October 27, 2012
Sugar Decomposition
Clues to a Chemical Reaction
Title: Decomposition of Sugar
Abstract:
Students observe the acid-catalyzed decomposition of common table sugar (sucrose). Students
recognize the evidence for a chemical reaction by observing the formation of new substances, a
color change, an increase in temperature, the formation of a gas, and a foul odor. The decomposition
of white sucrose, catalyzed by concentrated sulfuric acid, results in the formation of a black solid,
carbon, and water liberated as steam.
Key Concepts:
catalyst
products
reactant
Materials
Granulated Sugar - 110 grams or about 125 ml by volume; 650 grams for 5 classes
Sulfuric Acid - 18 Molar, 110 grams or 70 ml by volume; 350 ml for 5 classes
Sodium Bicarbonate (Baking Soda) - 300 g; 1.5 kilograms for 5 classes
Equipment & Supplies
Apron or lab coat
Beaker (400 ml); 5 for 5 classes
Beaker (600 ml); 5 for 5 classes
Glass Graduated Cylinder (100 ml)
Polypropylene Graduated Cylinder (100 ml)
Eye protection
Gloves (disposable, acid-resistant)
Glass stirring rod (40 cm)
Two Hazardous Material (HAZMAT) Containers (1-gallon each) with wide mouth (3.5 inches or
9 cm diameter) and screw cap
Tray
Paper Towels
Stainless Steel Rolling Cart or Table
Safety Precautions:
The sugar demonstration has many potential risks. It uses a strong acid and generates heat and
steam. Only perform this demonstration if you can be sure that all of the appropriate precautions can
be taken by the teacher and the students. Use eye protection, gloves, and an apron. Perform the
demonstration in a fume hood or outside the classroom. The steam that is released is hot and smelly.
Watch eyes and hands.
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Procedure:
This demonstration must be done in a fume hood or outdoors in an open area. Make sure the
students are at least six meters (about 20 feet) from your demonstration table.
1.
Put a 400 ml beaker on a tray or acid-resistant surface. Pour 110 grams (about 125 ml) of
the sugar into the 400 ml beaker and show the students that you are starting with table sugar,
known chemically as sucrose with the empirical formula C12H22O11. Ask them to describe
the nature of the sugar (solid, crystalline, white, water soluble).
2.
Measure 70 ml of concentrated sulfuric acid in a glass graduated cylinder and explain to the
students that it is a very strong acid and must be handled with acid-resistant gloves. Ask
them to describe the nature of the material (liquid, transparent, colorless).
3.
Pour the sulfuric acid into the beaker containing the sugar and stir with a glass rod until the
mixture turns black and begins to bubble. The color change and gas formation are two clues
to a chemical change. Remove the stirrer and ask the students to observe the beaker and
describe the nature of the mixture (black). Hold the bottom of the beaker with your gloved
hand and tell the students that the mixture is becoming hot; a third clue to a chemical
change. It will take a few minutes, but the reaction will suddenly become vigorous with the
formation of a gas with a pungent odor of rotten eggs (fourth clue to a chemical change)
and carbon, a new substance (fifth clue to a chemical change). Alternatively, insert a
thermometer into the beaker and tell the students the temperature that it reaches (typically
160 oC).
4.
As the sugar decomposes, water will be released in the form of steam, and carbon will
quickly grow like a mushroom out of the beaker. Explain that when the sucrose molecule
breaks down, water vapor is released and only carbon remains; it takes up more space than
the original sugar. The heat generated by the reaction causes a small amount of the sulfuric
acid to decompose into sulfur trioxide and sulfur dioxide, which are responsible for the odor.
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5.
Explain to the students that the sulfuric acid acts as a catalyst, so it is not consumed. The
black sponge-like mass of carbon is coated with sulfuric acid so while the students can
examine the material at close range, NEVER ALLOW a student to touch it.
Disposal:
Wearing acid-resistant gloves and using some paper towels, grasp the hot beaker and insert the
black, sponge-like carbon mass directly into the wide mouth HAZMAT container. Use the base end
of a 100 ml polypropylene graduated cylinder to break up the black mas into smaller pieces. Use a
spatula to scrape any excess carbon pieces from the beaker and pour directly into the HAZMAT
container. Add 300 grams (275 ml) of baking soda (sodium bicarbonate) from a 600 ml beaker to
the black mass in the container to neutralize the acid. Stir the mixture together with the base end
of the polypropylene cylinder. Carbon dioxide will be formed as the excess acid is neutralized by
the baking soda, so place the screw cap on the container, but DO NOT TIGHTEN to allow the gas
to be slowly released.
Wash all the glassware thoroughly with water. The black carbon residue in the beaker can be easily
removed with soap and water using a bottle brush or sponge. It will harder to remove if it is left
unattended for an extended period of time.
Further Discussion:
Show the structure of sucrose and the reaction sequence using visual aids. The sulfuric acid acts as
a catalyst to remove all the hydrogen and oxygen atoms from the sucrose in the form of water in a
process called dehydration. This leaves elemental carbon behind in a honeycomb-like structure.
A PowerPoint or other type of slide show can be used for this purpose. An example is shown on the
following pages.
Reference:
B. Z. Shakhashiri, 1983, Chemical Demonstrations: A Handbook for Teachers of Chemistry, vol.
1, pp 77-78.
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