HARDING MP Diagrams ed076p224 - I n the Classroom Melting...

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In the Classroom 224 Journal of Chemical Education Vol. 76 No. 2 February 1999 JChemEd.chem.wisc.edu Melting Point Range and Phase Diagrams— Confusing Laboratory Textbook Descriptions Kenn E. Harding Department of Chemistry, Texas A&M University, College Station, TX 77842-3012 The discussions of melting point range in many organic laboratory textbooks use a two-component phase diagram to illustrate melting point lowering and eutectic mixtures. This paper proposes use of a modified phase diagram to facilitate student understanding of the relationship between melting point range and purity. Most organic chemistry laboratory textbooks include melting point determinations as an early experiment to demonstrate characterization of compounds by their physical properties. Because of the importance of mixture melting points in providing strong evidence for nonidentity of two samples with similar melting points, these textbooks normally discuss melting point behavior of mixtures. These discussions include statements covering the relationship of melting point and melting point range to purity such as “Higher purity is indicated if the melting point becomes higher and the melting range narrower” and “With impure substances, the melting range becomes wider, and the entire melting range is lowered.” These generalizations are often the basis for laboratory quiz and final exam questions. Student confusion arises from the fact that many current laboratory textbooks ( 1 ) discuss this phenomenon with ref- erence only to an idealized two-component phase diagram (Fig. 1). These phase diagram discussions include a description of the melting behavior of a mixture of composition M. They point out that “melting” of the mixture will always begin at temperature t C and that all solid will disappear at tempera- ture t M . Some texts do a better job than others in pointing out that the “melting” at t C generally will not be detectable 1 (except for mixtures close to the eutectic composition) and that the first appearance of liquid will be at a temperature below t M and above t C . However, students find these gener- alizations about melting point range difficult to reconcile with the phase diagram. 2,3 The t C t M range is seen to be larger, rather than smaller, for a mixture containing less A (a more pure sample than mixture M). The graph appears to suggest that “nearly pure B” should melt over the broadest range ( t C ~t B ) rather than show a reasonably sharp melting point! The problem arises from using only a physically correct phase diagram to discuss the “observable” melting point range. The low end of a reported melting point range is not the point at which the phase diagram predicts the mixture begins “melting”; rather, it is generally recorded as the temperature at which
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This note was uploaded on 04/27/2011 for the course CHEM 233 taught by Professor Staff during the Spring '08 term at University of Illinois, Urbana Champaign.

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HARDING MP Diagrams ed076p224 - I n the Classroom Melting...

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