White Sugar - Black Carbon

Demonstration Experiment on Video

White Sugar - Black Carbon

Objectives: Decomposition of Organic Matter, Dehydrating Effect of Concentrated Sulfuric Acid

Peter Keusch

German version

Chemical:
conc. sulfuric acid

Supermarket product:
table sugar

Glass wares:
gas collecting cylinder h = 10 cm, d = 4 cm
measuring cylinder 25 mL
glass stirring rod

Hazards and safety precautions:

Conc. sulfuric acid is highly toxic. Causes severe burns. May be fatal if swallowed. May cause cancer through inhalation. Very destructive of mucous membranes.

Safety glasses, protective gloves, effective ventilation!

Experimental procedure:

30 g of table sugar placed in a gas collecting cylinder are mixed cautiously with 10 mL of conc. sulfuric acid while stirring with a glas rod.

Results:

The sucrose turns yellow and then red brown. After about one and a half minute the mixture changes color to black and a spongy mass of carbon grows well above the top of the cylinder. The formation of a carbon column is accompanied by the release of heat and evolution of vapor.

Video clip (Download RealPlayer .rm-Datei)

Discussion:

Concentrated sulfuric acid is a strong dehydrating agent. It dehydrates the carbohydrate sucrose with an exothermic reaction. Carbon and water are produced in this 'caramelization process' (1).

The heat released vaporizes the water. The equation above is an oversimplification. Concentrated sulfuric acid also acts as an oxidizing agent.

Some of the carbon is oxidized to carbon monoxide and carbon dioxide and some of the sulfuric acid is reduced to sulfurous acid (2). Sulfuric acid liberates sulfur dioxide from sulfurous acid. The hot steam and the produced gases create bubbles in the carbon making a carbon foam.

Note of caution: The fumes produced in this experiment are hot steam rich in unconverted sulfuric acid and produced sulfur dioxide. Do not breathe the fumes! The experiment should be performed in a fume-hood or well ventilated area.

Index of the Lecture Experiments