Roger C. Millikan University of California Santa Barbara. California 93106
A Discovery Experiment: C02 Soap Bubble Dynamics
Students often make a choice of maior based unon the view of a sul~jectthut t h w derive f n m thl, I q i n n i n g wurst,. (:t,nsnruentlv. m e u,ould like cod0 more than iust teach the basics in ; firstryear college chemistry course. o d e would like to help the students glimpse the pleasure that comes from understanding nature in fine detail, from applying that understanding to real problems, and from making unexpected discoveries that yield to analysis. This is very difficult, given the constraints of time and the limited background knowledge of first-year chemistry students. Rare indeed are suitable laboratory experiments which provide this excitement, this chance for discovery, and yet can be understood using basic chemical principles-all in a 3-hr lab period. Described below is one such exneriment which seems to provide the desired response. It excites some students, amuses others, and is less successful for a few. Since the experiment is largely qualitative, we depend upon other experiments in the sequence for the study of the quantitative aspects of equilibrium. We value this one as a change of diet, and for its positive motivational effect. Floating Bubbles Lab Instructor "Today weare going todo a different kind of experiment, a sort of open-ended one. The object is to have some fun, to make same observations, and maybe to tie them into the chemistry "
about.) "Bubbles filled with air, being very light, ought to float on a layer of heavier gas like a boat floats on water. I've often wondered if one could make them do that. Take CO?far example. It has a molecular weight of 44 compared to an average value of 29 far air. Thus Conis 44/29 = 1.5 times more dense than air. Could we float air bubbles an a layer of COz gas? Dry ice is frozen CO?, so this is something that would be easy to try. We could put a layer of crushed dry ice in the bottom of a large beaker or plastic container, wait a bit for the subliming COzto about half fill the container with cold gas, and then drop in a small air bubble. What do you think would happen? Would the bubble float at the invisible interface between the cold Congas layer below and the warmer air above? Or would the diffusion of each gas into the other mix them up so much that there is no such interface? We're going to find out some of the answers by trying it." "First some cautionary details. Dry ice can give you frost bite if handled with the hands, so use the scoop to put a layer of Dry Ice in the bottom of your beaker. Let your beaker sit quietly and undisturbed by air currents for about five to ten minutes while thedry ice sublimes. Youmaywant to cover the top about three quarters of the way with a sheet of paper to further keep air currents away. Also, the plastic wand that comes with the bubblestuffmakesbubbles that are too big for the size beaker you will be using. Isuggest you usea small pipet tip or medicine dropper far blowing small bubbles." "Make good notes in your lab h k on the different phenomena you observe. Not all the bubbles will behave the same wav. Somemav be more interesting than others. We will gather together a group again in about half an hour to discuss what you have observed, what it may mean, and what further experiments we should do to help answer the questions that are sure to arise from doing the first experiments. Go to it!" Initial Student Experiments The students setting up the experiment a t their lab benches meet with various frustrations. Some don't wait long enough for the CO2 gas layer to f o m . Many have the bubbles they blow miss falling into the beaker, or else they strike the walls.
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Some bubbles. not fullv blown. have a nendant dron of soan solution on tht. underiide. '1'h;s makes them so heavy that t h u sink ranidls . . to the bottom of the beaker. Yet. with repeated trials, success comes. The students eventually get a small. well-blown bubble that floats in the middle of the beaker for about ten seconds, sometimes bouncing up and down a little hit. This result often evokes a delighted crv of triumph. I t is a t this point that careful observ&ion hy'the student is rewired, for a number of different phenomena occur in the tt.n-st:cond lifetime nf the huhhle. or run ate^^, it iseasy tufloat new buhhlesrocheck on the reproducibili~y 18fwh;itu,as .;em Iwfor?. Typically, here is u,hnt thesrurlents see ~~
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The small (1cm diam) air bubble, shaken off the pipet tip from which it was blown, sinks slowly (5 cmls) into the beaker (1or 2 lval) containing a thin layer of Dry ice on its bottom. Psrt way down, the bubble stops, bounces up a bit, and floats as if invisibly suspended in the middle of the beaker. After about a second. the bubble beeins " to grow at an easily visible rate. It may almost double in size in 5 s. Subsequent behavior varies. The bubble may pop. If not, it slowly begins to sink. The soap film becomes cloudy as the bubble sinks toward the Dry Ice. Often an inward wrinkle appears in the enlarged bubble at about the same time as the bubble film freezes. Some bubbles remain intact as delicate frozen spheres sitting on the Dry Ice. Many fracture into fragments of spheres.
again with the lab instructor. Sooner or later, a student will ask the key question, "Why do the bubbles grow?" When this question comes, the instructor seeks to convince the group that this really is a very surprising observation. Although every student saw the floating bubbles growing, many thought nothing of it. The instructor produces another train of soap bubbles in the air by waving his wand. This time he points out that the bubbles do not crow. but seem to stav the same size until they pop. Quite acontrast to the bubbles floating on COz. Next the instructor reminds the students of the gas law of Charles and Guy-Lussac. Cooling gases contract. As the air bubbles sink into the cooler COegas layer, they will be cooled. Thus it seems that the bubbles ought to shrink. In looking for another explanation, the instructor mentions Graham's law of diffusion. It too gives the wrong answer. The air inside the bubble should diffuse out faster than the heavier COz diffuses in, if only molecular weights were important. Now it becomes clear to the students that their chance observation of bubbles that grow is a surprising one indeed. The instructor may wish to mention that such seeming paradoxes often point the way to the discoverv of unexnected Drocesses in nature. As the discussion s w i k , many suggestions will be offered as to what is going on. The more promising ones should he followed up, with the instructor guiding the discussion toward the question, "What additional experiments can we do now to give further information, or to test a proposed explanation?" As these are agreed upon, groups of students should go off to try them, with the agreement that in another 45 min,. everyone will once again gather to discuss what has been ob-
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Further Experiments Given the limited equipment and time available to the students, here are some of the further questions and experiVohme 55, Number 12, December 1978 / 807
ments they may be encouraged to try. (1) The inverse experiment. Blow C02 bubbles in air. Do they shrink? This can he done by sucking a pipet full of CO:! vapor from one of the beakers containing Dry Ice. Then dip the pipet tip in bubble stuff. Blow a bubble. These bubbles are so heavy, they sink to the bench top, usually forming a hemispherical bubble that rapidly shrinks to nothing. Repeating with air gives bubbles whose sizes do not change. The conclusion is that C 0 2 permeates the soap film ver; rapidly, while a mixture of 0 2 + NZ does not. (2) Other gases. Is this effect specific to COz? The students may blow bubbles using natural gas from the taps, or other gases available from cylinders. Results will depend on the eas chosen. Exceotine . ~.HIS. - . most eive constant size huhhles, indicating vrrv slow permeation rates. The efierr is uuiti, s ~ e r i f i cto CO, for smi) I~ubhlesas studied hers. ..'it change the soap soiution. his seemingly reasonable s"gl aestion leads to amhieuous results. While the reaction rates involved can be changed substantially by raising the pH, or by adding certain ionic catalysts, there is a complicating effect. Dilution of the soap solution leads to much thicker, heavier, and less stable bubhles. Any increase in the COz dissolution rate is offset by an increase in the diffusion path length through the thicker bubble. This makes the rate of bubble growth a poor indicator for changes in reaction rates. The difficulty can be turned into an advantage in the discussion bv ~ o i n t i n eout that in everv research oroiect . . there comes a t&e when-the previously used techniques no longer suffice. New methods or aooaratus must then be desiened in order to get the desired quantitative results. The students may be asked to devise and describe arrangements for more quantitative measurements as part of their write-ups. For example, a successful setup that has been frequently described in the literature replaces the bubble film with a stabilized aqueous film of constant thickness. I t takes the form of a flat piece of filter paper wet with the desired solution. C02 gas permeates through the filter-stahilized-film from a chamber of high partial pressure into a chamber that is constantly flushed free of C02. Measuring the time required for a given pressure drop in the COz-rich compartment gives excellent quantitative rates. What Is Going On?
If the students have already studied acid-base equilibria, and are familiar with the C O Z - H ~ C O ~ - H C O ~ - - Csystem, O~~they will be quick to suggest that this must have something todo with the enormous rate at which gaseous CO2 permeates aqueous soao films. What thev have discovered for themselves iia much sr;ldic.d hut not s&ly k n w n phenomenon called "t'acilitated trans~ort,"or scmetimes "carrier-mediated diffusion." This is ab active field of research with special relevance to physiology and to biochemistry. Many species are carried across cell membranes by a facilitated transport mechanism. These mechanisms are of chemical eneineerina ed ro separate mixintvreit too, tor they run IIP ~ ~ s ~ffvctivelg tures of certain gases. Schultz, rt al.' is a nirr rrrieu article covering many aspects of the wbject. The facilitated transport nl'('0~ through aqueous films has heen desrrihrd I q Enni,' and by Ward and Hobh.'The latter paprr is directly applicable tn the films used in these student experiments. One can conceive of the facilitated transport in terms of
808 1 Journal of Chemical Education
several steps. On the side of the film where the partial pressure of CO2 is high, we have
+
2
COz H 2 0 %H ~ C O ~ HC03-
+ HC
(1)
This reaction helps the COz to dissolve, and it sets up a concentration gradient for HC03- across the film. The bicarbonate ion consequently diffuses across, driven by this gradient. At the side of the film that is in contact with a very low partial pressure of CO2, the reaction reverses HC03-
+ H+ $-% H2C032 COz + HzO
(2)
This releases the CO2, destroys the bicarbonate ion on that side, and thereby maintains the concentration gradient as a driving force for more "facilitated transport." In parallel there is, of course, the diffusion of molecular C02, but this is very slow, just as it is for molecular 0 2 or N2 in these films. This is an oversimplified picture, no doubt, as there are many other orocesses eoine Nevertheless. this is the dominant mechanism; it is one the students c k understand using what thev have learned. and it exnlains their results. he specificity to COz arises from the fact that no similar reactions with water exist to produce carrier ions in the cases of 02, Nz, HZ,or CH4. For HzS, facilitated transport can and does occur. Hydration produces the HS- ion, which serves as the carrier. In fact, H2Sbubbles in air shrink even faster than CO:! bubhles. This shows that the rate determining step in the COz case is the slow hydration reaction of CO:!, and not the diffusion processes. The slow steps in reactions (1)and (2) can he catalyzed by many inorganic ions such as borate and sulfite. By far the best catalvst known is the enzvme carbonic anhvdrase. The mecianisms of catalysis arestill in d i ~ p u t esince . ~ the catalvtic effects are laree. thev are easilv studied. and mieht make good independentrkseaich for senior unr$ergraduates.
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Conclusion
This experiment has much to recommend it for use in freshman chemistry labs sometime after acid-base equilibria and diffusion have been covered in the lecture course. It uses safe. inexoensive chemicals and simole I t allows . anoaratus. -. a surpris&g discovery to he made from observations of familiar thinas. It emohasizes t o the students that thev must assess the meaning bf their observations carefully, leit they overlook important clues to possible discoveries. Lastlv, it dramatically introduces themto an important process oinature-that of "facilitated transport." Acknowledgment
The author thanks Dr. Peter Baine of Cal State University at Lone Beach for trvine this exoeriment in a second-quarter freshman chemistry"la6 session. His comments and ;uggestions were most helpful.
' Schultz,J. S., Goddard, J. D., and Suchdeo,S. R., AIChEJ., 20(3),
.- . - - .,.
A1 7 11 976) \ ,
Enns, T., Science, 155,44 (1967). W a r d 111, W. J., and Robb, W. L., Science, 156,1481 (1967).
'Kern, D. M., J. CHEM. EDUC., 37,14 (1960). 5Dennard, A. E., and Williams, R. J. P., J. Chem. Sac. (A), 812, (1966).
