Boiling and freezing simultaneously-with a feeble vacuum pump!

panded polystyrene cup, positioning this under a bell jar and evacuating the bell jar with a vacuum pump. As the pressure is reduced, the water will b...
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tested demonstrations Boiling and Freezing Simultaneouslywith a Feeble Vacuum Pump! submined by Mike Ellison Evergreen High School 14300 NE 18th Street Vancouver, WA 98684 James W. Long Unwemty of Oregon E~geneOR 97403 Hewitt (1) suggests demonstrating boiling and freezing a t the same time by placing a gram or two of water on a piece of plastic wrap draped over the open end of an expanded polystyrene cup, positioning this under a bell jar and evacuating the bell jar with a vacuum pump. As the pressure is reduced, the water will begin to boil, which reduces its temperature, because boiling is an endothermic process. If, a low enough pressure is achieved, the water will reach its freezing point and ice crystals will be formed concurrently with bubbles. I had heard of this, but when several students and I tried it with our old vacuum pump aRer school, we were unsuccessful. We hypothesized that the vacuum DumD was not lowering the Dressure enough. Previously, 1bad ihown the students how the temperatire of acetone could be lowered to around -20 'C by boiling it under reduced pressure with the same vacuum pump. a student brightly suggested mixing acetone with water and investigating the behavior of the solution. This yielded very interesting results, dramatically demonstrating the endothermic nature of the boiline ormess. We found ourselves in complete agreement wiih' Hewitt who remarks: "This must be witnessed to be a ~ ~ r e c i a t eFrozen d. bubbles of boiling water are a remarka& sightn(l). The Demonstration Materials 1expanded polystyrene cup or 250-mL beaker 1piece of black plastic, about 8-cm square 3 4 mL 60%acetone (vlv) in water 2 or 3 boiling chips Vacuum pump and bell jar Video camera and large screen monitor (optional)

Procedure Set the cup upright on the pump's plate and position the plastic sheet over the mouth of the cup. Place 3-4 mL of acetone-water mixture in the center of the plastic sheet. Add the boiling chips to the liquid. Position the bell jar over the cup and evacuate the bell jar. Our Observations

The mixture begins boiling innocently and then rather suddenly appears to freeze solid. However, bubbles continue to form intermittently, burst through the ice and betray the presence of liquid under the ice. This continues for a few moments, and then the ice thaws. The liquid sits qu-

GEORGE L. GILBERT Denison University Ganville. OH 43023

ietly for a moment, then bumps. At the moment the bubble forms, ice forms on the surface of the liquid. The ice then melts slowly, and the next time the liquid bumps the process repeats itself. This continues for about 5 min or longer. Hazards Acetone is flammable. Keep it away f r o m o p e n flames. Disposal Acetone can be evaporated in a fume hood. Discussion Phase changes can be used effectively to teach many basic concepts. Students are familiar with them and do not feel that thev are laboratow ~henomenathev will never see again. are easily &rested in deepening their understanding when thev realize how limited it really is. So, after theyGe seen boiiing by cooling (21,they are ready for another surprise: boiling is a cooling process. The endothermic nature of theboiling process is dimcult for students to believe. This demonstration effectively connects boilingwith cooling. This naturally leads to the question of how boiling can cause cooling. The explanation, of course, requires the concept of potential energy. So this demonstration helps students see the necessity for the extremely abstract idea of potential energy. Why does an acetone-water solution freeze under the same vacuum that would not freeze pure water? Because the acetone-water mixture has a higher vapor pressure than pure water, boiling will occur at a higher pressure and the rate of evaporation will be higher. This leads to more effective cooling. Also, the higher vapor pressure of the solution means more liquid will evaporate before the DumD is unsuccessful in continuing to cause boiling. This k u i t s in more cooling. The initial'free~in~ of muchof the liquid probably results from the selective crystallimtion of water.-The boiiing liquid (mostly acetone) left under the ice produces the spectacular explosions in the ice. As boiling continues, the mole fraction of water in the solution increases, causing the boiling rate to decrease. The temperature of the remaining liquid elevates until the ice thaws. However, bumping continues, and each time a bubble forms, the liquid is immediately cooled enough to cause visible crystals of ice to form on the surface of the liquid. This continues for at least 5 min and provides the clearest link between boiling and cooling. Even with the water-acetone mixture, our old vacuum pump will not cause freezing unless I get a good seal between the bell jar and the pump plate. I find that a black rubber pad made to place between the bell jar and the pump plate does not work as well as spreading a generous helping of vacuum grease on the bottom of the bell jar in v lace of the rubber r ad. I have sometimeshesitated to show this kind of demonstration to a large class, because they find it difficult to see. I have found that using a video camera connected to a large TV or monitor works satisfactorilv for this and manv other demonstrations. A camera with a macrolens setting is helpful.

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Volume 69 Number 4 April 1992

325

I present this demonstration to reinforce the concepts that I've introduced about energy effects in phase changes. It could be presented as a mystery for solution by individuals, smell groups, or the class. Acknowledgment The chemistry student who made the suggestion to add acetone to water was Rick Rudherg, class. of 1990, Evergreen High School. Also, the idea to use black plastic to make the ice more visible on the video screen came from my principal, Nancy Bush-Lange, who happened to visit during my first presentation of this experiment. Literature Cited 1.HeMn. P G. Concoptun1Physlra. 5th Ed.;Little. Bmwn & Ca.. 1985,p 253. 2. Summerlin, L R.;Borgtord, C. L.;Ealy. J. L.Ckmirn1Dpmonstmtione:ASourcebod for?hockrs, Vol 2. American Chemical Satiety, 1985, p 24.

The Howling Gummy Bear Dan M. Sullivan University of Nebraska Omaha, NE 68182

The concept of energy content of foods is one of the most difficult topics to explain to biochemistry students. Certainly, they realize that eating foods high in lipid or carbohydrate content contributes to expanded waistlines, but it's difficult to illustrate the fact that we really consume hieh-enerm foods in order to obtain the reduced carbon t G y cont&. I illustrate the caloric content of foods in two wavs. After we have studied glycolysis and the Krebs cycle, fignite a Snanish peanut on a stand made of a bent Daoer clip. The clip may be bent so that the large en2 firms atrianeular base and the small end forms a rouehlv circular rine &st smaller than the peanut. The peanut&;ally burns f; several minutes. releasing a ereat deal of heat. (The caloric content of is rel&;ily high, about 6.4 kcaVg (27 J / ~orI841 kcal per cue of Seanish-stvle peanuts, (11.The eRergy content i f slightly mire than four cups of peanuts is equivalent to that of one pound of body fat. After witnessing a drama&c demonstr&ion of the oxidation of wood splints by molten potassium chlorate at the 10th BCCE (21, I devised a similar experiment using candy gummy bears as the source of reduced carbon. I promise to reward the class for future work by making a gummy bear "howl" for them. At the appropriate time, I pass out samples of gummy hear candy. We identify and discuss the ingredients listed on the wrapper and again emphasize the fact that the reduced carbon is the source of energy. In a well-ventilated area behind a safety shield and over a fue-resistant surface, I set up a ring stand equipped with a heat-resistant test tube holder. A large (25- x 200-mm) Pyrex test tube containing approximately 15 g of KC103 is placed in the holder and heated until the KC103 melts. I ~ I U Da eummv bear into the molten KC10. and stand bv with a Ere ex&guisher for dramatic effect: The candy nites and burns furiouslv with a hieh-oitched roar. ~ m v o k ing exclamations of amazement from h e class andgasps of excitement from passersby. Caloric content of gummy bear-type candy differs with suppliers, hut a n average value is probably 95 kcdoz or 3.4 kcdg.' I have found that peanuts bum more slowly than gummy bears in molten potassium chlorate, but peanuts bum more rapidly than gummy bears in air. Gummy

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'Candy companies are relunant to pub icize the calor~ccontent of prod~cts.b ~ I rwi I supply sources of intormat~onupon reqLes1: most suppliers reported va Les Detween I00 kcal oz and 89.6 kcal oz. 326

Journal of Chemical Education

bears are difficult to ignite but burn vigorously once ignited; I suspect that the gummy bears are melted by the high temperature of the potassium chlorate and, therefore, present the possibility of a reaction occurring in a liquid phase rather than the liquid-solid phase observed for the buming peanut. Safety I've never had a fire or exolosion connected with this demonstration, but a fire e k i n g u i s h e r must h e available a n d t h e demonstration should h e conducted over a fire-resistant surface (small pieces of molten KC103 sometimes fly over the edge of the tube) and t h e demonstration must h e performed behind a safety shield (preferably within a hood) a n d i n a well-ventilated area. This demonstration produces quantities of smoke that I find innocuous but which might be irritating to those with sensitive lungs. The reaction can be videotaped for display in areas not equipped for demonstrations, but it is much more dramatic if seen in person. Do n o t a t t e m p t t h i s r e a c t i o n w i t h g r a n u l a t e d sugar; t h e high surface a r e a of t h e sugar particles might result in an explosion. A mixture of granulated sugar and potassium chlorate in a small cylindrical tube was once used for the purpose of splitting logs. Materials 25- x 200-mm heat-resistant test tube 15 g reagent grade KC10,

One gummy bear Safety shield and fire-resistantsurface Fire extinguisher Literature Cited I Chn.t#an..l .C.rryrr..l N u a . h n f o r l . r <