# Sulfur

Laplace’s Mini World · Plate 89 · Matter, Light & Sound · Matter

> Concentrated sulfuric acid on sugar: after a pause the sugar blackens and steams, and a porous black column of carbon rises out of the beaker.

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

## 01 The Sugar Turns Black

The beaker holds 50 g of white sugar, sucrose. In a fume cupboard a teacher pours concentrated sulfuric acid onto it, and at first nothing happens.

The Royal Society of Chemistry describes it like this: the sugar turns yellow, then brown, and after about a minute starts to blacken; a spongy mass of carbon rises up the beaker, steam comes off, and the carbon ends up two or three times the height of the beaker. How long the wait is varies from one account to another, from half a minute to several minutes.

The beaker gets very hot. Concentrated sulfuric acid causes severe burns to skin and eyes; this is a teacher’s demonstration, and this page does not show anyone how to do it.

*Yellow, then brown · black after about a minute · two or three times the beaker’s height*

## 02 What Goes Is Water

Sucrose is C₁₂H₂₂O₁₁: twelve carbon atoms, with just the hydrogen and oxygen to make eleven molecules of water. Concentrated sulfuric acid has a strong pull for water: it takes the hydrogen and oxygen out of the sugar in the proportions of water and leaves carbon. This is dehydration; the sugar is not burned black by a flame.

Textbooks write it as C₁₂H₂₂O₁₁ → 12 C + 11 H₂O. By that equation, 50 g of sucrose leaves about 21.05 g of carbon and gives up about 28.95 g of water. Carbon is only 42% of the sugar; more than half of its mass is water.

The real reaction is less tidy. The Royal Society of Chemistry calls the equation a simplification: some of the carbon is oxidised to carbon monoxide and carbon dioxide, and some of the acid is reduced to sulfur dioxide. The black solid left is not pure graphite either but a loose carbon-⁠rich residue.

*C₁₂H₂₂O₁₁ → 12 C + 11 H₂O · 50 g of sugar → 21.05 g of carbon + 28.95 g of water*

## 03 Blown Up by Steam

The reaction gives out a great deal of heat. Concentrated sulfuric acid gives out heat as it takes up water, and by the standard enthalpies of formation, splitting sugar into carbon and water gives out heat too. How the two share the heat in the real reaction has not been measured that we could find, so no figure is given here.

The heat turns the water taken out into steam. In the Royal Society of Chemistry’s words, the steam expands the remaining carbon into a porous, smoking, black column. If all the water in 50 g of sugar became steam at 100 °C, it would fill about 49 L, nearly two hundred times this 250 mL beaker.

Steam is not all that comes off: there is sulfur dioxide, carbon monoxide and carbon dioxide too, which is why it is done in a fume cupboard. The column is soaked with acid and is not to be touched afterwards.

*The water in 50 g of sugar → at most about 49 L of steam (100 °C) · blown into a porous column*

## 04 A Pale Blue Flame

Sulfur is yellow, as crystals or a powder, and melts at about 115 °C. Burning in the small cup of a deflagrating spoon it gives a pale blue flame; the US Coast Guard’s chemical data notes that the flame may be hard to see in daylight.

Sulfur and oxygen combine into sulfur dioxide: S + O₂ → SO₂. From the enthalpy of formation, each gram of sulfur gives about 9.26 kJ. 0.1 g of sulfur makes about 0.2 g of sulfur dioxide, about 75 mL at 20 °C. Pure sulfur has no smell; the sharp smell of burning sulfur is the sulfur dioxide.

Taken as 60 mm across and 300 mm high, this gas jar holds about 848 mL, and the oxygen in its air is about 178 mL: enough to burn at most about 0.24 g of sulfur. Industry burns sulfur into sulfur dioxide to make sulfuric acid, on the page “Sulfuric Acid from Sulfur”.

*S + O₂ → SO₂ · 0.1 g of sulfur → about 75 mL of sulfur dioxide (20 °C)*

## 05 A Choking Gas

Sulfur dioxide is colourless, with a choking, suffocating smell, and boils at −10 °C. Its molar mass is 64.06 g⁠/⁠mol, so it is about 2.2 times as heavy as air.

It dissolves readily in water: at 21 °C a litre of water takes up about 107 g, about 40 L of the gas. The solution is acidic and is called sulfurous acid, though the pure acid cannot be isolated.

NIOSH in the United States recommends no more than 2 ppm averaged over a working day and 5 ppm in any 15 minutes; 100 ppm is immediately dangerous to life or health. The 150 mL or so of sulfur dioxide from 0.2 g of sulfur is enough to bring about 75 m³ of air to 2 ppm. British school safety sheets say that more than test-⁠tube amounts belong in a fume cupboard, and that people with asthma should be kept away from the gas.

*Sulfur dioxide 64.06 g⁠/⁠mol · about 2.2 times as heavy as air · about 107 g a litre of water (21 °C)*

## 06 Faded, and Back Again

Damp red petals in sulfur dioxide slowly lose their colour. In the American Chemical Society’s demonstration film a red rose fades in sulfur dioxide until it is pink, and the sulfur dioxide from the sugar column bleaches a rose petal too.

Sulfur dioxide is a reducing agent. The 1911 Encyclopædia Britannica says that with water it often acts as a bleach, and that this bleaching is a reduction. Chemists now put it more exactly for anthocyanins, the red pigments of berries and red wine: in water sulfur dioxide becomes bisulfite, which joins onto the pigment to make a colourless compound. That step can be undone: an oxidant or more acid brings the red back.

Wool bleached with burning sulfur likewise turns yellow again when it is washed with soap or soda. Chlorine bleaches another way: with water it makes hypochlorous acid, which oxidises coloured substances to colourless ones, on the page “Chlorine Gas”.

*Sulfur dioxide fades damp red petals · an oxidant or acid brings anthocyanin’s red back*

## 07 Dried Fruit and Wine

Sulfur dioxide and sulfites go into food too: they hold back browning, enzyme reactions and the growth of bacteria, and they are antioxidants. In winemaking they kill unwanted bacteria, moulds and yeasts.

The European Union sets limits, all counted as sulfur dioxide: dried apricots, peaches, grapes, prunes and figs up to 2,⁠000 mg per kilogram; dried apples and pears up to 600 mg. For total sulfur dioxide in wine with less than 5 g of sugar a litre, red wine is limited to 150 mg a litre, white and rosé to 200 mg.

Since 1986 the United States has not allowed sulfites on fresh fruit and vegetables served raw, and sulfites at 10 mg per kilogram or more in a finished food must be declared on the label. The European Food Safety Authority set a temporary acceptable daily intake in 2016 and withdrew it in 2022: it found the toxicity data too thin to set one, and judged that the estimated intakes raise a safety concern.

*EU limits (as sulfur dioxide): dried apricots and others 2,⁠000 mg⁠/⁠kg · red wine 150 mg⁠/⁠L · white wine 200 mg⁠/⁠L*

## 08 From Chimney to Rain

The largest source of sulfur dioxide in the air is the burning of fossil fuels in power plants and other industry; sulfur is an impurity of coal. The US EPA describes how sulfur dioxide and the nitrogen oxides react with water, oxygen and other chemicals in the air to form sulfuric and nitric acids, which come down in rain. The nitrogen oxides are on the page “Copper in Nitric Acid”.

Normal rain has a pH of about 5.6; acid rain is usually between 4.2 and 4.4, with 16 to 25 times the hydrogen ions of normal rain. Volcanoes give off sulfur dioxide too, though only a small part of it: Mount Pinatubo’s 1991 eruption in the Philippines put nearly 20 million tons of it into the stratosphere. Volcanoes are on the page “Where Volcanoes Come From”.

Scrubbers in the flue take sulfur dioxide out with an alkaline reagent; wet scrubbers remove more than 90%. The power plants in the US Acid Rain Program emitted 96% less sulfur dioxide in 2023 than in 1990, and the EU-⁠27’s sulfur oxides in 2024 were 95% below 1990.

*Normal rain about pH 5.6 · acid rain 4.2⁠–⁠4.4 · US power plants’ sulfur dioxide 1990 → 2023: 96% less*

## 09 Everything on the Bench

Eight parts: the beaker with its column and heatproof mat, the deflagrating spoon, two gas jars, a jar of sulfur powder with its watch glass and spatula, a bottle of concentrated sulfuric acid, the side panel with its airflow monitor, the baffle and back wall. What matters most: every gas that comes off stays in the cupboard and is drawn away by the fan.

*Beaker · mat · spoon · gas jars · sulfur powder · acid · side panel · baffle*

## 10 Sugar and Acid

This is the page’s model, worked out in the page from the equations above. Choose the mass of sugar and the volume of concentrated acid and press “Pour it on”: the masses of carbon and water follow C₁₂H₂₂O₁₁ → 12 C + 11 H₂O; the steam is the most that water could make if all of it became gas at 100 °C; “water per mole of acid” is the ratio of the water taken out to the acid, in moles.

The column’s height is the model’s own: it goes with the mass of carbon, the column from 50 g of sugar standing 30 cm over the beaker’s floor, about two and a half times the beaker’s height. The wait and the rise are speeded up. The ranges are those the sources used: 20 to 60 g of sugar, 20 to 60 mL of acid. This is a model on a web page, not a way to do the experiment.

*The page’s model · masses from the equation · the column’s height is the model’s own*

**Try this**: In the lab, raise the sugar to 60 g: how much carbon is left, and at most how many litres of steam? Then raise the acid to 60 mL: how much water does each mole of acid get now?

## What the model leaves out

The bench is a model of a teacher’s demonstrations, not a copy of any one set: a section of a school fume cupboard 460 mm wide; the beaker drawn after the catalogue as a 250 mL tall-⁠form beaker (60 mm across, 120 mm high), where the Royal Society of Chemistry uses a 100 mL beaker; a 150 mm heatproof mat; gas jars 60 mm inside and 300 mm high; the deflagrating spoon after the catalogue with a 350 mm rod, an 18 mm cup and an 88 mm cap. The column’s shape, height and pores are drawn, not computed; its height is two and a half times the beaker’s, inside the sources’ two or three times, and its lean is only a shape. The steam, the flame and how fast the petals fade are drawn. How the spoon is lit is not shown. The equation is the textbook simplification: the real reaction also makes carbon monoxide, carbon dioxide and sulfur dioxide, and the model works out masses from the equation only.

## Easy to misread

The sugar is not burned black: the acid takes its hydrogen and oxygen out. But the reaction is very hot, and it does give off sulfur dioxide and carbon monoxide. The black column is not pure graphite. Concentrated sulfuric acid itself is colourless and oily. Sulfur has no smell of its own; the “sulfur” smell is sulfur dioxide. Sulfur dioxide is colourless, so the jar shows no colour. Sulfur dioxide’s bleaching can often be undone (anthocyanins with acid or an oxidant, stoved wool after washing); chlorine’s hypochlorous acid oxidises the pigment into something else. The sulfur dioxide in the air comes mostly from burning fossil fuels, and only a small part from volcanoes.

## Common questions

### Why does concentrated sulfuric acid turn sugar black?

Concentrated sulfuric acid has a strong pull for water: it takes the hydrogen and oxygen out of the sugar in the proportions of water and leaves carbon. This is dehydration; the sugar is not burned black by a flame.

See: [02 What Goes Is Water](https://world.lapla.si/en/sulfur/#water)

### Why is the carbon column from sugar and acid full of holes?

The heat turns the water taken out into steam. In the Royal Society of Chemistry’s words, the steam expands the remaining carbon into a porous, smoking, black column.

See: [03 Blown Up by Steam](https://world.lapla.si/en/sulfur/#heat)

### What colour is the flame of burning sulfur?

Sulfur is yellow, as crystals or a powder, and melts at about 115 °C. Burning in the small cup of a deflagrating spoon it gives a pale blue flame; the US Coast Guard’s chemical data notes that the flame may be hard to see in daylight.

See: [04 A Pale Blue Flame](https://world.lapla.si/en/sulfur/#burn)

### Can colour bleached by sulfur dioxide come back?

Chemists now put it more exactly for anthocyanins, the red pigments of berries and red wine: in water sulfur dioxide becomes bisulfite, which joins onto the pigment to make a colourless compound. That step can be undone: an oxidant or more acid brings the red back.

See: [06 Faded, and Back Again](https://world.lapla.si/en/sulfur/#bleach)

### What is the pH of acid rain?

Normal rain has a pH of about 5.6; acid rain is usually between 4.2 and 4.4, with 16 to 25 times the hydrogen ions of normal rain.

See: [08 From Chimney to Rain](https://world.lapla.si/en/sulfur/#rain)

## Sources

1. [Royal Society of Chemistry and Nuffield Foundation: Sulfuric acid as a dehydrating agent (about 50 g of sucrose; yellow, brown, black after about a minute; two or three times the height of the beaker; the equation is a simplification; a fume cupboard)](https://edu.rsc.org/experiments/sulfuric-acid-as-a-dehydrating-agent/999.article)
2. [C. Baker (2007): The dehydration of sucrose, Education in Chemistry, Exhibition chemistry (the steam expands the carbon into a porous, smoking, black column)](https://edu.rsc.org/exhibition-chemistry/the-dehydration-of-sucrose/2020073.article)
3. [OpenStax: Chemistry 2e, 18.9 (concentrated sulfuric acid is a good dehydrating agent; sulfur dioxide dissolves to sulfurous acid)](https://openstax.org/books/chemistry-2e/pages/18-9-occurrence-preparation-and-compounds-of-oxygen)
4. [OpenStax: Chemistry 2e, 18.10 (sulfur; the smell of burning sulfur is sulfur dioxide)](https://openstax.org/books/chemistry-2e/pages/18-10-occurrence-preparation-and-properties-of-sulfur)
5. [OpenStax: Chemistry 2e, 18.11 (hypochlorous acid oxidises coloured substances to colourless ones)](https://openstax.org/books/chemistry-2e/pages/18-11-occurrence-preparation-and-properties-of-halogens)
6. [OpenStax: Chemistry 2e, appendix G (standard enthalpies of formation)](https://openstax.org/books/chemistry-2e/pages/g-standard-thermodynamic-properties-for-selected-substances)
7. [Royal Society of Chemistry periodic table: Sulfur (yellow crystals or powder)](https://periodic-table.rsc.org/element/16/sulfur)
8. [PubChem: sulfur (HSDB, after the CRC Handbook: melting and boiling points; the US Coast Guard’s CHRIS: a pale blue flame, hard to see in daylight)](https://pubchem.ncbi.nlm.nih.gov/compound/5362487)
9. [PubChem: sulfur dioxide (colourless, choking; boiling point; solubility; NIOSH and OSHA exposure limits)](https://pubchem.ncbi.nlm.nih.gov/compound/1119)
10. [PubChem: sulfuric acid (Hawley’s: 98%, density 1.84; Merck: much heat with water, it chars sugar)](https://pubchem.ncbi.nlm.nih.gov/compound/1118)
11. [NIST Chemistry WebBook: fluid properties of sulfur dioxide (density at 20 °C and 1 atm)](https://webbook.nist.gov/cgi/fluid.cgi?ID=C7446095&Type=IsoBar&P=0.101325)
12. [CLEAPSS: Student Safety Sheets 22 (sulfuric acid) and 52 (sulfur dioxide) (corrosive; toxic if inhaled; asthmatics especially; a fume cupboard for more than test-⁠tube amounts)](https://science.cleapss.org.uk/resource/All-in-one-Student-Safety-Sheets.pdf)
13. [ACS Division of Chemical Education: Chemistry Comes Alive!, Chemical Properties of Sulfur Dioxide (a red rose fades to pink in sulfur dioxide)](https://chemedx.org/JCESoft/jcesoftSubscriber/CCA/CCA3/MAIN/SO2PROP/PAGE1.HTM)
14. [ACS Division of Chemical Education: Chemistry Comes Alive!, Formation of Carbon from Carbohydrate (the sugar column’s sulfur dioxide bleaches a rose petal)](https://chemedx.org/JCESoft/jcesoftSubscriber/CCA/CCA3/MAIN/FORMC/PAGE1.HTM)
15. [I. A. Smellie and I. L. J. Patterson (2024): Changing the Colour of Whole Fruits!, ChemEd X (bisulfite and anthocyanins form a colourless adduct; an oxidant or acid reverses it)](https://www.chemedx.org/article/changing-colour-whole-fruits)
16. [Encyclopædia Britannica, 11th edition (1911): Sulphur (with water sulfur dioxide often bleaches, by reduction)](https://en.wikisource.org/wiki/Page:EB1911_-_Volume_26.djvu/79)
17. [Encyclopædia Britannica, 11th edition (1911): Bleaching (stoved wool turns yellow again when washed with soap or soda)](https://en.wikisource.org/wiki/Page:EB1911_-_Volume_04.djvu/68)
18. [Commission Regulation (EU) No 1129⁠/⁠2011 (the food additives list; sulfites in dried fruit)](https://eur-lex.europa.eu/eli/reg/2011/1129/oj)
19. [Commission Delegated Regulation (EU) 2019⁠/⁠934 (total sulfur dioxide in wine)](https://eur-lex.europa.eu/eli/reg_del/2019/934/oj)
20. [US Code of Federal Regulations, 21 CFR 101.100 (sulfites at 10 ppm or more are declared)](https://www.ecfr.gov/current/title-21/section-101.100)
21. [US Code of Federal Regulations, 21 CFR 182.3862 (not on fruit or vegetables served raw; amended 1986)](https://www.ecfr.gov/current/title-21/section-182.3862)
22. [EFSA (2022): Follow-⁠up of the re-⁠evaluation of sulfur dioxide and sulfites, EFSA Journal 20(11)](https://doi.org/10.2903/j.efsa.2022.7594)
23. [US EPA: What is Acid Rain? (SO₂ and NOx become sulfuric and nitric acids; normal rain about pH 5.6, acid rain 4.2⁠–⁠4.4)](https://www.epa.gov/acidrain/what-acid-rain)
24. [US EPA: Power Sector Programs Progress Report, Emissions Reductions (2024; the Acid Rain Program’s plants 96 percent below 1990)](https://www.epa.gov/power-sector/progress-report-emissions-reductions)
25. [US EPA: Flue Gas Desulfurization (FGD) – Wet, Spray Dry, and Dry Scrubbers, EPA-⁠452/F-⁠03-034 (wet scrubbers above 90%)](https://www.epa.gov/sites/default/files/2020-08/documents/ffdg.pdf)
26. [EEA: European Union emission inventory report 1990⁠–⁠2024 (2026; EU-⁠27 sulfur oxides 95% below 1990)](https://www.eea.europa.eu/en/analysis/publications/annual-european-union-inventory-report-1990-2024-lrtap/)
27. [USGS: The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines, Fact Sheet 113-97 (nearly 20 million tons of sulfur dioxide into the stratosphere)](https://pubs.usgs.gov/fs/1997/fs113-97/)
28. [IUPAC CIAAW: standard atomic weights](https://www.ciaaw.org/abridged-atomic-weights.htm)
29. [NIST: CODATA 2022 values (the molar gas constant)](https://physics.nist.gov/cgi-bin/cuu/Value?r)
30. [NASA: Earth Fact Sheet (air’s mean molecular weight and its oxygen)](https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html)
31. [DWK: DURAN tall-⁠form beakers (250 mL: 60 mm across, 120 mm high)](https://tienda.dicsa.es/beakers-glass-duran-tall$B$einfo253_lang_uk.htm)
32. [Philip Harris: Deflagrating Spoon and Cap (a 350 mm rod, an 18 mm cup, an 88 mm cap)](https://www.philipharris.co.uk/product/lab-equipment/labware/deflagrating-spoon-and-cap/b8r07697)
33. [Philip Harris: Gas Jar: Cylindrical - 60mm x 300mm (60 mm inside, 300 mm high)](https://philipharris.co.uk/product/chemistry/gas-preparation-and-analysis/gas-jar-cylindrical-60mm-x-300mm/b8a64130)

## Data

Sulfur’s, sulfur dioxide’s and the acid’s properties from the CRC Handbook, the Merck Index, Hawley’s and the US Coast Guard’s CHRIS (via PubChem’s HSDB records); exposure limits from NIOSH (via PubChem); sulfur dioxide’s density from the NIST Chemistry WebBook; atomic weights from IUPAC’s CIAAW; the enthalpy of formation of sulfur dioxide and the chemistry of sulfuric and sulfurous acid and of hypochlorous acid from OpenStax Chemistry 2e; air from NASA’s Earth Fact Sheet; R and the standard atmosphere are the exact CODATA 2022 values. The masses, volumes, heats and ratios are computed here; the lab is the page’s own model. The demonstrations after the Royal Society of Chemistry’s Practical Chemistry (with the Nuffield Foundation) and C. Baker’s Exhibition chemistry (Education in Chemistry, 2007); CLEAPSS Student Safety Sheets 22 and 52; the Journal of Chemical Education’s Chemistry Comes Alive!; I. A. Smellie and I. L. J. Patterson in ChemEd X (2024); the Encyclopædia Britannica, 11th edition (1911), on sulphur and on bleaching; food and wine after EU Regulations 1129⁠/⁠2011 and 2019⁠/⁠934, the US Code of Federal Regulations (21 CFR 101.100 and 182.3862) and EFSA (2022); acid rain after the US EPA (What is Acid Rain?, the power sector progress reports, the scrubber fact sheet), the EEA’s inventory report 1990⁠–⁠2024 and the USGS on Mount Pinatubo. The cupboard is this page’s model of a teacher’s demonstrations: a section of a school fume cupboard, a 250 mL tall-⁠form beaker of 60 × 120 mm (after DURAN’s table) on a 150 mm heatproof mat, gas jars of 60 × 300 mm with 75 mm covers, a deflagrating spoon with a 350 mm rod, an 88 mm cap and an 18 mm cup (after Philip Harris’s catalogue); 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

The text, pictures and models on this page are licensed CC BY-NC 4.0: free to use and adapt for teaching and other non-commercial purposes, with credit to world.lapla.si. Third-party material listed above keeps its own licence.

https://creativecommons.org/licenses/by-nc/4.0/deed.en

## Cite this plate

Laplace’s Mini World, Plate 89, “Sulfur”, https://world.lapla.si/en/sulfur/, CC BY-NC 4.0, updated 2026-11-03.