solvated ions. As indicated by Figure 2, the relationship is not exact; a decent fit is found using a second-order polynomial, but over-fitting of the data is perhaps something to be avoided for such a simple exercise. But the results of the four combinations of salts used here are making a statement: the less massive ion travels a farther distance than the more massive ion, and a better quantitative understanding of this phenomenon is perceived when a solvation sphere is considered. This is equivalent to the results of the gas-phase diffusion experiment mentioned above, and is sufficient to serve a s a laboratory experiment in a general chemistm lab setting. The idea ofhiffusion has been studied theoretically; however, i t seems difficult to apply in this liquid-phase case. There are several generalizations of gaseous behavior described bv Fick's laws (3):they include, for concentration gradients:
where J;. is the flux of comoonent i in the r direction. the concentration gradient is &;i 62,and D is a proportionality constant called the diffusion coefficient. For diffusion in a n ideal solution, the force causing the diffusion is balanced by a frictional force proportional to the viscosity of the liquid; the diffusion coefficient can be derived as (3, 4)
where n is the viscositv of the liauid and r i s the radius of the sphkrical particle;ihe other iariables have their classic definitions. This expression of the diffusion coefficient does not, however, depend on the mass of the ion, which is one of the easiest variables to define for experimental systems like these. It is also the variable of interest for gasphase diffusion. Simple diffusion theory does not address specifically why heavier ions diffuse faster than less massive ions, although i t is thought that the qualitative relationship should be obvious. Conclusion We have developed a laboratory experiment to measure the diffusion of ions in the liquid phase and to show that the relative distances of diffusion are related qualitatively to the inverse of the mass of the solvated ion. For the instructional lab, this experiment promises to be less noxious than the gas-phase experiment, although less theoretically obvious. However, the reproducibility of the data and the conclusions one can derive from the data show that the point that 'heavier moves slower'can be predicted and proven using the liquid phase.
The "Magic" Flask Rubin r at ti no' and John J. Fortman Wright State University Dayton, OH 45435 Pirketta scharlin2 University of Turku SF 20500 Turku, Finland Often demonstrations of even elementary concepts, such as the color changes of various indicators or using the scientific method to predict the sequence of color changes in a series of beakers (the spectral order of colors), can catch the attention of more "sophisticated" students by using an unexoected "trick" or a n element of "maeic." Because effective lecturing involves a n element of theater, why not make use of s i m ~ l tricks e to intrigue a class? Of course. our goal i s not to mystify and we7'explain" how the trick worked either a t the time of use, or a t a later time, if you wish, to give students the opportunity to figure the trick out on their own. The "magic" flask demonstrations described in this paper are a lot of fun to do, and student response is quite gratifying. There are several kinds of "magic" vessels that can be purchased from magic suppliers that appear to be empty on first pour, but can be used to pour repeatedly liquids while going "empty" each time. The commercially available one we have is made from aluminum and cost $45 over 10 years ago. I t has approximately a 2-L capacity, but the design is such that i t is difficult to use and is not a good container for the dilute solution of NaOH we normally use. In this paper we will describe two versions that work well and some demonstrations that may be performed with them. Our elegant version was fabricated for us by a skilled glassblower out of a 2-L round-bottomed flask. The estimated cost would range from $50 to $100 depending on your glass-blower. The dimensions given in Figure 1 are not critical, but the data presented in the table refer to these dimensions. The inner tube is 43 mm i.d. and 25 cm long. The annular space is about 3 mm and the clearance between the inner tube and the flat bottom of the flask is 5-8 mm. Although the drawing shows a 6 x 12 mm slot at 'Author to whom correspondence should be addressed 'Visiting Associate Professor.
Acknowledgment Sunuort for this ~ r o i e c from t the State of Ohio Board of ~eg&t.s is acknowiedged. We thank the reviewers for some excellent suggestions in revisine . ... ..the manuscriot. This rrrojecr is tin rxr(nsion i,f the Honor; (;ener:~l ('hemistry Prom:im ;it Cl~vclnndStatr I:nivrrsitv. ;~drnmistcredb y John R. Luoma. Additional support from David G. ~ e h e m a n n and Robert L. R. Towns is appreciated.
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Literature Cited 1. Alycn. H. N.: uutton. E E. Tested Dmo~trirnlionrin Chrmr.%
6th od. Journal of Chemical Education Press: Earton. PA, 1965. 2. Ref. 1 , p202There isapparentlyamicmrealeTOPSexp~~im~nt~AIy~~,Micro-Chcmistry P~ertcd-TOPS#I511 us in^ s Petri dish to d~monrfratediffusion. but wa have bcen unable to locab, the rrference for s~leciFicr.We thank Lhe reviewers for btinfing thlr to our attention. 3. Atkins, P.i'hysiml Ci>miist!:v;W. H. Freeman and Company: Snn Francisco. CA. Jobn Wiley and Sons: New 1990:Alberry. R. A : Silbey. R. W. Ph.ysIcnl Chem~r#ly: Ydk. NY 1992. 4. Laoffer M. A. J. Cheln. Edrrc. 1981. .58, 250.
Figure 1. The all-glass magic flask. Dimensions in millimeters
246
Journal of Chemical Education
Average Volumea in Milliliters Delivered for Five Trials Pouring
Glass
Plastic I
Plastic I I
1 2 3 4 5 6
300 180 150 122 108 104
7
90
8 9 10
78 70 64
292 198 144 118 100 92 80 70 62 54
234 200 162 I36 122 98 90 76 64 62
'Average deviation for all pourings about f5 mL.
your thumb when pouring. Volumes delivered for two different plastic bottles are given in the table for successive pourings. The plastic cup lid needs to be cut down so i t can be screwed hack on the bottle due to the caulk filling. Occasionally, you may find t h a t the spout when re-seated does not give a tight seal. This can be remedied by using a bead of caulk on re-seating. This inexpensive modified plastic liquid detergent bottle works very well as a "magic" flask. We typically use the magic flask for our "Rainbow Board" demonstration. We use an illuminated board that is easier to use and to make and more adaptable than that described by Olesen and Leaton ( 1 ) .Our light board is made from a four-foot standard two-tube fluorescent fixture that has a flat translucent plastic covering. Depending on the make. vou mav wish to add stabilizine feet and an in-line switch." Our s.Erl~#r. 1984.61, 172.
31nstruc1ions are from a handout by G. L. Gilbert. Volume 72 Number 3 March 1995
247
