A New Road to Reactions Part 2 Wobbe d e Vos and Adri H. Verdonk Department of Chemistry Education, Utrecht University. P.O. Box 80.069. Utrecht, The Netherlands Precipitation a s a Chemical Phenomenon If a solution of lead(I1) nitrate is added to a solution of potassium iodide, or vice versa, a beautiful yellow precipitate of lead(I1) iodide is formed. This is a good method for the preparation of lead iodide:Also, it enables a student in analytical chemistry to distinguish quickly between solutions of lead nitrate and, say, zinc nitrate or between solutions of potassium iodide and potassium chloride. It is, however, not a good way to draw the attention of students of a heginning chemistry 'course to the formation of a precipitate as a chemical phenomenon. The alternative that we chose gives better results in'terms of students' inclination to discussing fundamental aspects of the chemical reaction. This precipitation experiment follows in our syllabus shortly after the yellow powder experiment discussed in our previous article.'
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The ~ u r ~ ; i s lYellow n ~ Line Again, students are workingin groups of three or four. Each group is provided with a Petri dish of 8 cm diameter containing a very thin layer of distilled or deionized water. In addition, each group is given two spatulas, a watch glass with a little lead nitrate, and a second watch glass with a little potassium iodide. (For reasons mentioned in our first article, lead nitrate for the time being is called by its pseudonym "white minium" while ~otassiumiodide is labelled "courtoisite" after the ~renchmanC'ourtoiswho discovered iodine.) Students are instructed 1,) r h r e the Petri dish in a nerfectlv horizontal position on their lab bench and to p u t a smafl amount of the "white minium" (lead nitrate) carefully into the water close to one side of the dish. Next, using a clean spatula, some potassium iodide is placed exactly opposite the lead nitrate on the other side of the dish. Students are then told to take a few minutes to draw this arrangement schematically on paper, without touching the Petri dish. While drawing, students suddenly notice a thin yellow line in the middle of the dish. As they watch, the line slowly grows in length as well as in width, and after a few minutes the water layer appears to he divided into two roughly equal parts. In ~
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WoMm de Voe is a lecturer in chemicaleducationat WecM University in the Netherlands. He studied inorganic chemistry and psychology at Groningen University and holds a doctw's degree in chemical education from the University of Utrecht. Aner teaching chemistry for many years in secondaryschoais in Holland andtwo years in Uganda, he isnaw involved in research projects in chemicai education. He is panicuiariy interested in the spontaneous ideas that children have about the mrpustular nature of matter and is convinced that careful listening to st"dents' ~ ~ n ~ e r s a lcan i o nhelp ~ to solve problems connected with the teaching of chemistry. During his researchds Vos wrote a school textbook on elementary chemistry called "Chemie in Duizend Vragen" (Chemistry in a Thousand Ouestians). He is also involved in the presewice and-in-Sewi~e training of chemistry teachers. in addition, he is an editor of a Dutch journal on science education.
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their sketches, most students draw the line more or less equidistant from the places where lead nitrate and potassium iodide have been put into the water. Since potassium iodide migrates considerahlv faster than lead nitrate. it is essential that lead nitrate he piaced in the water first to h o w it a head start. Students are puzzled by the appearance of the yellow line and by its gradual expansion. The instructions for the'experiment are followed by a number of questions designed to make use of the curiosity provoked. The answers show that most students consider the line to indicate the places where the two suhstances have met. This implies the assum~tionof transport of both substances throughthe water. Migration In many groups, primitive corpuscular models of matter are used spontaneously throughout the discussion. The development of these ideas is probably encouraged by the ohservation that the yellow line is composed of numerous small, occasionally glittering particles that on closer examination are found to he in contitiuous motion. In a frequently occurring interpretation, "molecules" of the substances starting from either side of the dish travel through the water and, upon colliding, comhine or change into molecules of the vellow suhstance. Inspection of the yellow line with a magnifying glass while the reaction is in progress does not reveal anv motion in the water on either side ofthe line. Students explain this by referring to the extremely small size of the molecules. The idea that a suhstance can he transported through the water is interesting from the teaching point of view, since it is not hased on direct observation hut on an interpretation including a hypothetical element. In our opinion, it is important that students discuss their reasons for accepting transport as an essential Dart of their exnlanation. Analysis of group discussion recorded on audiotape showed that in manv..proups . the miaration of the dissolved substances is regarded as resulting from mutual attraction. This idea of a force of attraction working a t a distance often occurs in association with the animistic view that these two suhstances wish to comhine and do not mind traveling a long distance for that purpose. We feel that the teacher should face this complication by arranging for a confrontation of the idea of attraction with contradicting evidence. T o organize such a cognitive conflict, the students are invited to state explicitly whether they thought attraction to he the cause of substance migration through the water. This question, answered affirmatively by a majority of the groups, contains the hidden message to students that the other possihility is a t least conceivable and thus opens the possihility of further investigation. Students are then instructed to repeat the experiment, this time first putting some potassium iodide in the water and waiting three or four minutes before adding lead nitrate at the other side of the dish. T o their surprise, the yellow suhstance results immediately upon the addition of the lead nitrate. Students are asked to explain this latter observation hased ~
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' De Vos, W. and Verdonk, A. H., J. CnEM. EDUC., 62,238 (1985). 648
Journal of Chemical Education
on the nreviouslv voiced attraction theorv. With verv few exceptions, the groups quickly reject this possibility, arguing that the spreading out of one substance does not depend on the presence of the other and, probably, vice versa. This spontaneous soreadina can now be referred to as diffusion o;migration, names, ofcourse, made available by the teacher. Strictlv speakina, diffusion is onlv a minor effect. If the water layer-is thick enough, the salt sdution, being relatively heavier than watrr. can he seen flowing away from the solid across the hottom of the dish. Whrn a a h r e d snlr is nsrd this process can easily he ol~icrvedwhen lnukin~at the dish fro& the side Advantage of This Method The Petri dish precipitation of lead iodide is a more powerful way of teaching the concept of chemical reaction than the test-tube version for several reasons. In the first place, student reactions seem to indicate that the process in the Petri dish appeals to a sense of beauty, thereby lowering the threshold to analysis and discussion of the phenomenon observed. Secondly, the reaction in the test tube is an instantaneous process, leaving students with the problem of analyzing something that has happened, while in the Petri dish the reaction is taking place a t a rate that allows students to follow it. Not only the product but the process itself can be studied. (In the test-tube version, after the reaction has taken place, students are witnessine"the orecioitate . . settle down. a orocess which to inexperienced observers gives the impression that the amount of vellow suhstance is actuallv decreasind Thirdly, th; Petri dish experimeut separates the three processes dissolution, migration and reaction spatially. All three can be pointed a t and discussed while they are taking place. In the test-tube version this is not possible. Foundations for Further Experlments Several test-tube reactions can be converted into Petri dish versions, many of them exhibiting beautiful results to those who can appreciate them. Two examoles, thowh not the most spectacula; ones, now have a fixed place in the curriculum. Both aim at a further understanding of the chemical reaction. Shortly after the Petri dish experiment with lead nitrate and potassium iodide, students are instructed to repeat the experiment, this time using ordinary sugar and salt. Of course, no precipitate is formed. The questions in connection with this experiment lead students to a better distinction between dissolution, migration, and reaction. Students have some difficulty in acknowledging that salt and sugar not only dissolve and migrate in the water. but that there are also actual collisions between sugar molecules and salt "molecules" eveu though no reaction is taking place. They seem to believe that careful execution of the experiment should be rewarded with some visible result. The failure of salt and sugar to react can help to make them realize that the chemical reaction is not a feature of the experimeut itself but of the combination of substances in the experimental conditions. In the second additional example, the Petri dish experiment is repeated using lead nitrate and tahle salt. This time a white line is produced due to the formation of sparingly soluble lead chloride. The white line is less spectacular than a yellow one; nevertheless, its formation surprises many students since they do not expect ordinary tahle salt to be capable of such a thing. One group wrote, "We learned that salt is a chemical after all." This is a remarkable advance for these students. The experiment, however, has its major influence on the curriculum when somewhat later powdered lead nitrate and powdered kitchen salt are mixed in a mortar. This time there is no visible change no matter how long the mixture is rubbed with a pestle. The apparently meaningless effort is followed by the question, "Do
you think that a chemical reaction has taken place in the mortar?" This auestion relates the exoeriment not onlv to the Petri dish version with the same twosalts, but also to the analogous exoeriments with lead nitrate and potassium iodide. G r o w discussions show that groups considering all four experiments are able to interpret the (lack of) result in the last case as a possible invisible chemical reaction. Referring to the yellow substance that could be formed in two ways, they regard the formation of a new, white substance in the mortar as likely. Some groups state very cautiously that what is true for a yellow substance does not necessarily apply to a white one. Others, making use of their corpuscular models, argue that since the reaction anoarentlv results from collisions hetween lead nitrate molecuies and sait molecules, it will probably take place in the mortar as well as in the Petri dish. (It is difficult to say how much lead chloride is actually formed when lead nitrate and sodium chloride are rubbed together in a mortar: students' answers, however, should not be judged from this point of view, but by the level of argumentation that has been achieved.) The Payofls There is, of course, still a long way to go before students have acquired sufficient understanding of the concept of chemical reaction. Direct presentation of chemical facts and definitions as a teaching method will certainly take less time. One of our reasons for choosing the harder and longer road to chemical reactions is that we expect understanding of this fundamental concept to he more profound and more widely applicable if it is a result of a personal struggle of the student for active communication with others. I t is a common fear among teachers that giving away the initiative t o students is an irreversible process leading to inefficient use of time and to other troubles. Our attempts to avoid such results have been focusine " on the use of intellectuallv and emotionallv . ao~ealina .. experiments accompanied by challenging questions within the scooe of students' interests. fi has become increasingly clear to us that these attempts are more likelv to he successful to the extent that thev are based on a thorough understanding of students' specific learning problems and possibilities. Many of the more imaginative ideas now functioning in the curriculum can he traced back to questions and speculations spontaneously formulated hv -,students. Listening to students is of crucial importance. This explains why in our approach, curriculum development is closelv inteerated with research into chemistrv learning " processes as well as with training of chemistry teachers. Additional Experiments Apart from the reactions already mentioned there are a larae number of other reactions aivina beautiful and interpetri dish instead of a test esting results when carried out in ; tube. Colored salts, like nickel or cobalt sulfate and potassium chromate provide students with a good opportunity to check their ideas on migration of salts in solution. Several of these reactions make an excellent performance on the overhead oroiector, especiallv if some examples are included that do not . . react. New possibilities arise if a solution of one of the reactants is used instead of water. A crystal of washing soda in a thin layer of very dilute acetic acid solution will surround itself by a slowly expanding ring of bubbles of carbon dioxide gas. With acid-base indicators beautiful results can be obtained. There are manv other interesting and eveu snectacular examples of Petri dish reactions. In fait, there are f& too many for an introductory chemistry curriculum. The ones that are chosen deserve a well-thought-out educational approach if more than iust another niece of recreational chemistrv is the aim. Each of them carries the message that chem~stryis a fascinating subject.
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Volume 62 Number 8 August 1965
649
