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JChemEd.chem.wisc.edu • Vol. 75 No. 6 June 1998 • Journal of Chemical Education 787

The vapor pressure lowering (VPL) that results when anonvolatile solute is added to a solvent is a topic covered invirtually all freshman chemistry textbooks. That this phenom-enon occurs is an experimental fact. However, contrary tothe impression given in most texts, VPL seldom occurs inaccordance with Raoult’s law. In fact it has been stronglyargued by Hawkes (1), that this “law” has little practicalapplication and should be omitted from introductory chem-istry courses.

Explanations offered for the phenomenon of VPL areinevitably wrong or, at the best, misleading. The VPL causedby addition of a nonvolatile solute brings about a reductionin the rate of evaporation of the solvent that will lead, viathe liquid–vapor equilibrium, to a reduced vapor pressure.The fundamental error perpetuated in numerous chemistrytexts is to attribute the reduced rate of evaporation to theoccupation and consequent “blocking” of surface sites bynonvolatile particles. These texts typically argue that nonvola-tile particles in the solution surface will block some solventmolecules from escaping to the vapor phase but will notimpede vapor molecules that are condensing back into thesolution. Some texts go so far as to support this incorrectexplanation with detailed diagrams (2).

At an elementary level the blocked surface site theorymay be easily dismissed by considering the following simpleexperiment. Float a few corks on the surface of a liquid. Surfacesites will certainly be blocked, but the vapor pressure willobviously remain unaltered.

The alarmingly widespread occurrence of the fallacious“blocked surface sites” theory was noted by Mysels (3) as longago as 1955. Although he supplied a comprehensive list ofreasons for the incorrectness of the blocked surface sitestheory, he offered no alternative explanation. Since then, thisand other erroneous explanations have proliferated rather thanvanished. As a result, it is necessary once again to bring thematter to the attention of educators and writers in the fieldof introductory chemistry.

Despite its extensive use, the blocked surface site theoryis not the only incorrect explanation used to explain VPL. Aless common but still erroneous explanation is illustrated bythe following example: “Owing to the extra attraction that thewater molecules feel for the dissolved particles, they find itharder to escape from a solution than from pure water” (4 ).Unfortunately, this simple and attractive explanation is alsowrong. The molecular origin of VPL is not found in the en-ergy of interaction between the solvent and solute particles.Ideal solutions, for which the solution enthalpy is zero, aswell as solutions that form endothermically or exothermically,all produce a VPL that is independent of both the sign andthe magnitude of the solution enthalpy.

Since vapor pressure lowering is not a result of enthalpyeffects, it must then be a result of entropy effects (5). We have

found only one freshman chemistry text (6 ) that gives anadequate explanation of VPL by making use of a simpleentropy-based argument. This is an approach that teachersand authors of future textbooks would do well to emulate.It is outlined very briefly below.

A solution is more “disordered” than a pure solvent. Thusthe entropy difference between a pure solvent and its vaporwill be greater than the entropy difference between a solutionand its vapor. As a result, solvent molecules will have a greatertendency to leave the pure solvent than they do to leave thesolution and the solution will have a lower vapor pressure. Itwill be noted that this simple explanation has nothing what-ever do with the blocking of surface sites by nonvolatile par-ticles or with the energy of interaction between solute andsolvent particles, and also avoids each of the objections raisedby Mysels (3).

At a nonmolecular level, vapor pressure lowering maybe correctly explained in terms of the chemical potentials, µ,of the pure solvent and the solution (7 ). However, thisapproach is generally beyond the level of the average chem-istry freshman.

If students are not familiar with the basic concepts ofentropy, then no attempt should be made to explain VPL. Itshould simply be stated as an experimentally verifiable fact.No explanation at all is better than a faulty one. We don’t,for example, try to explain to freshmen why opposite chargesattract each other. We simply note this phenomenon as anexperimentally observable fact and leave it at that.

CommentIn the above discussion, when referring to entropy, the

word “disordered” is used in quotation marks. This is donebecause the linking of entropy to molecular disorder mayproduce significant misconceptions. This has been discussedin numerous articles, of which the one by Lowe (8) is of par-ticular excellence.

Literature Cited

1. Hawkes, S. J. J. Chem. Educ. 1995, 72, 204–205.2. Atkins, P. W. The Elements of Physical Chemistry, 2nd ed.; Oxford

University Press: Oxford, 1996; p 131.3. Mysels, K. J. J. Chem. Educ. 1955, 32, 179.4. Matthews, P. Advanced Chemistry 1; Cambridge University Press:

Cambridge, 1992; p 379.5. Atkins, P. W. Physical Chemistry, 5th ed.; Oxford University Press:

Oxford, 1994; p 223.6. Chang, R. Chemistry, 5th ed.; McGraw-Hill: New York, 1994; p 493.7. Atkins, P. W. Physical Chemistry, 5th ed.; Oxford University Press:

Oxford, 1994; p 222.8. Lowe, J. P. J. Chem. Educ. 1988, 65, 403–406.

Vapor Pressure Lowering by Nonvolatile Solutes

Gavin D. PeckhamDepartment of Chemistry, University of Zululand, Private Bag X1001, Kwa Dlangezwa, 3886, South Africa