In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz
Kutztown University Kutztown, PA 19530
Paramagnetism Paradoxes: Projectable Demonstrations submitted by:
Frederick C. Sauls* Department of Chemistry and Physics, King’s College, Wilkes-Barre, PA 18711; *
[email protected] Ed Vitz Department of Chemistry, Kutztown University, Kutztown, PA 19530
checked by:
Charles Malerich Department of Natural Sciences and Chemistry, Baruch College, New York, NY 10010
Students’ understanding of paramagnetism and diamagnetism may be strengthened if they are challenged to interpret a series of demonstrations that may seem paradoxical. These demonstrations are done with a minimum of material on an overhead projector with a convenient procedure that encourages comparisons and heightens contrasts. Recent demonstrations of paramagnetism (1, 2) make use of several transition-metal compounds. The paramagnetism of oxygen has been demonstrated by condensing atmospheric oxygen and allowing the liquid to flow between the poles of a magnet (3–5), but this method is inconvenient for many teachers. A demonstration of the apparent repulsion of a candle flame by a magnet recently appeared (6). We propose some refinements to the simple and effective bubble demonstration (7–9) and its extension to new effects as a result of the action of a magnetic field on a solution of manganese(II). We also note that projectable, qualitative demonstrations of paramagnetism are surprisingly simple to do by dragging powdered transitionmetal compounds with a strong magnet. Demonstration 1: Paramagnetism Paradox Four Petri dishes (A–D) are placed on an overhead projector. Dish A contains a drop of saturated MnSO4 solution1 under a layer of mineral or vegetable oil.2 Dish B contains a drop of oil floating on a layer of saturated MnSO4(aq), dish C a drop of water in a layer of oil, and dish D a drop of oil in a layer of water. A transparency identifying the contents may be placed under the dishes to aid identification. A strong magnet3 brought near to the first two dishes will attract the drop in dish A and repel the drop in dish B. What might cause these effects? Observers may reasonably hypothesize that it is paramagnetism that leads to the attraction of the of Mn(II) solution, but it is less clear that the diamagnetism of oil leads to its apparent repulsion. It could also be that the oil bubble is apparently repelled because it is displaced as the Mn(II) solution is drawn toward the magnet. As both explanations predict the motion of Mn(II) toward the magnet and the oil away, we cannot tell which effect predominates for either droplet. The expected weakness of the diamagnetic effect (typically one-thousandth the strength of paramagnetism) suggests that diamagnetic repulsion is not a major contributor, but an empirical separation of the effects would be more convincing. The magnet has virtually no effect on the drops in dish C or dish D. This demonstrates that it must be the presence of the
paramagnetic solution that causes the effect. But it still might be argued that roughly equal diamagnetic repulsions of the oil and water drops would lead to no net repulsion in either C or D. A more careful examination in D shows that the oil drop rises above the level of the water solution. The oil displaces ~90% water and ~10% air. Because paramagnetic O2 in air is attracted to a magnet we cannot exclude the possibility of an apparent repulsion of the oil drop. Simply stating that this effect is negligible is unsatisfactory and observing the effect of the magnet on gas bubbles may provide additional clues. Demonstration 2: Bubble Paradox A Petri dish containing detergent solution4 is placed on an overhead projector and a bubble of oxygen, nitrogen, or methane is created in the solution with a syringe or pipet.5 The oxygen bubble is attracted when a magnet is brought near, the air bubble is unaffected, and the bubble of nitrogen or methane is repelled, displaced by the air (20% O2). As with the first demonstration, the repulsion may hypothetically be due to paramagnetic attraction of O2 in air displacing the bubble or diamagnetic repulsion of N2 or methane. As with the liquids the effects cannot be distinguished because in each case we observe the difference between the magnet’s effects on the bubble and the air. A paradoxical effect is observed if a magnet is brought near an air bubble on the surface of a saturated MnSO4 solution (with or without detergent). Counterintuitively, in light of the observations above, the bubble is repelled by the magnet, [naïvely, the Mn(II) in the walls of the bubble might be expected to cause some attraction, and the air inside displaces air, leading to no differential effect]. Further experimentation shows that the air bubble may approach the magnet or move away from it depending on how the magnet is held. If it is held vertically and close to the bubble, the bubble moves away from the magnet. If the magnet is held closer to the solution and farther from vertical, the bubble moves toward the magnet. The puzzling behavior of the bubble is explained by slight deformation of the surface of the Mn(II) solution created by the magnet. The bubble slides downhill, but the direction of “downhill” seems peculiar. These surface deformations may be examined by the demonstrations below.
• To show that a magnet can alter the shape of a surface, a flat-based (e.g., scintillation) vial filled with saturated MnSO4 solution may be placed on the projector. Two strong magnets brought adjacent to the surface on opposite sides will distort the circular
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 4 April 2008 • Journal of Chemical Education
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In the Classroom magnet
image (the focus may require adjustment to give the best effect) because the meniscus will rise at the approach of the magnet. A vial filled with water shows no such effect.
• The surface deformation may be directly observed by viewing the reflection of fluorescent lights (especially those with rectangular grids) as the magnet is brought close to the surface of the Mn(II) solution. Depending on the orientation of the magnet, the distortion may be a simple symmetrical depression or shaped like an annulus when the magnet is below the dish and like a symmetrical hill or raised ring when above. If the magnet is held at a slant, the surface deformation may be parabolic. This is because the magnetic lines of force (viewable with ferrofluid6 or iron filings) are concentrated at the circular edges of the face, and the paramagnetic solution is drawn to the area of greatest magnetic flux. If a pointed steel pole cap is placed on the magnet to concentrate the lines of force, the solution surface takes a nearly Gaussian shape. The surface deformation may also be viewed by placing a few drops of the Mn(II) solution on an opaque black surface and holding the magnet above it, but this gives less information on the detailed shape of the changes.
This works best when done by students individually. It requires some practice before the observers see the distortion, incandescent lights or fluorescent fixtures with diffusers make the distortion more difficult to see, and we have found no way to show it to a large audience.
• A laser pointer (or other suitable laser) is mounted so that the beam reflected from the solution surface hits a wall or screen (Figure 1). Moving a magnet held perpendicular to the liquid surface above (or below, but the solution depth must be small) the point where the laser hits the solution changes the angle of both the reflected and refracted beams in response to the changing shape of the surface. The point where the reflected beam hits the far wall moves, as does that where the refracted beam hits the dish. The motion is readily interpreted in terms of the shape of the surface. It is also possible to fix the magnet above the solution or below the Petri dish and move the laser held in a clamp on a ring stand so the beam “scans” the surface. Replacing the solution with deionized water abolishes the movement completely; thus there is no surface change.
To summarize, these experiments show that the attraction due to the paramagnetic Mn(II) causes effects large enough to obscure effects due to diamagnetism. Thus the phenomena can be best explained by paramagnetic attraction and surface deformation of the solution, rather than diamagnetic repulsion. Demonstration 3: Paramagnetic Powders The projection technique allows a quick demonstration of the paramagnetism of various transition-metal compounds. If a few crystals of the compound are placed in a Petri dish on an overhead projector stage, they are attracted to a strong magnet. A document camera may be used to show this effect or to show movement of the crystals on a sheet of paper when a magnet is moved below it. The attraction is observable with crystals of MnSO 4∙H 2O, MnO 2, Fe 2(SO 4) 3∙9H 2O, FeSO 4∙7H 2O, CoCl2∙6H2O, and to a lesser extent CuSO4∙5H2O, CuCl2∙2H2O, NiCl2∙6H2O, and NiSO4∙7H2O.7
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laser
Petri dish
MnSO4(aq)
Figure 1. Solution surface deformation demonstration.
Conclusion These demonstrations are ideally suited as attention grabbers for introducing magnetochemistry. The relative strengths of the paramagnetic and diamagnetic effects are demonstrated in thought-provoking ways. Careful observation of these effects can provide a foundation for understanding aspects of quantitative magnetic measurements, for example, the necessity of a diamagnetic correction in measurements of paramagnetism. Notes 1. Saturated (approximately 3 M) MnSO4 solution may be prepared by adding 60 g MnSO4∙H2O to ~120 mL of deionized water and stirring 20 minutes with gentle warming (30–40 °C). 2. The drop of saturated MnSO4 solution should be added after the oil, and the Petri dish should not be too clean; otherwise the solution will wet a clean glass surface and become “stuck”, thus unable to move. 3. Use a rare earth magnet. A suitable ½ in. × 2 in. rod magnet is available from National Imports (San Jose, CA), catalog #NSN0553. The more powerful field will improve the demonstration. A rare earth disk magnet held with plastic forceps works. 4. Commercial dish detergent (e.g., Palmolive) works well. Large, long-lived bubbles will form from a solution made by adding ~25% glycerin to the detergent and making a 5% solution of the resulting mixture in distilled water (10). Larger bubbles yield greater sensitivity in demonstrations of magnetic effects. 5. Gases from tanks can be carried to the demonstration in Mylar party balloons. They may be removed from the balloons by using a needle and syringe. Transparent tape will close the hole. Alternatively, the balloon can be stretched over a one-hole rubber stopper that is pushed onto the glass tubing of a #2 or #4a stopcock. A rubber hose connected to the opposite end of the stopcock can be connected to the source to fill the balloon, then connected to a pipet for blowing bubbles. If tanks of gases are not available, the gases may be prepared easily in syringes (11). 6. Ferrotec Home Page. http://www.ferrotec.com/products/ ferrofluid/?gclid=CN2_uM__v4sCFSYHQQod81vRag (accessed Dec 2007). 7. Similar demonstrations are presented in Chemistry Comes Alive, Volume 2 (12), but we use a magnet available to everyone, and simplify the method considerably.
Journal of Chemical Education • Vol. 85 No. 4 April 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Classroom
Literature Cited 1. Malerich, C. J.; Ruff, P. K. J. Chem. Educ. 2004, 81, 1155. 2. Cortel, A. J. Chem. Educ. 1998, 75, 61. 3. Saban, G. H.; Moran, T. F. J. Chem. Educ. 1973, 50, 217. 4. Shakhashiri, B. Z.; Dirreen, G. E.; Williams, L. G.; Smith, S. R. J. Chem. Educ. 1980, 57, 373. 5. Lethbridge, J. W.; Davies, M. B. J. Chem. Educ. 1973, 50, 656. 6. Taikawa, Y. http://www.exploratorium.edu/explore/trythis_flame. html (accessed Dec 2007). 7. Tsunetaka, S. Yukagaku 1985, 34, 251. 8. Shimada, H.; Yasuoka, T.; Mitsuzawa, S. J. Chem. Educ. 1990, 67, 63. 9. Matsuyama, Y.; Yasuoka, T.; Mitsuzawa, S.; Sasaki, T. J. Chem. Educ. 1997, 74, 943. 10. (a) Exploratorium. http://www.exploratorium.edu/ronh/bubbles/ formulae.html (accessed Dec 2007). (b) Boys, C. V. Soap Bubbles:
Their Colours and the Forces Which Mould Them; Dover Publications, Inc.: New York, 1959; p 170. (c) Katz, D. Chemistry at the Toystore. http://www.chymist.com/Toystore%20part1.pdf (accessed Dec 2007). 11. Mattson, B.; Microscale Gas Chemistry. http://mattson.creighton. edu/Microscale_Gas_Chemistry.html (accessed Dec 2007). 12. Jacobsen, Jerrold J.; Moore, John W. Chemistry Comes Alive! Volumes 1 and 2. J. Chem. Educ. 2000, 77, 671; also available at JCE Web Software, http://www.jce.divched.org/JCESoft/ jcesoftsubscriber.html (accessed Dec 2007).
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