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.
The "Chemistry Teacher Connection" (CTC) is especially for high school chemistry teachers. For only $40/year, it offers an online-only subscription to CLIC along with membership in the Division of Chemical Education, normally $65/year. CTC subscribers receive access to all articles and supplements from 1996 through the current issue.
Through special arrangement with the ACS, JCE High School CLIC is now able to provide subscribers with online access to Chemical & Engineering News articles that have been selected specifically for secondary science instructors and their students.
Occasionally, collections of JCE back issues become available for donation to individual teachers, schools, or libraries. JCE matches collections with interested recipients. Recipients pay shipping costs or pick up the collection.
For journals in other fields of chemistry visit ACS Publications.
Citation
Always in the