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I find it distressing to encounter a lecture demonstration that seeks to make a valid scientific point through deception. I refer to the Tested Demonstration, “A Reaction That Takes Place in Beakers but Not in Conical Flasks: A Catalysis-Related Demonstration” (1). The surreptitious introduction of Fe(II) into the beakers but not the conical flask does indeed produce the indicated effects, as the title states. The take-home message, however, is not one of integrity. By the time matters are all sorted out, and the demonstrator confesses his trick, the innocent observer is not likely to retain much of a scientific message. Nor, perhaps, a highly favorable impression of chemistry. This is unnecessary. An honest, effective, and impressive demonstration makes the point correctly and directly. One should mix the Cr(VI), Fe(II), and KI–starch solutions in different orders. It will be perceived that only those in which KI–starch is the first or second component will yield the I2–starch complex upon mixing. Decolorization of the I2–starch with thiosulfate can also be incorporated if one desires. From there, the issue of induced reactions, specifically the trapping of the Cr(V) transient by iodide, will provide plentiful discussion. Literature Cited- White, C. J. Chem. Educ. 2004, 81, 364.
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