JAMES 0.SCHRECK COLORAW G~EELEY. C0 80639
UNIVERSITY OF NORMERN
Preparation of Sterno from Blackboard Chalk A "Safe" Student-Teacher Activity Michael J. Welsh Illinois State University, Normal. IL 61761
The Department of Chemistry at Illinois State University has, for several summers, conducted a flexible interactive program for secondary and elementary science teachers. The program is aimed a t giving the participants a chance to develop teacher aids, demonstrations, visuals, laboratory projects, etc. Several of the participants expressed the desire to develop "safe" demonstrations and laboratory experiments. The important objective of a "safe" chemical activity for vounser investieators is that the activity hold the attention bf thestudent. This can ultimately be dine by incorporating as much mixing, grinding, stirring, filtering, color change, etc., as possible. The younger the investigator, the greater the need for tangible evidence to indicate chemical and physical change. The excitement of chemical reactions lies in the feeling of control over that reaction, and accomptishment when the reaction is a success. The preparation of a Sterno-like material from blackboard chalk is a student-teacher activity that incorporates the objective described above. Reacllons The heart of this activitv is the ael formation of alcohol and a saturated calcium acptate soiution into a Sterno-like sub~tance.~:' The "solid" alcohol is then ignited w further illustrate its similarity to Sterno. To makk this reaction a more appealing activity for an elementary or a high school studen;,rhe s i r c e ofthesaturated calciumacetate solution will be the product of the reaction between chalk and vineear. And while the alcohol of choice in 95%ethvl alcohol. 91% Tsopropyl alcohol works very well and is more economically obtained from any drug store. The major components of blackboard chalk are CaC03 and CaSOa. Either the white or yellow "sight saver", antidust, hard-pressed chalk made by Binney and Smith has ample CaC03 for this project. Colored chalk is composed of little to no CaC03 and will not work. I would recommend that the investigator use yellow "sight saver" chalk and white distilled vinegar. This combination will enhance the filtering stage by giving a light yellow foam and chalk susnension before filterine. and the filtrate will be clear to slightly cloudy. While wkte chalkand red vinegar work well, they do not give a color change during the filtering stage.
' Billlnaer. R. D.~ J. Cham. Educ. . 494 ~ ~ - .. . . 1034. ~ -i.f. ., ~. ,~
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EaIIey, P. S.; Bailey, C. A.; Anderson. J.: Koski, P. G.; Rechsteiner, C. J. Chem. Educ. 1975, 52, 524. Surnrnerlin, L. R.; Ealy. J. L. ChemicalDeronstmfions:A Source Book for Teachers; 1985: Vol. 1, p 132
Aqueous calcium acetate, carbon dioxide, and water are produced from chalk and vinegar by the following reaction: CsCOds) + 2CH3COOH(aq) impure vinegar chalk
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Ca(CH3COO)daq)+ COdg) + HzOU calcium carbon water acetate dioxide
The calcium acetate solution is either boiled or evaporated down to eive a saturated solution of calcium acetate. A small amount i f this solution is added to 91% isopropyl alcohol and within 20 s a gel forms. Ethyl alcohol can be used, but it must be 95% alcohol. The gel stage will depend on a saturated calcium acetate solution and a concentrated alcohol solution. The gel will not form if too much water is present in either component. The resulting gel is similar to the commercial product Sterno. The flame produced by the combustion of the isopropyl alcohol is quite hot, but the combustion is limited to the gel. The formation and structure of the gel is not clear. Most likely the gel forms as a result of a network of calcium acetate, which traps the isopropyl (or ethyl) alcohol molecule~.~ While the exact nature of this gel is not known, there are some points of discussion concerning the nature of gels that may accompany this activity. First, a gel is acolloidal system characterized by a liquid (alcohol) dispersed in a solid (calcium acetate).3 Second, a colloidal system that resembles a liquid is called a sol, whereas a colloidal system that resembles a solid is called a gel. The terms solation and gelation are used to describe the reversible transformation of sols into gels and vice versa. Third, gels may he classified by the nature of their network linkaees. Gels mav be linked chemically through covalent b o n d l ~ t ~ r e and n e acrylamide used for electro~horesisare examdes of eels that are chemicallv linked. ~n'contrast, gels may be lilked by physical forces such as hydrogen bonds, van der Wads forces, or hydrophobic and ionic interactions. The Sterno-like material in this activity is an example of such a eel as well as the familiar Jello dessert. ina all;, substances such as jelly (pectin), agar, and the precipitates of aluminum hydroxide and iron(II1) hydroxide are-additional examples of gels that can be easily obtained and compared to the gel produced in this activity. Materials 600-mLbeakers or 16-ozplasticcups, stirringrod or plastic spoon, filter paper or coffee fdters,funnel, aluminum pie pan.
Volume 67 Number 4
ADri1 1990
337
Chemicals Blackboard chalk, white distilled vinegar, 91%isopropyl alcohol. Actlvlty Procedure Crush approximately 25 g of yellow chalk (2.5 sticks) into a coarse powder. This can be done with a mortar and pestle, or the chalk can be wrapped in a piece of cloth and crushed with a hammer. Place the chalk in a 250-mL beaker or a 16-02 clear plastic cup. Add a total of 100 mL of white distilled vinegar to the chalk in two 50-mL additions,otherwise the foam produced by the escaping COz will overflow the container. After the f i t addition stir the mixture with a stirring rod or s plastic spoon until the foam has subsided; after the second addition, continue to stir the mixture for approximately 8 more minutes after the foam subsides. All of the chalk will not dissolve, hut you will produce plenty of calcium acetate for this demonstration. ~ ~ ~ ~ ~ ~ ~ ~ Gravity filter the rhalk-vinegar mixture through filter paper or two coffee filters. The resulting calcium acetate solution will be eitherdear or slightly cloudy, bur the yellow rvlured chalk should be fdtered out of the solution. A saturated solution of calcium acetate is required for the formation of the gel, so either evaporate or boil the solution down to approximately 15 mL. If the calcium acetate solution is concentrated by boiling the solution, use commercialSterno as the heat source. At the conclusion of the activity point out to the students the irony of using commercial Sterno to make the Sterno like gel. If the calcium acetate solution is concentrated by evaporation, and an ~
338
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evaporating dish is not handy, an aluminum pie pan works well. It will take hetween 24 and 36 h for the solution to evaporate down to the desired volume. To make the Sterno, place 5 mL of the saturated calcium acetate solution in a 50-mL beaker or plastic cup. Add 30 mL of 91%isopropyl (or 95%ethyl) alcohol to the calcium acetate solution. In about 15-20 s the mass in the container will solidify. Place the gel in an evaporating dishor an aluminum pie pan, and ignite it with ametch. The gel will burn for about 15miu, until all the alcohol has been used up leaving behind charred calcium acetate salt. Conclusions Virtually all of this project can he done by an elementary school student with the suoemision of the instructor without any fear of the student hkng harmed. Supplying safe student-teacher activities for secondary and elementary school science teachers is a n ongoing concern of the Chemistry Department a t Illinois State Universitv. The goal of our prigram is exposing students to safe, exciting, hands-on activities that illustrate simple, basic chemistry. The format of our program is simple-the elementary i d secondary school teachers provide us with the needs of their classrooms and labs. and we ~ r o v i d ethe teachers with the technical resources to meet ihose needs. Hands-on student activities in science is the kev to maintainine student interest in science.
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