Chemistry in Pictures: Barbenheimer reaction

Patrick Lutz, an assistant professor at St. Lawrence University, adapted a classic iodine clock demonstration for his General Chemistry class to evoke the starkly different vibes of the summer’s two biggest movie events: Barbie and Oppenheimer. The demo starts with Lutz pouring a test tube full of a Barbie-pink solution comprised of vinegar, food coloring, starch, iodide, and a small amount of ascorbic acid into a beaker containing clear, colorless hydrogen peroxide. For several seconds, nothing happens. Then the rose-colored liquid abruptly turns a purple-black color reminiscent of Oppenheimer’s dour color palette.

At the time, Lutz didn’t discuss the chemistry in detail with his students, who were just learning what atoms and molecules are. But here’s what happened behind the scenes: iodide (I^-) reacts with hydrogen peroxide in the presence of acid to form molecular iodine (I₂). Starch reacts with I₂ to form a black complex—but ascorbic acid reacts with I₂ faster, delaying the starch complexation and the color change. The students will eventually do the reaction in the lab portion of the class as an introduction to stoichiometry.

Lutz says he likes the demo because all the supplies can be purchased inexpensively from a supermarket or a drugstore, and the naturally colorless starting solution can be dyed to fit any theme.

Credit: Patrick Lutz

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