edited by GEORGE L. GILBERT Denison University Granville, Ohio 43023
An Explosive Demonstration
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D. W. Rehfeld
Nonhern State College
Aberdeen, SO 57401
Mlchael Barondeau h Roscoe H l ~ School Roscoe. S O 57471 James W. Long UOlvW611Y 01 OWgOn Eugene. OR 97403
Students often have difficulty bridging the gap between the classroom and the "real" world. This demonstration of a dust explosion may help them because students in all parts of the country have heard news reports of grain elevator explosions. dust explosion or the hurning 07 fine particulate matter has direct application when cowring the topicof reaction rates. Fm example, when Kotz and Purcelll discuss reaction rates in their recent texthook, they include a picture of lvroiwdium dust hurnine. However. the demonstration of " . a dust explosion will be much more dramatic, and therefore the students mav retain the "chemistrv" for a loneer period of time. A dust explosion can be easiliand safely simulated in the classroom, and the needed materials are very inexpensive and readily available. For 15 years, one of the authors (MB) has used a dust explosion apparatus consisting of a coffee can, ruhher tubing, and a thistle tuhe. The modified apparatus described below eliminates the thistle tube. Elimination of the thistle tube is beneficial because each apparatus constructed with a thistle tube did not always work and not all teachers, particularly elementary and secondary teachers, have access to thistle tubes. The materials needed to construct the dust explosion apparatus are as follows: a 3-lb coffee can with plastic lid, a plastic medicine vial (2.1 cm diameter and 6 cm high), ruhher tubing (1 cm outside diameter and about 55 cm long), a candle (either votive or taper), and a frozen juice can (about 5.5 cm diameter) made of cardboard. All of these materials can be purchased a t either a grocery store or a hardware store. To construct the dust explosion apparatus, first make a 1.0- to 1.5-cm-diameter hole near the bottom of the coffee can. An electric drill or a hammer and screwdriver may he used. The hole should he laree enoueh so that the rubber tubing canslide through i t rel&vely easily. Likewise, make a similar-size hole near the bottom of the iuice can with a sharp pointed scissors. Also cut the juice c& off so it is about 4 cm high. The purpose of the juice can is to help keep the medicine vial in an upright position. The hole near the hottom of the plastic medicine vial should he about 0.8 cm or small enough so that the rubber tuhing fits tightly in the hole. This hole can be made with an electric drill or a pair of sharp pointed scissors. All of the hole sizes may need adjusting depending upon the size of the rubber tuhing used. Next thread the ruhher tubing through the coffee can, then through the juice can and into the medicine vial. The general setup of the apparatus is shown in the figure. The dust explosion is conducted as follou,~: discard the lid
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' Kotz. J.
C.; Purcell, K. F. Chemistry & Chemical Reactivity; Saunders: New York. 1987; p 469.
894
Journal of Chemical Education
Dust explosion apparatus.T is 3-lb coffeecan. R Is rubber tubing. C iscandle. J is cuf-off juice can, and M is medicine vial. Drawing is not to scale but shows general placement ol objects. See text for more details.
to the medicine vial, and place dry cornstarch or flour in the medicine vial until the vial is one-third to one-half full. T ~ D the vial to settle the powder. Set the vial, which is inside thk juice can, to one side of the coffee can, and pull the excess rubber tuhing out of the coffee can. Place the candle on the other side of the coffee can and light the candle. Place the plastic lid on the coffee can and quickly blow hard and long into the ruhher tubing. The explosion will blow the lid as high as the ceiling and some red flames will shoot out of the can. Caution: he sure the person doing the blowing stands hack and does not have his or her head over the can. To assure that a successful explosion will occur, the following suggestions are made: (1)The candle should have a reasonable-size flame. Sometimes after several explosions the candle will not burn well and cleaning the wick off periodically will allow for a good flame. The short votive candles may he used, or a tapered candle that has been cut to about 4 cm high will work. Some wax can be melted around the shortened tapered candle to hold i t in an upright position in the coffee can. A handy way to light the candle in the coffee can is to use a long tapered candle. (2) The cornstarch or flour must he drv. One should drv i t in an oven for several hours or overnight. The dry powder may then he stored in a container with a tieht lid. If the ex~losiontechnioue fails to work, try redryinithe powder. ~ i powder y is very important, and i t may look dry but will not work every time in the dust explosion apparatus. (3) The person blowing into the plastic tuhe must be ready before you place the plastic lid on the coffee can. The heat from the burning candle in the coffee can will melt the plastic lid if the explosion does not occur soon enough. The explosion itself does not damage the lid. T o introduce this demonstration, one may call a student up to the front of the class and tell him or her that you are eoine.. to test his or her breath bv havine him or her blow into .. the rubhertubing. After theex~losion,;hestudentsnaturalIv srart askina ouestims. l'his demonstration can thus reinforce the scientific tools of observation and cause and effect.
What caused the explosion? Is it repeatable? Will a smaller candle flame effect the results? Will cornstarch or flour that has not been dried work? Will the cornstarch or flour burn by itself? (Try lighting some in a metal spoon with a candle.) These are some of the questions that may be raised. Many variables can be considered and a series of experiments could be conducted to study the variables. Now the "chemistry" can be brought hack to real life. Why do elevators explode? What conditions must be present? In summary, the dust explosion apparatus is easily constructed from cheap material and can he used in annmber of ways to make your classroom "explosive."
Styrofoam or waxed paper cups Plastic spoons or glass rods Candy molds Watch glass or Petri dish Graduated cvlinders: one 10 mL and two 25 mL Plastic sandwich baes Safety goggles Latex gloves Safety Precautions Latex glwes and safety goggles should be u o m when squeezing out thcexcew liquid in Prmedurer.4 and H.Sodium silicate solurron is a skin irritant. Wosh the pulsmer with water beiore pnsaing it around the class Procedure
Silicate-PVA Polymers SUBMITTEDBY
Barbara A. Burke Calltornla State Polytechnic Unlverslty, Pomona 3801 West Ternde Avenue CHECKED BY
D. T. Haworth and M. F. Raab Marqualte Unlvenlty Mllwaukea, W153233
Various forms of silica are known to interact with alcohols.' In a recently ~uhlisheddemonstration a "super ball" type of polymer was prepared from sodium silicate and water-soluble, low-molecular-weight alcohols such as ethanol.% The reaction of poly(viny1 alcohol) (PVA) and borate ion to form a gel ("slime") has been used as a chemical demonstrat i ~ n .It ~ was . ~ of interest, therefore, to investigate the interactions between PVA and silicate ion. The demonstration Dresented here involves a polymerization reaction between'^^^ and sodium silica6 inaqueous solution. The polymer formed exhihits properties that are dependent upon the amount of silicate ion used. Small amounts of silicate result in a polymer that is elastic and can be stretched into thin, almost transparent sheets, whereas larger amounts yield a polymer with significantly less elasticity, but which can be molded into various shapes.
Materials Poly(viny1alcohol),4% (by wt) aqueous solution. A 4% PVAsolution is made by dissolving with heat 4 g of PVA in 96 g of water. Poly(viny1 alcohol), 98% hydrolyzed, Avg MW 88,000, Aldrich Chem. Co., Cat. No. 18,966-9. Sodium Silicate Solution, 40-42Be, Technical Grade, Fisher, Cat. No. SO-S-338.
Iler. Ralph K. The ChemistryofSilica; Wiley: New York. 1979; pp 295. 296. 387, 395. 704. 706. Gardner. M.: Summerlin, L.; Borgford, C. "Selected Demonstrations from Berkeley I.C.E. Summer, 1985". Cassassa. E. 2.:. Sarouis. . A. M.,Van Dyke, C. H. J. Chem. Educ. 1986, 63, 57. Sarquis, A.M. J. Chem. Educ. 1986.63.60
A. Formation of Polymer Sheets To a cup containing 25 mL PVA solution, add 5 mL of sodium silicate solution with stirring. (Use any convenient 5:l ratio.) The polymer forms immediately and after 30 s to 1 min of stirring, it can he lifted out of the reaction mixture on the spoon (glass rod). Pull the polymer mass offthe spoon, squeeze out the excess liquid, knead the mass with your fingers for about 1 min, and then spread the polymer out on an inverted watch glass or Petri dish. Let stand about 10-15 min, until the sheet is strong enough to be lifted. Wash the polymer with water. Pass the sheet (on the watch glass or Petri dish) around the class. For comparison, a sample of this polymer sheet that has been air dried for about 24 h can also be shown. The dried polymer no longer exhibits much elasticity, but it is flexible and quite strong. Alternatively, the polmer mass can be passed around the class without making a sheet. Note that the strength and elasticity of the polymer increases, reaching its maximum in about 3045 min. The polymer can be stored indefinitelyin a plastic sandwich bag to retain its elastic properties. 6. Formation of a Molded Polvmer To 25 mT. PVA solution. add 25 mL sodium silicate solution with stirring. (Use any convenient 1:l ratio.) The polymer forms immediately and after 30 s to 1min of stirring, it can be lifted out of the reaction mixture on the spoon (or glass rod).Pull the polymer off the spoon, squeeze out the excess liquid and immediately press the polymer mass into a small candy mold. After about 10 min, remove the polymer from the mold. Wash the polymer with water. Then pass the molded polymer around the class. This polymer remains flexible after moldineand can be stored indefinitelv bae. . in a olastic . Afterairdrying fmatmut 24 h, the polymer shrinksamnewhat. but it retarns its shape and must of its flexibilir?. ~
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Discussion The silicate ion in these polymers probably cross-links the PVA strands, similar to those presumed to be present in "slime" 3. I n the presence of small amounts of sodium silicate. onlv a few silicate ions are available to bridee the PVA strands, yielding a weak, but quite elastic polymer. When larger numbers of silicate bridges are formed, astronger, but far less elastic polymer is formed. Thus the polymer from ~rocednreB can be molded but not stretched to any areat degree, whereas the polymer from procedure A can be stretched into thin sheets but not molded.
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Volume 65
Number 10
October 1988
895
