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Ronald L. Rich’s letter clearly alludes to two common concerns for those of us who teach. The first is that general dependences in science can be assumed mistakenly from highly specialized cases. A second is that these misleading generalizations often appear in the classroom. In light of these concerns, I would agree with Rich’s proposal for teaching the topics in question. As an historical note, the tendency to over-generalize is addressed by early Enlightenment thinkers such as Francis Bacon who warned of the idols of the mind. Among these include the idols of the tribe, which assume more order than actually exists in nature. As Rich suggests, a presumed dependence of boiling point upon molecular mass implies a general dependence that is not observed. With regard to the specifics of the dependence of boiling point upon mass in molecular hydrogen, our article points out that several other explanations for the effect were considered. However, none of these are able to explain the observed results. The actual working model treats each hydrogen molecule as residing within a harmonic potential energy well where more massive isotopes lie deeper within the well. As a result, greater energies (or higher temperatures) are required to initiate boiling as the molecular mass increases. Such a model, indeed, explains the observed boiling point dependence upon mass in molecular hydrogen, but, as Rich points out, the effect is rather small and is not universal throughout chemistry.
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