What property of concentrated sulphuric acid is in action when sugar turns black in its presence?

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Carbon snake is a demonstration of the dehydration reaction of sugar by concentrated sulfuric acid. With concentrated sulfuric acid, granulated table sugar (sucrose) performs a degradation reaction which changes its form to a black solid-liquid mixture.[1] The carbon snake experiment can sometimes be misidentified as the black snake, "sugar snake", or "burning sugar" reaction, all of which involve baking soda rather than sulfuric acid.

A column of porous black graphite formed during the experiment.

Carbon snake experiment

Concentrated sulfuric acid can perform a dehydration reaction with table sugar. After mixing, the color changes from white to brownish and eventually to black. The expansion of the mixture is the result of vaporization of water and CO2 inside the container. The gases inflate the mixture to form a snake-like shape, and give off a burned sugar smell.[1] The granularity of the sugar can greatly affect the reaction: powdered sugar reacts very quickly but sugar cubes take longer to react.[2]

When sucrose is dehydrated, heat is given out to the surroundings in an exothermic reaction, while graphite and liquid water are produced by the decomposition of the sugar:[3]

C12H22O11 (s) + H2SO4 (aq) + 1/2 O2 (g) → 11 C (s) + CO2 (g) + 12 H2O (g) + SO2 (g)

As the acid dehydrates the sucrose, the water produced will dilute the sulfuric acid, giving out energy in the form of heat.

C12H22O11 (s) → 12 C (s) + 11 H2O (l)

Paranitroaniline can be used instead of sugar, if the experiment is allowed to proceed under an obligatory fumehood.[4] With this method the reaction phase prior to the black snake's appearance is longer, but once complete, the black snake itself rises from the container very rapidly.[5] This reaction may cause an explosion if too much sulfuric acid is used.[6]

Elephant's toothpaste
Black snake (firework)
Chemical volcano
Diet Coke and Mentos eruption

^ a b Shakhashiri, Bassam Z.; Shreiner, Rodney; Bell, Jerry A. (2011). "1.32 Dehydration of Sugar by Sulfuric Acid". Chemical Demonstrations a handbook for teachers of chemistry volume 1. University of Wisconsin Press. pp. 77–78. ISBN 978-0-299-08890-3.
^ Don't Try This at Home - 3 - Sulfuric Acid and Sugar, retrieved 2022-01-31
^ Roesky, Herbert W. (2007). "Experiment 6: Sugar coal by splitting off water from sugar with sulfuric acid". Spectacular Chemical Experiment. Wiley. p. 17. ISBN 978-3-527-31865-0.
^ Summerlin, Lee R.; Ealy, James L. (1988). "Experiment 100: Dehydration of p-Nitroaniline: Sanke and Puff". Chemical Demonstrations: A Sourcebook for Teachers Volume 1 (2nd ed.). American Chemical Society. p. 171. ISBN 978-0-841-21481-1.
^ "Carbon Snake: demonstrating the dehydration power of concentrated sulfuric acid". communities.acs.org. 2013-06-06. Retrieved 2022-01-31.
^ Making a carbon snake with P-Nitroaniline, retrieved 2022-01-31

Wikimedia Commons has media related to Carbon snake.

Dehydration of sucrose with sulfuric acid ChemEd X
Sugar snake (Sugar and Baking soda) MEL science
Sugar Snake: Sulfuric Acid and Sugar youtube.com

Retrieved from "https://en.wikipedia.org/w/index.php?title=Carbon_snake&oldid=1082271130"

Sucrose (table sugar) is placed in a beaker and concentrated sulfuric acid added. Soon an exothermic reaction takes places during which a column of carbon rises from the beaker with a cloud of steam and sulfur dioxide.

The name carbohydrate derives from the formula of sugars such as sucrose (C12H22O11 or C12(H2O)11) in which the formula appears to be a hydrate of carbon. In the presence of concentrated sulfuric acid sucrose is dehydrated to produce carbon and water. The heat of the reaction vaporizes the water causing the column of carbon to form.

Citation
- Shakhashiri, B. Z. Chemical Demonstrations; University of Wisconsin Press: Madison, 1983; Vol. 1, pp 77-78.
Design, Text and Demonstrator
- Gary Trammell, University of Illinois at Springfield, Springfield, IL 62794
Videographer/Editor
- Steve Dykema, University of Illinois at Springfield, Springfield, IL 62794
Voice
- Margaret Biddle, University of Wisconsin–Madison, Madison, WI 53706
Audio Production
- Greg Minix, University of Wisconsin–Madison, College of Engineering, Madison, WI 53706
- Jerrold J. Jacobsen, University of Wisconsin–Madison, Madison, WI 53706

One of the most spectacular chemistry demonstrations is also one of the simplest. It's the dehydration of sugar (sucrose) with sulfuric acid. Basically, all you do to perform this demonstration is put ordinary table sugar in a glass beaker and stir in some concentrated sulfuric acid (you can dampen the sugar with a small volume of water before adding the sulfuric acid). The sulfuric acid removes water from the sugar in a highly exothermic reaction, releasing heat, steam, and sulfur oxide fumes. Aside from the sulfurous odor, the reaction smells a lot like caramel. The white sugar turns into a black carbonized tube that pushes itself out of the beaker.

Dehydrating sugar by reacting it with sulfuric acid makes for an entertaining and educational chemistry demonstration.
The reaction produces a growing "snake" of black carbon, a lot of steam, and the odor of burning caramel.
The demonstration illustrates an exothermic reaction and a dehydration reaction.

Sugar is a carbohydrate, so when you remove the water from the molecule, you're basically left with elemental carbon. The dehydration reaction is a type of elimination reaction.

C12H22O11 (sugar) + H2SO4 (sulfuric acid) → 12 C (carbon) + 11 H2O (water) + mixture water and acid

But wait... sugar does not contain water, does it? How can it get dehydrated? If you look at the chemical formula for sugar, you'll see a lot of hydrogen and oxygen atoms. Combining two hydrogen atoms with one oxygen atom makes water. Removing the water leaves behind the carbon. Although the sugar is dehydrated, the water isn't 'lost' in the reaction. Some of it remains as a liquid in the acid. Since the reaction is exothermic, much of the water is boiled off as steam.

The sulfuric acid and sugar reaction is a popular chemistry demonstration for high schools, colleges, and science enthusiasts. But, it isn't the kind of project you should do at home.

If you do this demonstration, use proper safety precautions. Whenever you deal with concentrated sulfuric acid, you should wear gloves, eye protection, and a lab coat. Consider the beaker a loss, since scraping burnt sugar and carbon off of it isn't an easy task. It's preferable to perform the demonstration inside of a fume hood because the reaction releases sulfur oxide vapor.

If you're looking for other dramatic exothermic demonstrations, why not give one of these a try?

Steel Wool and Vinegar: Soaking steel wool in vinegar is something you can do at home. Basically, the acetic acid in vinegar reacts with iron in the steel wool in an oxidation reaction. It's rust formation, but it occurs much more rapidly than waiting for natural processes.
Barking Dog Reaction: This barking dog reaction gets its name for the sound it makes. Igniting a mixture of carbon disulfide and nitric oxide in a long glass tube forms a flame. The flame travels down the tube, compressing gases in front of it until they have nowhere to go and explode. The small explosion doesn't break the tube, but it does produce a loud "bark" or "woof" and it glows bright blue.
Dissolving Laundry Detergent in Water: While not as exciting as the sulfuric acid and sugar reaction or the barking dog reaction, dissolving laundry detergent is something you can try the next time you wash your clothes. Hold a bit of dry detergent in your hand and dampen it with water. It gets warm!
Elephant Toothpaste Demonstration: If elephants used toothpaste, it would be the size of the foam produced by this chemical reaction. The reaction between hydrogen peroxide and potassium iodide produces a lot of gas. A bit of detergent added to the mixture traps the gas and makes a steaming, bubbly foam. Adding food coloring customizes the color.

Roesky, Herbert W. (2007). “Experiment 6: Sugar coal by splitting off water from sugar with sulfuric acid”. Spectacular Chemical Experiments. Wiley. p. 17. ISBN 978-3-527-31865-0.
Shakhashiri, Bassam Z.; Shreiner, Rodney; Bell, Jerry A. (2011). “1.32 Dehydration of Sugar by Sulfuric Acid”. Chemical Demonstrations: A Handbook for Teachers of Chemistry Volume 1. University of Wisconsin press. pp. 77–78. ISBN 978-0-299-08890-3.