More Chemistry in a Soda Bottle A Conservation of Mass Activity Daniel Q. Duffy and Stephanie A. Shaw Department of Curriculum and Instruction, The Florida State University, Tallahassee, FL 32306 William 0. Bare and Kenneth A. ~ o l d s b ~ ' Department of Chemistry, The Florida State University, Tallahassee, FL 32306 The law of conservation of mass generally is introduced early in introductory chemistry and physical science courses. Some texts offer experimental illustrations of conservation of mass such as burning a candle in a closed oxygen atmosphere (1)or burning magnesium in a flash bulb (2,3). Experiments of this type are difficult to carry out as student activities or even as classroom demonstrations. Instead experimental illustrations of conservation of mass typically involve carrying out a reaction such a s a precipitation and showing that the combined masses of the reactants is equal to the mass of the product mixture (4).While such reactions serve to illustrate the conservation of mass, thev fail to ~ r o v i d ea n amreciation of why conservation of .. mass was ever questioned. Consider the apparent loss of mass when a piece of wood is burned. Aristotle (384-322 BC), extending the earlier work of Empedocles (-493--433 BC), proposed a complete philosophy of nature based on the four-element composition of matter: earth, water, air, and fire (5).I t was observed that earth and water tended to move down (the principle of gravity), while air and fire tended to rise (the principle of levity). Chemical and physical processes were often explained in terms of the elements finding their natural levels. According to the four-element theory of matter, when a wooden log is burned, the fire and air are rrlwwd, leaving the earth land ocrasionally some watw Iwhind. The Aristotc,li;in vitw of matter gcnernlls was JCcepted well into the middle ages and ~ a s - ~ e r h a p best s articulated by Dante (6). Another theory of the composition of matter was the phlogiston theory formulated by Stahl (1660-1734) and polarized by Becher (1635-1682) that explained combustion by the existence of a substance (phlogiston) contained in all flammable material that was released in the form of fire when the material was burned (7). The phlogiston theory easily explained the apparent loss of mass in many combustion reactions; however, it could not explain why certain metals, when heated in air, gained weight. This aberration was the theory's ultimate downfall (8). Only after these theories of matter were challenged could chemists recognize that mass was conserved. The first experimental demonstration of conservation of mass is generally attributed to Lavoisier (1743-1794). Working with sugar, yeast, and water in known amounts, Lavoisier allowed t h e mixture to ferment. By determining the amounts of carbonic acid gas (COz) and water given off, Lavoisier was able to show that the weight of the initial reagents was the same a s the weight of the final residue and products. Thus, Lavoisier was the first to write down the law of conservation of mass, though it was implicit in the earlier works of Black and Cavendish (9). '~uthorto whom correspondence should be addressed
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Figure 1. Apparatus used for initial reaction: (A) baking soda. ( 8 )18 test tube, (C) vinegar, and (D) 400-mL beaker.
x 150 rnm
Given the evolution of thinking that led to the law of conservation of mass, it seems that a more effective demonstration would involve a reaction in which there is a n apparent loss of mass. Typically such a reaction would involve evolution of a gas. We describe here an activity in which the products of a gas-evolving reaction are contained in a 2-L soda bottle. I n nrinciole this activitv could Ilc c;irri(:d out with any numhc:r ofgui-evolving rcuctims. Wt: illustrate the l m l c ;iooroach hr. eivlne the. soecific conditions to carry out thisactivity fbrthe reaction between vinegar and baking soda. Procedure Initially the reaction should be carried out in a n open vessel such a s a 250-mL heaker (Fig. l).The baking soda, vinegar, and glassware are weighed together and the mass is recorded to the precision of the scale. After pouring the vinegar into the beaker, the empty test tube is returned to the beaker, and the mass is measured again. An initial mass of 2 g baking soda and 20 mL of vinegar will give a n apparent mass change of about 0.6 g, which is detected easily on common laboratory scales. This preliminary activity is conveniently carried out a s a demonstration and provides the instructor a n opportunity to generate class discussion. For example, students can be asked to speculate on the nature of the reaction and the meaning of the change in mass. One also can ask students to design procedures for collecting the gas so that it can be used in later experiments. The actual conservation of mass activity essentially involves repeating the above procedure in a 2-L soda bottle (Fig. 2). Given the availability and economy of the required materials, we generally have students do the activity
Collateral Topics This activitv was develo~edfor use in a n introductorv physical science course, and while i t could be used in a& science course to illustrate conservation of mass. i t also can be used to introduce a number of important topics in chemistry. Pressure and Gas Laws
Students generally notice the increase in pressure when the vineear and baking soda are combined in the soda bottie, progding a n opportunity to comment on the relationship between the volume, pressure, and amount ofgas. We have reported the use of a pressure gauge attached to the plastic bottle cap to measure pressure-volume relationships i n a Cartesian Diver experiment (12). The modified cap can be used in this activity to allow students to read the pressure inside the bottle for various initial amounts of baking soda or vinegar. Thus, one can measure the increase in pressure a s a function of the mass of COz released to develop a quantitative relationshiv between pressure and amount of gas a t constant volurne~
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Acid-Base Chemistry
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actlvttv 1Al bakFlaure 2 Apparatus used for conseivat~on-of-mass ing soda, ( ~ ) ' 1x8 150 mm test tube, (C) vinegar, (D) 2-L sbda bottle. and (E) cap.
themselves. Roughly 2 g of baking soda is transferred to a n empty 2-L bottle, followed by a 18 x 150 mm test tube containing 20 mL of vinegar. The test tube is gently slid down the sides of the bottle to avoid spilling the vinegar. The bottle i s tightly capped and the bottle, along with its contents, is carefully placed on a scale. Once the initial mass has been measured, the bottle is inverted. combining the vinegar and baking soda. Asecond measurement of mass is taken, generally showing little or no change in mass.2 The bottle iB opened, and a t&d measuremencis made indicating a decrease in mass consistent with the loss of gas. Initially the apparent loss of mass is only about 0.4 g because the atmosphere in the bottle is rich i n carbon dioxide relative to air. Tilting the bottle and squeezing slightly several times to flush out the COz gives the expected loss of mass of roughly 0.6 g. If a greater change in mass is desired, more baking soda and vinegar can be used. The amount of vinegar can be increased by using a longer tube of comparable diameter or by using more than one test tube. Alternatively, a more concentrated solution of acid could be used provided the necessary safety precautions are taken.
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Caution should he taken to avoid excessive pressures.
The conditions presented here give final pressures of ,~ is well roughly 3 psi above atmospheric p r e ~ s u r e which below the typical pressures in bottles of carbonated beverages a t room temperature-roughly 100 psi (11). 1 a ow-preclson aooratory oa ance s dseo n tnls act v ly. the measxement error can oe an apprec aole fracttonof !he overa mass change. For example, an economy triple-beam balance gave masses to f 0.05 g compared to the overall change in mass of 0.6 g, and Consistent positioning of the bottle on the weighing pan was required to reproducible measurements within the error of the balance. While the error in measured mass may provide an opportunity to introduce error analysis to this activity, the relative error can be reduced by increasing the amounts of the reagents used (see discussion in text). in pressure was measured using a pressure gauge 3 ~ h increase e fitted to a plastic bottle cap, as described in ref 10. The observed increase in pressure agrees well with the value calculated assuming the production of 0.6 g of carbon dioxide gas in a two-liter bottle.
The reaction between acetic acid and sodium hvdroeen . ,, carbonate used in this activity prowdci u natural platform u p m which to d ~ i c u s .wid-hxr s vheni~stn.Measurinv the ; of of the vinegar before the reaction occuk and the H the final solution shows that an acid-base reaction has occurred and provides a n introduction to conjugate acids and bases. By adding phenolphthalein to the vinegar before i t is placed in the soda bottle, the change from acid to base can be observed during the course of the activity, provided that acetic acid is the limiting reagent. Chemical Calculations and Dynamic Equilibria
This activitv can be used to illustrate the use of chemical calculations to determine the extent of reaction. Assuming vinegar to be a 5% solution of acetic acid in water and baking soda to be essentially pure sodium hydrogen carbonate, the reaction is carried out under roughly equimolar conditions. One can calculate that the loss of mass is roughly two-thirds of that predicted for the reaction between acetic acid and sodium hydrogen carbonate assuming the reaction goes to completion and no carbon dioxide remains dissolved i n the resulting solution. The discrepancy could be due to a number of factors including the solubility of COz in water and the eauilibrium distribution of reactants and - products in reactions between weak acids and weak bases. I n fact. the chemistrv involved i n this activitv is m i t e complex. For example, p " effects ~ the solubility of C& so that the addition of more vineear ~ r o d u c e smore carbon dioxide gas, even if the sodium h;idrogen carbonate i s completely consumed. Because this activitv normallv would be carried out early in the course, i t canprovide a n introduction to chemistry that will revisited later in the course. ~
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Reactivity
In principle, any of a variety of gas-evolving reactions could be used in this activity (e.g., active metal plus acid; hydrogen peroxide plus catalyst). Again, hazards associated with the reagents and products must he considered, especially considering the possibility of s~ravinnwhen the bottle is vented. Again, Eaution should he taken to avoid excessive pressures. Summary and Conclusions The conservation of mass activity described here was developed and used i n a restructured physical science course Volume 72 Number 8 August 1995
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for prospective elementary school teachers. The philosophy of the course was based on a n interactive learning model emphasizing hands-on activities and group work. Students were encouraged to make broad observations during the activity and to ask questions. Ideally, the student will ask the important questions, such a s 'Why does the mass change?hnd "Is mass conserved?" The instructor can generate discussion by asking the students questions; for example, "Why was the observed change in mass different from that computed using staiehiometry?" or "Have you really proved the Law of Conservation of Mass?"
In addition to illustrating the law of conservation of mass, this activity can serve as a springboard to many important topics in a chemistry or physical science course, and the simple, inexpensive materials make it easy to setup and adapt to any classroom.
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Journal of Chemical Education
Acknowledgment We wish to acknowledge the National Science Foundation (under grant #TPE-9150031)for support of course development. Literature Cited 1. Wilbraham, A. C.; Stsley. D. D.; Simpson. C. J.; Matt., M. S. Chemislry. 2nd ed.; Addiron-Wesley Publishing: Menlo Park. CA, 1990, p 16. 2. Ebbing, D. D. Ganernl Ciremislry, 3rd ed.; Houghton Mifnin: Boston, 1990 p 5. 3. Hankins. W.; Hanlkins. M. hrlmduclion l o Chemistry; C. V. Mosby: St. Louis. 1974;
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4. watt.G.W.:Hltch, L. F.:Lagowski,J.J.Chamidryinth@Labom*ory; W WN0rton:
Naw York, 1964; p 19. 5. Gjertsen, D. The Clawics dScirnrm. A Study oflbeiua Enduring Scienlifle Works; Lllian Barber: New York, 1984; p 16. 6. Buttefield. H.In TheHisfory ofSciencp; Ori#ins ond Results ofLhoScientific Reua1"l;on:Cohen & wesf: London, 1951: chapter 1. 7. McKe, D.In The HidoryofScienrr;OriginsortdR
