Sulfuric Acid and Sugar Demonstration

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.

Chemistry Demonstration

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.

Safety Precautions

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.

Other Exothermic Chemistry Demonstrations

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.

Sources

• 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.