 0.3 ppm.

So making chlorine and these demonstrations happen in a fume cupboard, done by a teacher, everyone in eye protection. A fan draws air in at the front: the standard for British schools is a face velocity of at least 0.3 m⁠/⁠s with the sash open 400 mm, ideally 0.4 to 0.45 m⁠/⁠s.

The baffle at the back keeps the air speed even across the opening, and new cupboards must have an airflow indicator. This page does not show how chlorine is made; industry electrolyses brine, on the page “Electrolysis of Brine”.

*Face velocity: at least 0.3 m⁠/⁠s (sash at 400 mm) · ideally 0.4⁠–⁠0.45 m⁠/⁠s*

## 04 Damp Fades, Dry Does Not

The jar holds dry chlorine, and two strips of red paper go in: one dry, one damped with water. The damp strip soon fades to white from its edges; the dry one stays red.

A paper read to the Royal Society of Edinburgh in 1848 quotes Davy: dry litmus suffers no change in dry chlorine; when water is admitted the colour soon disappears. An 1851 report adds that dry litmus paper kept in dry chlorine out of the sun stayed unbleached for more than eight months, while in sunshine its colour was gone after six weeks.

Damp blue litmus paper in chlorine turns red first, then white: red for the acid made, white for the bleaching.

*Dry chlorine · dry red paper · damp red paper*

## 05 The Bleach Is Hypochlorous Acid

Chlorine dissolves in water and part of it reacts: Cl₂ + H₂O ⇌ HCl + HClO, hydrochloric acid and hypochlorous acid. The reaction is reversible, and at any moment only about a third of the dissolved chlorine has reacted.

The bleach is the hypochlorous acid: it oxidises coloured substances to colourless ones. Paper with water on it makes hypochlorous acid; dry paper has no water, and so none.

Fresh petals hold water, so they fade in chlorine too. Scheele, who first made chlorine in 1774, noted that it took the colour out of leaves and flowers. At 20 °C a litre of water dissolves about 7 g of chlorine.

*Cl₂ + H₂O ⇌ HCl + HClO · about a third of the dissolved chlorine reacts*

## 06 Sodium Burns in Chlorine

Cold sodium in chlorine hardly reacts at first: its surface is coated, and it is not hot enough. In the Royal Society of Chemistry’s version a piece of about 0.3 g is put on a dry brick and heated from above with a Bunsen burner: the sodium melts, the oil left on it burns off, and then the metal itself catches fire.

Then the cover comes off an upside-⁠down jar of chlorine and the jar goes over the burning sodium: the yellow flame brightens and white smoke fills the jar. 2 Na + Cl₂ → 2 NaCl. The yellow is sodium’s flame colour, on the page “Flame Tests and Spectra”.

They do not use a combustion spoon: a steel spoon gives brown smoke of iron chloride, a brass one copper chloride. Gram for gram, sodium gives about 17.9 kJ in chlorine, about 2.2 times as much as in water; sodium and water are on the page “Sodium in Water”.

*2 Na + Cl₂ → 2 NaCl · about 17.9 kJ for each gram of sodium*

## 07 The White Smoke Is Salt

The white smoke is the sodium chloride made: tiny grains of salt that fill the jar and settle on its wall as a white film.

This 0.3 g piece of sodium uses about 0.46 g of chlorine, about 157 mL, and makes about 0.76 g of sodium chloride, giving out about 5.4 kJ.

The jar holds about 826 mL of chlorine, about 2.4 g: enough for about 1.6 g of sodium, so this piece uses less than a fifth of it. A metal soft enough to cut with a knife and a toxic yellow-⁠green gas have become white table salt.

*0.3 g of sodium + 0.46 g of chlorine → 0.76 g of sodium chloride*

## 08 Under a Gram a Tonne

Chlorine is the most common disinfectant for drinking water, and it is used in swimming pools too. In water it becomes hypochlorous acid and hypochlorite.

The WHO’s guidelines: for effective disinfection there should be at least 0.5 mg of free chlorine a litre after at least 30 minutes’ contact at a pH below 8.0, and at least 0.2 mg a litre where the water is delivered. The health-⁠based guideline value is 5 mg a litre; some people can taste it at 0.3 mg.

At 0.5 mg a litre, a tonne of water needs only 0.5 g of chlorine, about 170 mL of the gas, about a fifth of this jar, before anything in the water uses some up. The higher the pH, the less hypochlorous acid: at pH 7.5 hypochlorous acid and hypochlorite are about equal. The “chlorine” smell at a pool is mostly chloramines, made when chlorine reacts with sweat, urine and the like.

*WHO: at least 0.5 mg⁠/⁠L after 30 minutes · at least 0.2 mg⁠/⁠L where delivered · guideline value 5 mg⁠/⁠L*

## 09 1810: A Name for a Colour

In 1774 Carl Wilhelm Scheele, at Uppsala in Sweden, heated hydrochloric acid with the mineral pyrolusite, manganese dioxide, and got a dense greenish-⁠yellow gas with a choking smell that bleached litmus paper and took the colour out of leaves and flowers.

For years after it was called oxymuriatic acid, thought to contain oxygen. On 15 November 1810 Humphry Davy told the Royal Society that nothing showed it held oxygen and that it was an element, and proposed a name taken from one of its obvious properties, its colour: Chlorine, from the Greek χλωρος, greenish yellow.

Three years earlier, in 1807, Davy had made sodium for the first time, with electricity, on the page “Sodium in Water”. Both elements of table salt passed through his hands.

*1774 · Scheele makes it · 1807 · Davy makes sodium · 1810 · Davy names chlorine*

## 10 The Bench, Piece by Piece

Eight things: the jar turned over and its cover plate, the brick, the Bunsen burner, the two jars with the flower and the red strips, the side panel with its airflow monitor and gas tap, and the baffle with the back wall. What matters most: the chlorine stays in the fume cupboard from start to finish.

*Gas jars · cover plate · brick · Bunsen burner · side panel · baffle*

## 11 What Goes in the Jar

This is the page’s model, computed in the page from the equations above. The jar can hold pure chlorine or chlorine mixed with air. Damp paper and the flower take chlorine into their water and make hypochlorous acid, and their colour fades; how fast is the model’s own choice, slower with less chlorine; dry paper has no water and does not fade in the model. Burning sodium burns by 2 Na + Cl₂ → 2 NaCl until the sodium or the chlorine is used up, its heat from the enthalpy of formation and the jar’s colour from the chlorine left; cold sodium does not react in the model.

Each press of “Put it in” starts afresh. It is a model on a web page, not a way to do the experiment.

*The page’s model · masses and heat from measured values · the fading’s speed is the model’s*

**Try this**: In the lab, take burning sodium and raise it to 2 g: which runs out first, the sodium or the chlorine, and how much chlorine is left in the jar? Then try the dry paper: does it fade?

## What the model leaves out

The bench is a model of a teacher’s demonstration, not a copy of any one set: a section of a school fume cupboard, its worktop 440 mm wide, the side panel and back wall cut at 400 mm; gas jars 60 mm inside and 300 mm high and 75 mm cover plates after a catalogue, a 215 × 102.5 × 65 mm brick; the glass’s thickness is the model’s. The sash is not drawn; one of the Royal Society of Chemistry’s versions is done with the sash down. The two Royal Society of Chemistry pages differ on the piece of sodium, about a 3 mm cube in one and about 0.3 g in the other; this page uses 0.3 g. The gas’s colour is worked out from the absorption cross sections for white daylight and the standard observer; the glass, the room’s light and the eye’s adjustment to its surroundings are not. The flame and the smoke are drawn, not computed. The jar filling from the bottom is drawn after downward delivery; the sources only say the jar is full enough when it looks distinctly yellow-⁠green. In the lab the speed of the fading is the model’s own.

## Easy to get wrong

Chlorine itself does not bleach; the hypochlorous acid it makes with water does. Dry paper in dry chlorine kept out of the sun can keep its colour for months, though sunlight slowly bleaches it. Damp blue litmus turns red first, then white. Chlorine is yellow-⁠green, not green, and a thin layer looks almost colourless. Sodium has to be hot before it burns in chlorine. The “chlorine” smell at a pool is mostly chloramines; a stronger smell does not mean more chlorine. Household bleach is a solution of sodium hypochlorite, not chlorine; with acids it gives off chlorine, so it is never mixed with other cleaners. Not all dissolved chlorine turns into acid: only about a third of it reacts.

## Common questions

### What colour is chlorine gas?

A molecule of chlorine is two chlorine atoms joined together, Cl₂. At room temperature it is a yellow-⁠green gas, often described as pale yellow-⁠green.

See: [01 A Yellow-⁠Green Gas](https://world.lapla.si/en/chlorine/#colour)

### How much heavier than air is chlorine?

Chlorine’s molar mass is 70.90 g⁠/⁠mol; air’s is about 28.97 g⁠/⁠mol on average. At the same temperature and pressure a gas’s density goes with its molar mass, so chlorine is about 2.45 times as heavy as air.

See: [02 Heavier Than Air](https://world.lapla.si/en/chlorine/#heavy)

### Why does damp paper fade in chlorine but dry paper does not?

The bleach is the hypochlorous acid: it oxidises coloured substances to colourless ones. Paper with water on it makes hypochlorous acid; dry paper has no water, and so none.

See: [05 The Bleach Is Hypochlorous Acid](https://world.lapla.si/en/chlorine/#hclo)

### What is the white smoke when sodium burns in chlorine?

The white smoke is the sodium chloride made: tiny grains of salt that fill the jar and settle on its wall as a white film.

See: [07 The White Smoke Is Salt](https://world.lapla.si/en/chlorine/#salt)

### How much chlorine does drinking water need?

The WHO’s guidelines: for effective disinfection there should be at least 0.5 mg of free chlorine a litre after at least 30 minutes’ contact at a pH below 8.0, and at least 0.2 mg a litre where the water is delivered.

See: [08 Under a Gram a Tonne](https://world.lapla.si/en/chlorine/#water)

## Sources

1. [Royal Society of Chemistry and Nuffield Foundation: Heating group 1 metals in air and in chlorine (sodium burning on a brick, the jar of chlorine turned over it, why not a combustion spoon, a teacher’s demonstration only)](https://edu.rsc.org/experiments/heating-group-1-metals-in-air-and-in-chlorine/732.article)
2. [D. Fleming (2022): The reaction between sodium and chlorine, Education in Chemistry, Exhibition chemistry (about 0.3 g of sodium lit under a Bunsen first, the yellow flame brightening and the white smoke, the sash down)](https://edu.rsc.org/exhibition-chemistry/the-reaction-between-sodium-and-chlorine/4015463.article)
3. [Royal Society of Chemistry: Generating, collecting and testing gases (chlorine made in a fume cupboard and collected by downward delivery)](https://edu.rsc.org/experiments/generating-collecting-and-testing-gases/693.article)
4. [Royal Society of Chemistry: Microscale reactions of chlorine with water or halide ions (damp indicator paper turns red then bleached; HOCl does the bleaching)](https://edu.rsc.org/resources/microscale-reactions-of-chlorine/530.article)
5. [Royal Society of Chemistry periodic table: Chlorine (Scheele’s gas of 1774; the name from the Greek for greenish yellow)](https://periodic-table.rsc.org/element/17/chlorine)
6. [CLEAPSS: Student Safety Sheet 54, Chlorine (toxic if inhaled, a respiratory irritant; amounts in drinking water and pools)](https://science.cleapss.org.uk/resource/All-in-one-Student-Safety-Sheets.pdf)
7. [CLEAPSS: GL123, Fume cupboard buying guide (2022; face velocity, the baffle, the airflow indicator)](https://science.cleapss.org.uk/resource/GL123-Fume-cupboard-buying-guide.pdf)
8. [ACS Division of Chemical Education: Chemistry Comes Alive!, Reaction of Chlorine with Sodium (cold sodium does not react at first)](https://chemedx.org/JCESoft/jcesoftSubscriber/CCA/CCA3/MAIN/NACL/PAGE1.HTM)
9. [G. Wilson (1848): On the Action of the Dry Gases on Organic Colouring Matters, Trans. R. Soc. Edinburgh 16 (dry litmus unchanged in dry chlorine)](https://doi.org/10.1017/S0080456800022444)
10. [G. Wilson (1851): abstract of the same, Proc. R. Soc. Edinburgh 2 (over eight months unbleached in the dark, bleached after six weeks of sun)](https://doi.org/10.1017/S0370164600036257)
11. [H. Davy (1811): The Bakerian Lecture: On Some of the Combinations of Oxymuriatic Gas and Oxygene, Phil. Trans. R. Soc. 101 (read on 15 November 1810; the name Chlorine)](https://doi.org/10.1098/rstl.1811.0001)
12. [OpenStax: Chemistry 2e, 18.11 (a greenish-⁠yellow gas; with water it gives hypochlorous acid, which oxidises coloured substances to colourless ones; molten sodium ignites in chlorine)](https://openstax.org/books/chemistry-2e/pages/18-11-occurrence-preparation-and-properties-of-halogens)
13. [OpenStax: Chemistry 2e, appendix G (standard enthalpies of formation)](https://openstax.org/books/chemistry-2e/pages/g-standard-thermodynamic-properties-for-selected-substances)
14. [OpenStax: Chemistry 2e, appendix H (the Ka of hypochlorous acid)](https://openstax.org/books/chemistry-2e/pages/h-ionization-constants-of-weak-acids)
15. [J. Clark: Chemguide, Atomic and physical properties of the halogens (about a third of the dissolved chlorine reacts with the water)](https://www.chemguide.co.uk/inorganic/group7/properties.html)
16. [PubChem: chlorine (HSDB, after the CRC Handbook, the Merck Index and Kirk-⁠Othmer: colour, density, boiling point, solubility; NIOSH exposure limits)](https://pubchem.ncbi.nlm.nih.gov/compound/24526)
17. [R. Atkinson et al. (2007): IUPAC evaluated data for atmospheric chemistry, volume III (Cl₂ absorption cross sections)](https://doi.org/10.5194/acp-7-981-2007)
18. [MPI-⁠Mainz UV/VIS Spectral Atlas: Cl₂ (the NASA JPL 2011 recommended values)](https://uv-vis-spectral-atlas-mainz.org/uvvis/cross_sections/Halogens+mixed%20halogens/Cl2.spc)
19. [CIE: the 1931 standard colorimetric observer and illuminant D65 datasets](https://doi.org/10.25039/CIE.DS.xvudnb9b)
20. [NASA: Earth Fact Sheet (air’s mean molecular weight)](https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html)
21. [IUPAC CIAAW: standard atomic weights](https://www.ciaaw.org/abridged-atomic-weights.htm)
22. [NIST: CODATA 2022 values (the molar gas constant)](https://physics.nist.gov/cgi-bin/cuu/Value?r)
23. [WHO: Guidelines for Drinking-⁠water Quality, 4th edition incorporating the 1st and 2nd addenda (2022; table 8.17 and the chlorine fact sheet)](https://iris.who.int/handle/10665/352532)
24. [WHO: Chlorine in Drinking-⁠water, background document (2003; hypochlorous acid and hypochlorite about equal at pH 7.5)](https://cdn.who.int/media/docs/default-source/wash-documents/wash-chemicals/chlorine.pdf)
25. [US CDC: Preventing Eye Irritation from Pool Chemicals (the “chlorine” smell at pools is chloramines)](https://www.cdc.gov/healthy-swimming/prevention/preventing-eye-irritation-from-pool-chemicals.html)
26. [Philip Harris: Gas Jar: Cylindrical - 60mm x 300mm (60 mm inside, 300 mm high, heavy foot, ground flange)](https://philipharris.co.uk/product/chemistry/gas-preparation-and-analysis/gas-jar-cylindrical-60mm-x-300mm/b8a64130)
27. [Philip Harris: Gas Jar Cover - 75mm (sheet glass ground on one side)](https://www.philipharris.co.uk/product/chemistry/gas-preparation-and-analysis/gas-jar-cover-75mm-pack-of-10/b8a64154)

## Data

Chlorine’s absorption cross sections from the IUPAC (R. Atkinson et al., 2007) and NASA JPL (2011) evaluations, both after D. Maric, J. P. Burrows, R. Meller and G. K. Moortgat (1993); the colour worked out here against the CIE 1931 observer and illuminant D65 (CIE datasets) with the CSS Color 4 matrices; chlorine’s density, boiling point, solubility and the species in chlorine water from the CRC Handbook, the Merck Index and Kirk-⁠Othmer (via PubChem’s HSDB records); exposure limits from NIOSH (via PubChem); air’s mean molar mass from NASA’s Earth Fact Sheet; atomic weights from IUPAC’s CIAAW; enthalpies of formation and the Ka of hypochlorous acid from OpenStax Chemistry 2e; R and the standard atmosphere are the exact CODATA 2022 values. The heats, masses, volumes, colours and pH shares are computed here; the lab is the page’s own model. The demonstrations after the Royal Society of Chemistry’s Practical Chemistry pages, D. Fleming’s Exhibition chemistry (Education in Chemistry, 2022) and Inspirational chemistry; CLEAPSS (Student Safety Sheets 54 and 100, buying guide GL123); the Journal of Chemical Education’s Chemistry Comes Alive!; J. Clark’s Chemguide; drinking water after the WHO Guidelines for Drinking-⁠water Quality (2022) and its chlorine background document (2003); pools after the US CDC; the dry and damp papers after G. Wilson (Royal Society of Edinburgh, 1848 and 1851); the history after the Royal Society of Chemistry and H. Davy’s Bakerian Lecture, read on 15 November 1810. The cupboard is this page’s model of a teacher’s demonstration: a section of a school fume cupboard, gas jars of 60 × 300 mm with 75 mm cover plates (after Philip Harris’s catalogue), a 215 × 102.5 × 65 mm brick and a Bunsen burner; not a copy of any one set. Material presets and the model kit as on the site’s other model plates (public/looks/CREDITS.md).

## Licence CC BY-NC 4.0

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## Cite this plate

Laplace’s Mini World, Plate 87, “Chlorine Gas”, https://world.lapla.si/en/chlorine/, CC BY-NC 4.0, updated 2026-11-03.