Density of Antifreeze-Water Mixtures A General Chemistry Experiment in Compositional Analysis Paul A. Flowers Pembroke State University, Pembroke. NC 28372 Density is a function of composition. People make use of this fact frequently in everyday life, some perhaps unknowingly, to distinguish different substances. The ecologically aluminum cans minded who recvcle earhaee easilv .senarate . from tin-coated steel cans based on the difference in densities of the two metals (i.e., aluminum is "lighter" than tincoated steel). Likewise, clear plastics and glasses, identical in appearance, may be distinguished by virture of their densities (glass is "heavier" than plastic). Although these examples are for pure substances, the composition dependence of density also applies to mixtures. This behavior is the basis for many compositional analysis procedures, for example, screening blood samples for iron content (does the drop of blood sink or float in the nurse's test liquid?). Even more common is the use of hvdrometen to determine the comuosition of your car's battery and radiator fluids (how many of those little plastic balls float in a sample of fluid?). The information in the preceding paragraph is perhaps common knowledge t o those with even limited backgrounds in chemistry; to many first-year chemistry students, however, this is a new and insightful observation. The first-semester general chemistry laboratory thus provides an ideal setting for demonstrating this principle by having students prepare and measure the densities of several binary mixtures and use their data t o determine the composition of an unknown mixture. Such an experiment represt& a twist on density measurement exercises involving pure substances1 and carries the "dry" lab described by Feinsteinz a step further by involving students in acquisition as well as analysis of the data. The experiment described herein employs antifreeze and manv other hinarv water as the binarv . svstem. Althoueh " systems may serve equally well, the antifreeze-water system was chosen for its "real world" anueal. As is eenerallv the case, students are more receptive id the study i f fundamental chemical principles when presented in the context of a practical application. In addition to providing experience in some standard laboratory procedures (pipetting and weighing), this exercise exposes students to graphical analysis, a method of data treatment used frequently in both science and nonscience disciplines.
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Exoerlmental Caution: Ethyleneglycol, the majw component ofantifreere, isa sweet tasting, poisonour liquid. Studentuahuuld be advised w avoid accidentnl ingestion hy following standard laboratorysafetyprecautions. Reagents and Apparatus Commercialantifreeze (-97% ethylene glycol)and tap water were used to prepare the mixtures. If available, graduated pipets (e.g., 10 mL with 0.1-mL divisions)are recommended, as they permit convenient delivery of varied quantities of the mixture components. Alternatively, volumetric pipets calibrated "to contain" (TC) may be used. in which case the eomoonent ouantities cited in the orocedure below may he revised to arcommodatr the volumes of the available pipe-. The use of T n wlumetric pipeta is discouraged since the mixtures t~ecomeincreasingly viscous withnntiireezerancentmtic,n, making volume measurements with such pipets tedious. Weighings ~
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Journal of Chemical Education
were performed on electronic, top-loading balances that possessed O.W1 gprecision in the employed mass range. Procedure Weigh a 50-mI. beaker or other container to he used as a weighing vessel for the mixtures. Into five separate containers pipet precisely 1.00. 1.50.. 2.00.. 2.50.. and 3.00 mL of antifreeze. Into these same eonkiners ~ 4.W. 3.50.. 3.00.. 2.50. and 2.00 mL of . .i m ~reciselv .t water, respertively, (totaladded wlumeof5.00 ml. in eachconraina,, and agitate to ensure thorough minilag. The resultant mixtures arc nominnlly 20. :30.40. 50 and 609 antifreere by volume, respectively. Sequentially transfer precisely measured aliquots of each standard mixture to the weighing vessel, obtaining the weight of the vessel and mixture after each transfer. Finally, weigh a precisely measured volume of an antifreeze-water miiture if "unknown" composition (between 20% and 60%) provided by the instructor. Discard all waste antifreeze in a properly marked waste container. ~~~
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Data Treatment Use the mass and volume data to calculate densities for eachof thestandard mixtures.and d o t thecalculated densities against volume percent of anhfreeze. Draw the "best possible" straight line through the plotted data points by sighting along the paper a t an oblique angle. The density varies in a fairly linear fashion over the range of 20-60% antifreeze as shbwn by an instructor-generated graph (see fipure).3Because of the nonlinear variation in mixture densit i a t low and high concentrations of antifreeze, the plot may not be extrapolated beyond the limiting data points. Finally, determine the composition of the unknown mixture hy cross reference on the density-composition graph. Results and Discussion The aualitv of student results has twically varied from to shockingly bad. The extremely poor results are very more often a result of sloppy laboratory technique rather than data treatment, m& times invol&ng blatant disregard of the stated experimental procedure. Among the hetter results of students who followed the procedure carefully, however, values for the unknown's composition are generally within a few percent of the accepted value. For example, one lab section's results for an unknown containing 45% antifreeeze showed percent errors of 0,4, and 8%. This experiment provides the professor with an opportunity to discuss the many applications of density measurements to compositional analysis, including, if desired, descri~tionsof various hvdrometers4as wellas the use of density badients.6 ~ d d i t i o n a l l ~this , exercise may serve to motivate discussion of other chemical principles. For example, some students may pose the question "Why is i t important to know the composition of one's radiator fluid?", the - -
' For example, Richardson, W. S.; Teggins, J. E. J. Chem. Educ.
l988,65, 1013-1014. Chem. Educ. 1972. 49. 111. Feinstein. H. I. J. -~~ Seealsd Dean, J. A.. Ed. ~ange&&ndbcok of Chemishy; McGraw-Hill: New York, 1985, p 10-75. Footnote 3, pp 10-86-10-87. Oster, G.; Yamamoto, M. Chem. Rev. 1963, 63, 257. ~~
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answer is that volumes, unlike masses, are not always additive in mixtures. In this particular case, the mixture's volume is less than the sum of the component volumes (i.e., the solution "contracts" upon forming2). If volumes were additive, the density would vary linearly with antifreeze concentration according to the equation
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where dm(,= mixture density, %V,r = volume percent antifreeze, d.f = antifreeze density, and d, = water density. This relation is depicted by the dashed line in the figure. It is of additional interest to note the correlation between this observed postive deviation from "ideal" behavior and the negative deviations from Raolt's law observed for some binary liquid mixtures. In both cases, the deviations result from the greater attractive force present between the different component molecules (e.g., water and ethylene glycol) relative to that molecdes of the same comnonent. ~ - - between -~~ - ~ ~ The experiment as described represents an exercise desiened for the first semester of eeneral chemistrv. Modifications to the procedure could be made t o yield laboratory exercise more suitable for the second semester, for example, by measuring the boiling or freezing points rather than the densities of the mixtures (in which case the discussions of colligative properties and ionideality mentioned in the preceding paragraph would be more appropriate). Finally, the professor may wish to emphasize concepts of statistical data analysis by pooling student data and/or by performing linear regression rather than "eyeballing" to generate the straight line. ~~~
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)and ~ecwetical(----)plats of mixture denExperimental-I sity versus nominal volume percent antifreeze(an for waterfantiheere mixtures. The theoretioal cuwe was calculated assuming additive volumes. answer to which will involve a discussion of colligative properties. The question may also arise "Why can't we just add the water and antifreeze to a beaker and weigh it?" The
Mendeleev Communications: A New Journal
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Menddeev Communications, a new international rapid wmmunications journal is to be launched in.lnnunry 1991 asa collaborativeventure between the Royal Society of Chemistry and the Academy of Sciences of the IISSR. The jonrnal will cover all branches of rhemirtrv and will he similar in furmat and range of subiert matter to the RSC's well-estnhlished iournal Chemical ~ommunications ,~ Tlntil now., the results of Soviet chemical research have been available mainlv throueh the translation of articles ~~~already published in Russian in the IISSR. Menddceu Communmrtonc will puhlish orrginal papers directly in English, avoiding the delays inhtrenc in the eonrentiunal communication channels. For the first time, the international chemical community will have rapid access tocurrent 3ovict chemical research. Cummunicatiunswill appear within three mont hs uf their receipt in the UK. There are EditorialBoardsin both theUK and the USSR, who will advise on refereeingpolicy to ensure that only the highest quality research is reported in the journal. For iurrh~rdetailsconrsct: S. .lane Davies. L'K StaffKditor, Mendeleev Communicatims. Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road. Cambridge CB4 4WF, UK. Te1: ( t 4 4 (U122J 42006fir. ~~~~~~~~
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Volume 67
Number 12 December 1990
1069
