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In “A Thermodynmic Analysis to Explain the Boiling-Point Isotope Effect for Molecular Hydrogen” (1) the title is just right, but we need to point out clearly, and perhaps often, that the mass effect, which is mentioned repeatedly, and properly, throughout the article, is very small in the rest of chemistry. Too many otherwise well-prepared chemists still teach and write about a supposed general dependence of boiling point on molecular “weight” or mass, and some readers may take this article as supporting that. For the related variable of melting point, for example, the molecular mass as such is cited currently (2) and repeatedly as a relevant independent variable. This is misleading for melting points too, although the importance of symmetry is well elucidated in that article. Even for molecular hydrogen, the difference in boiling points between 20.4 K for diprotium and 23.5 K for dideuterium, although important at these low temperatures, is perhaps not striking for a mass ratio of 1 to 2. Let’s give students the useful and interesting information in both of these articles, together with a perspective (3) that incidentally exposes the uniqueness of molecular hydrogen with regard to the importance of mass. Literature Cited- Baker, D. B.; Christmas, B. K. J. Chem. Educ. 2000, 77, 732–734.
- Brown, R. J. C.; Brown, R. F. C. J. Chem. Educ. 2000, 77, 724–731.
- Rich, R. L. J. Chem. Educ. 1995, 72, 9–12, on using polarizability to predict boiling points.
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