The author replies to Duhr, et al.

I wish to thank Duhr et al. for raising two main issues with respect to our KClO₃/EtOH dehydration of sugar demonstration (1): watch glass breakage leading to ignition of paper towels, and the option of omitting KClO₃ from the mixture. Having performed this demonstration well over 50 times, I must report that I did once encounter a cracked watch glass, although that did not result in ignition of the paper towels underneath. Apparently, low quality glass and/or insufficient mixing of the solid KClO₃/sucrose leading to hot spots at the bottom of the pile can on rare occasions crack the watch glass holding the mixture.

I admire the ingenuity that Duhr et al. have shown in devising their 3 in. clay pot setup. Although clay lacks the transparency of glass, they overcome this problem by partially filling the pot with a bed of sand. This makes the KClO₃/sucrose pile more visible to the class as it protrudes over the top of the pot, and it also serves to absorb heat from the combustion. For those who prefer to use equipment more commonly found in a chemistry stockroom when setting up their demonstrations, three other solutions to the cracked watch glass problem come to mind:

1. Use a Pyrex petri dish placed on top of a watch glass to hold the KClO₃/sucrose pile.
2. Use two watch glasses: If the top one does crack, the bottom (cooler) one will still hold.
3. Use a nonflammable tabletop covering in place of paper towels, e.g., a metal tray or an asbestos pad.

Regarding Duhr et al.’s suggestion of performing the demonstration without KClO₃, we have tested this many times and found it to be unsatisfactory. Because the combustion occurs at a substantially lower temperature in the absence of KClO₃, sucrose dehydration is much less complete. The result, a rather unimpressive black mound approximately the same size and shape as the original solid mixture, does not grow into the dramatic black column afforded by the presence of KClO₃.

Literature Cited

1. 1. Silverstein, T. P.; Zhang, Y. J. Chem. Educ. 1998, 75, 748.