In the Left Corner…Most sugar from a grocery store is granular and pure white. If the grains are large enough—existing as large crystals—it is immediately apparent that sugar isn’t even white; it is totally colorless and transparent. Sold at candy stores, we call it rock candy.
In the Right Corner…Concentrated sulfuric acid is thick and nearly colorless—oily—much like the Karo Syrup one puts on pancakes. It is an extremely dangerous acid and dehydrating agent, capable of inflicting severe burns. One of my college chums, working in a laboratory during a work-cooperative assignment, caught just the bottom of a large bottle of the stuff on a laboratory bench. It landed on his blue-jeans. In just seconds he was under a safety shower. Yet, he suffered 3rd-degree burns!
Let the Fight Begin!Pour some concentrated sulfuric acid¹ into a beaker containing sugar granules, and a dramatic reaction takes place.² The sugar turns brown, then changes to black. The volume of the sugar increases greatly. What happened? What’s the black stuff and what is the mist? Why is the beaker so hot?
Knockout: What Happened?The heat is due to the release of a tremendous amount of energy. All that heat vaporizes water producing steam—the mist is mostly water.³ Be careful to avoid the mist, as it doubtless contains traces of acid. Did you notice from the structural formula of sucrose that it contains atoms of hydrogen and oxygen? Combine them, and you get the water, formula H₂O.
Just one molecule of sucrose contains 11 molecules of water. If you take those out, all that remains of a sugar molecule are 12 atoms of carbon. The black residue is mostly carbon with some excess acid.
Sulfuric Acid Turns Sugar BlackSulfuric acid is a super-strong dehydrating agent, it sucks up water, or the components needed to produce it. Hence, the ether groups (-C-O-C-) and hydroxyl groups (-OH) in sucrose are stripped away. The overall reaction is written:
C₁₂H₂₂O₁₁ → 11 H₂O↑ + 12 CTurned to colorless water and black carbon, the sulfuric acid turns sugar black.
¹ The sulfuric acid needs to be concentrated to exert its dehydrating effect on sugar.
² This reaction should not be attempted, except by properly-trained laboratory personnel.
³ Carefully avoid contacting the mist. It is mostly steam, but may contain traces of acid.
References: ← Back to QS Nuggets