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application/ and analogie~
edited
by:
RON DELORENZO Middle Georgia College Cochran, Georgia 31014
An Analogy for Soluble and Insoluble Mfxtures: Sand and Magnetic Iron Filings Ira B a t r a
Garde
The Phillips Exeter Academy Exeter. NH 03833
In a previous note in this Journal,' Richard A. Kjonaas described a marble-magnet analogy to illustrate the difference between soluble and insoluble mixtures. In that analogy, mixing red and blue marbles or red and blue magnets served to demonstrate a soluble mixture of nonpolar molecules or of polar molecules, respectively. Both combinations resulted in comnlete intersoersion of the "like" obiects of different colors. Also, in Kjonaas' analogy mixing marbles with maenets demonstrated an insoluble mixture in which a " nonpolar substance and a polar substance were combined. When I used this analom in my Introductory Chemistry classes to illustrate the differencks between soluble and insoluble mixtures, my students were not comfortable with the use of uniformly interspersed red and blue marbles to represent a soluble mixture composed of two nonpolar substances. They felt that even though the red and blue marble mixture had uniform composition throughout, it did not seem like a soluble mixture a t the macroscopic level because the individual red and blue marbles were large enough to be distineuished readilv. not compatible with .. a ohenomenon . the students' experience of soluble mixtures. One student volunteered a modification to the red-blue marble analogy that the other students preferred. He suggested thoroughly mixing fine sand of two different colors (Fig. I), perhaps blue and yellow. He predicted that, from a distance, the overall texture and color of the sand would appear to be homogeneous because of the small size of the individual sand grains. For my students a mixture of sand of different colors served as a better analogy because the overall homogeneous visual impact was consistent with their exoerience of soluble mixtures. Indeed. in our experience wKen two substances like sugar and hottea, or heiane and octane (to use Kjonaas' example), are dissolved in each other, while the molecules do not lose their individual identities in solution. the individual components of the soluble mixture cannot be visually distinguished. In addition to being effective a t the macroscopic level, the sand analogy also works at the microscopic level. If one looks closely at the sand mixture, the individual particles are distinguishable. Therefore, the sand analogy can be used to point out the particulate, molecular nature of solutions. In addition. the relative amounts of blue and vellow sand can be varied to illustrate the distinction between a solute and a solvent. One limitation of the use of sand particles to represent nonpolar molecules is that the analogy does not illuminate the van der Waals attractions, hydrophobic interactions, and other intermolecular interactions which do occur between nonpolar molecules. The sand analogy for nonpolar solutions developed spon-
' Kjonaas, Richard A. J. Chem. Educ. 1984, 61, 765. ISA
.lournal nf Chemical Education
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MACROSCOPIC VlEW
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Figure 1. Sand of two different colors: a soluble mixture of nonpolar substances.
Rod
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Figure 2. Magnetic iron filings aftwa different colors: a soluble mixtureof polar Substances.
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Stirring Rod
MiCROSCOPlC VIEW MACROSCOPIC VlEW Figure 3. Sand and magnetic iron filings: an insoluble mixture of a nonpolar and a polar substance.
taneouslv and with little difficultv. However, i t is much more challenging to develop an analogy for a polar substance which is effective a t both the macroscopic and microscopic levels. In order for the analogy to be effkctive a t the macroscopic level in the way that sand was, the objects used in the analogy need to be very small and visually indistinguishable from a distance when mixed. Yet a close-up, microscopic view should reveal the narticulate nature of the solution. and also the nature of the inter-particle interactions. Small spindle-shaned maenets the size and shaoe of iron filines. hereafter refkrred t o k "magnetic iron filings2', fulfill tcese criteria. Therefore, to illustrate a soluble mixture of two polar substances, ask your students to imagine combining iron filings of two different colors (Fig. 2). Though the iron filings of two different colors intersperse completely when mixed, upon a closer view the specific orientation of the iron filings with respect to one another, and their inter-particle attractions and repulsions, become apparent. This microscopic view illustrates the particulate, molecular nature of the solution, and the electrostatic forces acting between the polar molecules. As in the case of Kjonaas'analogy, i t is important to emnhasize to students that electrostatic and maenetic forces are not the same. In order to illustrate an insoluble mixture composed of
polar and nonpolar substance, ask your students to imagine pouring tiny magnetic iron filings, followed by sand particles. into a glass beaker. Initially, two separate layers are apparent. upon stirring the mixt&, one can imagine small clumpsof iron filings breakinnoff from the main hody of iron filings and becorn& interspersed in a sea of sand (pig. 3), much the way micellar globules of oil float in water when a mixture of common cooking oil and water is shaken. Then ask the students to examine the mixture clos~ly.At this microsconic level it becomes anoarent that the sand and magneticiron filings do not inteisperse uniformly due to the nreferential attraction of iron filines for other iron filines. and the relative lack of attraction of the iron filings for & nonpolar sand particles. IA summar;, the sand/magnetic iron filing analogy, through its use of small particles, improves upon the marble/ magn& analogy because it is effective not only at the rnicn,. scopic level, but also at the macroscopic, experiential level. his greatly enhances acceptance of the analogy hy high school students, many of whom are just beginning to venture into the world of molecules. The molecular level lies far beyond experience, and for some i t is therefore difficult to erasn. Since the sand/maenetic iron filine analoev is effech e i t both the experientsl and molecular level,% can help bridge these two planes, thereby easing the transition to thinking molecularly.
