edited by TOMTIPTON University of Nebraska Lincoln. NE 68508
Try This Solution C. W. McLaughiln Science Department Chairman Central High Schwl St. Joseph. MO 64501
Nearly every secondary level chemistry course includes a unit dealing with solution chemistry. When I teach this unit t o introductory chemistry classes, I always include such standard concepts a s chemical steps i n t h e formation of a solution, heat of shution, co~~i~ativebroperties, etc. I also want my students t o realize t h a t the solution process itself is dynamic. This concept is difficult for t h e students t o visualize. They are only used to reading about other dynamicprocesses. There is also the familiar persistent argument that atoms are just so small t h a t students cannot actually visualize them doine anvthine. dvnamic or otherwise. ~ c a;tivi&s e "presented here involve students in the learnine nrocess bv reouirine them t o b e art of the ~ h v s i c a l of the"dekonstr&tions. hey also lowe; t i e abstraction level of t h e concept. Materials Required Bucket of old tennis balls (Contact the tennis coach.) Prepared definitions of "unsaturated," "saturated," "dynamic equilibrium," and "supersaturated" (A minimum of ten students are needed for these activities.) Actlvily One: Unsaturated Soiution (1) Have several students sit an the floor in the center of the room around a bucket of tennis balls. The remainder of the students stand in a circle around the sitting students. Each student represents a solvent particle, and each tennis ball represents a particle of a crystalline solute placed in the solvent. (See Figure 1.) (2) At the sound of "Go" (or "dissolve") from the instructor, the inner solvent people begin gently tossing the tennis balls up and over their beads. Eacbof the outer solvent people may catch atennis ball-solute, hut he must toss it back if another "solute" comes his way to catch. Tennis balls an the floor should be picked up and tossed in the air again by the seated students. This random action is allowed to continue for about 60 seconds or until almost all the tennrx Idl-solute particles have been "diuiolvrd," t i r . . held in smnwne's hand). At the sound oi"Stop," acuunt is made of all the tennis bdh in students' handsand uf thore un the flwr.'l'he~e numbrrs arr rrcorded, and then the arri\,iry is repeated. ( 3 1 The numher of tenn~shnlis "dirsolved"eould artunlly I,? higher if eachmrdent "red h t h hands. This i i when I p m t ,rut the term "unsaturated" ISPEClAI. NOTE: F19r t h w sectiun. make sure more hands than tennis halls are present.] Students are asked to make an analogy between why they did not always hold onto the same ball and how real solvent particles are in motion. The exchange of solute particles with other solvent partides is also discussed. Activity Two: Saturated Solution and Dynamic Equilibrium (1) Add more tennis balls to the bucket and instruct the solvent people that they now hold two tennis balls hut, as before, must toss one back when a third ball approaches. (2) Remind the inner solvent group to toss the tennis halls, not to throw them, as the handsof those in the outer solvent group will soon be occupied. Tennis balls ending up an the floor (precipitating) are to be picked up and tossed again. This should continue for about 60 seconds. Then a count is recorded of tennis balls in udvenr hnndsnnd thowon the flgnr. Afwr repeating thisnctivity, the ~Iudrntsllegin to see that a maxrmum numher of dissolved tennis ball-solutes is approached each time.
404
Journal of Chemical Education
S1' solute"
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Figure 1. Unsaturated solution.
lnstr~ctorplaces more tennis balls here ,stc.
Each student holds two tennis batla while extending arms toward center
Figure 2. Supersatvrated solution.
(3) By comparing the number of tennis balls dissolved in this activity
to the number dissolved in the "Unsaturated Activity," students easily see that saturated means more solute dissolves in the saturated solution than in the unsaturated solution and that a certain maximum is reached when no more solute will dissolve in the solution. If more solute is added above this maximum. it will precipitate. When the" I h k back on thr prows* of tennrs halls flying in huth dirrctirmi tto and from thc ~ I I C ~ P they I , , haw a firsthand erpentn~sufequilil,riumandthemovement that muit be happening at the molecular level when a solution is forming. Activity Three: Supersaturated Soiution (1) This concept can be difficult for many first-year chemistry students to understand. After a brief discussion of a textbook's definition of the term, each student is given two tennis balls. (They are now "saturated.") The inner circle solvent people are still seated, but the outer circle people are instructedtomove in very close and allow their outstretched hands, still holding the tennis balls. to touch. (See Fieure 2.) This formation can be a little uncomfortable. (2) The instructor now begins to place the remaining tennis balls in the places where people's hands are touching. These tennis halls would be ones that would end up on the floor during the "saturated" demonstration, and they would be on the floor again except that this is a special solvent-solute interaction. To ilustrate that this special interaction may not he very stable, the instructor selects one student, places one more tennis ball in the structure that has formed, and at the same time nudges that student's arm so that a sudden "orecioitation" occurs. . . (3) Armnparirm i,f thatutal numbrr of tennis halls supported here to the numtwr dirwlved i n the "catumt~d"nrtiviry should demonstrate that what was uncc conqiderrd t h mnximum ~ mlncentration could be exceeded if special considerations are used.
I have been using these activities for several years and have been pleased with the way students are able to remember the concepts. I have also found t h a t many students like the way these activities make the atomic world less of a n abstract concept.
