e ' ,- A New Road to Reactions Part I

Wobbe d e Vos and Adri H. Verdonk Utrecht University. P.O. Box 80.069, Utrecht. The Netherlands

Much attention has been paid to guided discovery as a teaching method, however, teachers have been given little concrete advice on how to carry this out. In this paper, we attempt to guide "the guide" on a new road to the concept of chemical reactions.

Communicating the Basics Education in chemistry begins relatively late in Holland:

students are 14 or 15 when they first see a chemistry teacher teach. These students. orobablv iust like those in other . . . . countries, have great difficulty in grasping and understanding the concept of "chemical reaction." Failure at this basic level usually means a frustrating experience for the student and teacher alike. The program to he described herein involves a conscience effort to address and solve this problem.

In learnine chemistrv, we feel, students should be encour- aged IO discuss rhemical phenomena among themselves using their own termmoloav. The teacher should rreate situations in which students discover deficiencies in their own vocabulary when trying to communicate their observations and ideas. A new meaning of a word such as "reaction" is not understood by just listening to a teacher using or even defining the word. To grasp its full meaning, the students have to use the word themselves in several relevant situations. Initially, their ap- plication may be ambiguous or not work a t all, in which case the concept has to be refined or readjusted until under- standing is achieved. This is one reason why we have our students work in groups of three or four.

We also believe that imnrovement in chemical education depends largely on how weli we listen to our students. Whereas chemical nroblems can be solved hv studvinn chemical sub- stances, &aching problems can be soived b i &dying students. On the one hand, of course, i t is easier to experiment with suhstances than with children; but on the other hand, suh- stances cannot talk. I t is easier to studs and learn from stu- dents who are actively engaged in communication about chemical problems than from students who just sit and listen. This is an additional advantage of group work. Yet, the main reason for introducing group work is the one mentioned above.

We try to encourage students to tackle the confusing field of chemistrv hv develooine their own ideas on the corouscular " . . - nature of substances. Even if these ideas are very primitive by comparison for instance with Dalton's atoms and molecules they should nevertheless be taken seriously and not be dis- missed as expressions of so-called nalve realism. By allowing students to put their ideas into words and to compare and contrast them with their own observations of chemical phe- nmena , we hope to stimulate a kind of to-and-fro thinking between fact and fantasy, which, at this level, might be just as scientific as this sort of thinking can be in quantum me- chanics.

The most notable feature of this program is that the teacher hardly ever seems to teach in front of the class. Groups of students spend their time carrying out experiments and trying to answer a great number of questions, many of them open- ended. These questions with their corresponding answers (which are corrected as necessary by the teacher) form the main body of knowledge upon which the students are tested. Little other information is presented directly to the students,

thus the classical lecture technique of teaching does not play a dominant role in this program.

DO Not Blind Them, Make Them Think The following is a detailed look at one of the typical ex-

oeriments in which students are confronted with this dis- covery 'technique to investigate the concept of chemical change. As previously noted, a precise definition of the chemical reaction is not what is wanted in this discovery-ori- ented aooroach, rather the essence of chemical change is to be discovered and formulated by the students thems&es.

There is a laree repertoire of experiments that can he used to demonstrate ihe difference between chemical and physical phenomena. However, all are not created equal in our effort to nrohe the cnriositv and understanding of students. The - combustion of magnesium ribbon, the decomposition of am- monium dichromate. the exolosion of eunnowder and the electrolysis of acidified water &e all very spectacular examples of chemical reactions. What makes them spectacular, however, is not the fact that substances change into other suhstances. Fascinated (and blinded) by the bright light of burning magnesium, students fail to notice the white powder that is left behind bv the process. In manv examples, i t is the pro- duction of energy and not the conversion of substances that attracts students' attention.

On the other hand, demonstrations involving the rusting of iron. the hurnina of a candle, or the frvina of meat are too commonplace and-thus do not stimulate curiosity among students. The results of such experiments are taken for granted, and any questions about what is happening to the suhstances come only from the teacher, not from the students themselves ~~~~ ~~ ~

What was needed were experiments that could intrigue purely by the change of substances into other substances and that would not display any distracting phenomena. The best example we have used involves the mixing of solid lead nitrate and potassium iodide with a pestle in a mortar. Rubbing these two white salts together immediately results in the appearance of the beautiful yellow-colored lead iodide. The important thine is that nothing else hapnens. There is no electricitv. flame, noise, or anyuother notkeable energy effect and no solvent, catalyst, or any other auxiliary substance involved.

From White To Yellow This reaction, however, does not in itself teach anything.

What i t can do is help us to create a situation in which stu- dents state exolicitlv that a new substance has come into beine - during the experiment. This statement should not consist of a renhrasine in the students' own words of somethine that has be& put i r k his or her mouth by the teacher, but-it should be an exnression of what the student feels to be a nennine - discovery.

This is the first in a series of articles that will emphasize the use of chemical readions to iMuce and reinlwce chemical concepts through a "Guided Discovery Approach.'' It is hoped that others who have de- veloped similar concrete examples representing chemistry through reactions will share lheir methcds in this series.

238 Journal of Chemical Education

We were surprised to find that several experiments in well-known elementary Chemistry textbooks do not really induce students to discuss their observations in terms of substances changing into other substances. Instead, these experiments provoke discussions in terms of properties of a suhstance being lost or gained. Rust, in this view is not a new substance hut just "rusted" iron, and therefore it is still iron. Very often a clear choice is evaded by just talking about "it" that has turned red or become solid. Students' use of language suggests that they are, in their heart of hearts, reluctant to accept the possibility of substances changing into completely new substances.

The Swiss psychologist Jean Piaget bas pointed out1 that children develop the idea of substance conservation together with the conservation of weight and volume about the age of 10, that is, well hefore their first chemistry lessons. The meaning that Piaget gives to the word suhstance may differ somewhat from the chemist's, hut it seems that most children a t the age of about 14 still firmly adhere to an unspoken and unconscious idea that each individual substance is conserved whatever happens to it. However, this idea of substance con- servation, described by Piagetians2 as another step forward in cognitive development, turns out to be a stumbling block on the road to understanding the chemical reaction.

The purpose of the yellow powder experiment was to bring ahout a head-on collision between fact and conviction in the students' mind. To achieve this, the experiment must he part of a carefully arranged teaching situation. Several details, which may appear a t first to he rather pointless, have been found to be of crucial importance.

Each group consisting of three or four students is provided with two pestles, two empty mortars, and two watch glasses, each containing a few grams of a white powder. One of the powders is lead nitrate. However, hecause the name "lead" tends to draw student's attention away from the experiment without adding additional relevant information a t this point, the powder is labelled "white minium." The other powder, potassium iodide, is unlabelled. Students are warned that both powders are poisonous and told not to touch them.

The first instruction to the students is to put about half the amount of "white minium" in one mortar and about half of the other unlahelled powder in the other mortar. Once the transfer is accomplished they are told to rub each powder separately with the pestles. This allows for a familiarity with the equipment while it also provides a reference for the change that will eventually be observed. Of course, no change in color is observed a t this point. The students are then instructed to pour the contents of one mortar into the other. As they do, some students see a pale yellow color where the two powders touch. One movement with a pestle now produces a brilliant yellow color, which is usually hailed with astonishment and admiration. The reactions of most students show that the conflict between expectation and observation is both an in- tellectual and an emotional experience.

The unexpected color change will not easily he forgotten; however, when the ohserved facts do not fit in with existing theories, so much the worse for the facts. Some students ac- tually attempt to deny the existence of the yellow color or to explain it away. A usual comment is that one of the powders was yellow already. This is why the students were told to use only half the amount of each powder, leaving the other half for comparing the yellow with the original color.

Very often students will invent the theory that the little grains of white powder are like tiny little eggs: if you break them by rubbing the powder with a pestle, the yolk will come out and determine the color. This is why we ask students first to rub each powder separately. A reminder of this activity rules out this theory.

Now there isno "escape" theoryleft for the students. Some groups try to make the confusing facts more acceptable by adding audible to visual information; for instance, on one of

our audiotapes four girls working in agroup together mention the yellow color 22 times before they dare to write their ob- servation down. Finally, however, nearly all students choose the same phrase to describe what they have seen: "It turns yellow," without specifying what "it" is.

How to Guide Them Now the time has come for the teacher to be present and to

teach. He (or she) should not try to explain the formation of solid lead iodide but should ask the right question at the right moment. The question that should be asked in a rather casual sort of way is: "Which of you put that yellow substance into this mortar?"

The question produces general confusion in the group, students pointing at each other almost accusingly:

"You did!" "No, I didn't, it was you!" Of course nobody will admit having put the yellow sub-

stance into the mortar. Then it is the teacher's turn to he surprised: "When I gave you that mortar there was no yellow suh-

stance in it, and now there is. So who, may I ask, put it there?"

Perserverance is often rewarded. The question seems a fair one and can be repeated a few times if necessary. Then, sud- denly, one student, backed up sooner or later by others, finds a way out:

"None of us nut it there: it is somethim new and it just appeared." his student has discovered a i d formulated an essential feature of the chemical reaction: a new substance has been formed. He or she did this in answer to a question that had been deliberately formulated so as to present the old conservation view: if there is a yellow powder in the mortar, it must have come from somewhere else and have heen out into the mortar.

The imoortant thine is that the student, by dvine the an- swer thai tbe yellow suhstance is new, abanbons his or her former conviction and bases his or her opinion on a new in- terpretation of observations. The role of the teacher is to make it harder, not easier for the student to abandon his or her former idea. The new view on substances should be a personal victory of the student and something to he proud of, not something obtained without difficulty.

This is the best possible result, hut it is not always achieved. If no student gives the desired answer. the teacher mav sueeest it. The sugg&ion is u&ally accepted immediateiy h;ihe eroun. as if students were afraid to sav i t themselves. The .. name chemical reaction, used by chemists to refer to this peculiar phenomenon, is of course always supplied by the teacher.

Further on in the syllabus, students may sometimes revert to using the language of substance conservation or use am- biguous expressions (it changes), but each time the yellow powder experiment is referred to, either by the teacher or one of the students, the group is reminded of and accepts the possibility of chemical change.

It Did Not Stand Alone Prior to the reaction of lead nitrate and potassium iodide,

we introduced a few similar experiments in which no reaction takes place. In these experiments the mixtures show predid- able colors. ex.. red and white chalk oowders eive a oink . - . "

mixture. Under a microscope the grains of each of the ingre- dients can still be distineuished. The aim of these introductow "

experiments is to make the yellow powder experiment contrast even more stronelv aeainst a familiar and uncomdicated - - - background, ' Piauet. J., and lnhelder. B., "The Child'sConstruction of Quantities.''

~outledqe 8 Keoan Paul. London. 1974. ~eais. J. ~. ;e t al., 'Cognitive Development;' John Wiley 8 Sons,

New York. 1978.

Volume 62 Number 3 March 19115 910

The first white powder was n m e d "white minium." but the other powder has not been given a name. The teacher there- fore can ask students whether this second white powder might also be white minium. After the appearance of the yellow color, all students immediately rule out this possibility. Why? This question can initiate a lively discussion among students on what exactlv hanwens when the two nowders touch. . ..

The reaction ~ i l e a d nitrateand potnssium iodide will also take place when the twodt*are shaken in a closed test tube. However, this method has son,? educational disadvantages. In the first place, thrre is a glass wall between the mixture and the student ' eye. This wall, although transparent, puts the chemicals into a differrnt world; mysteries hehind glass are more easily taken for granted. In the second place, the actual reaction takes place while rhe mixture is being shaken, but ohservation of the process at that very moment is difficult. Rubbing with a pestle gives a more intense feeling of direct contact with what haonens: when I ruh. the vellow culur will . . , " appear here and now. Action and reaction are closely related. This reinforces the sensation of ~ersonal involvement in the process.

A final remark concerns the develonment and the use of corpuscular ideas hy students. The yellAw powder experiment itself does not normally induce students to reach sponta- neously for corpuscular theories to explain the color change. When asked for an o~ in ion on what h a o ~ e n s to the "mole- cules," they usually say that the little white molecules turn yellow when they meet. This, of course, can hardly be called an explanation. The yellow powder experiment in our cur- riculum plays a role in the development of theidea that each substance corresponds to a particular molecular species. The concept of chemical reaction can then be interpreted on the molecular level.

Additional Experiments Lead nitrate and potassium iodide is not the only combi-

nation of solids that react as powders in a mortar. Since this type of experiment presents the essence of the chemical re- action in a verv direct and unembroidered wav. it seems worthwhile ha;ng a few other examples in res&ve. Other possible reactions include not only double decompositions,

but also oxidation-reduction, acid-base, and complex-for- mation reactions. In the following list we restrict ourselves to reactions that can safely be carried out by inexperienced students. We try to avoid the expensive chemicals. Of course, students should be informed never to mind substances in a mortar unless so directed by the teacher.

Double decompositions. Instead of lead nitrate, other lead salts, e.g., lead acetate or lead carbonate hydroxide (white lead). can be used. Some mercurv(I1) salts give beautiful re- sults &th potassium iodide, but they b e ratier expensive and verv noisonous.

~ n b t h e r example is the reaction of potassium hexacy- anoferrate(I1) with hydrated iron(II1) chloride. The mixture is first yellow, then green, and finally dark blue, due to the formation of Prussian Blue. (These reactions among solid salts are not sharply distinguishable from the ones mentioned under complex formation.)

Oxidation-reduction reactions. Potassium iodide reacts with copper(II) sulfate pentahydrate. The mixture turns dark brown and smells of iodine. The color and the iodine smell can be made to disappear completely if an excess of sodium sulfite heptahydrate is added and rubbed together with the mixture. I t seems that copper(I1) is reduced to copper(1) by iodide. The iodine (or triiodide) formed in this reaction is in turn reduced by sulfite. (With oxidation-reduction reactions care should be taken to avoid the formation of ex~losive mixtures of the gunpowder type.)

Acid-base reactions. Sodium carbonate decahvdrate (washing soda) reacts easily with oxalic acid and with several other solid organic acids like citric and ascorbic acid. The mixture changes into a foam from which gas evolution can be seen and heard.

In another example, an extremely small amount of white phenolphthalein powder, "diluted" with sodium chloride, reacts with calcium hvdroxide formine a nink mixture.

Complex formation. Hydrated ironi11i) chloride forms a scarlet Daste with sodium acetate. annarentlv due to the for- mationbf the iron(II1) acetate co&pi~x. ~mmonium iron(II1) sulfate dodecahvdrate (iron alum) reacts in the same wav. An almost black mixture results fromthe reaction of each of ihese iron salts with potassium thiocyanate.