nakhleh 1992 why some students don't learn chemistry

Why Some Students Don't Learn Chemistry

Chemical Misconceptions

Mary 6. Nakhleh Purdue University, West Lafayette, IN 47907

Many students at all levels struggle to learn chemistry, but are often unsuccessful. Diswvering the reasons has been the target of many studies. One possible answer that is beginning to emerge is that many students are not con- structing appropriate understandings of fundamental chemical concepts from the very beginning of their studies (I). Therefore, they cannot fully understand the more ad- vanced concepts that build upon the fundamentals.

In this article, I first present a cognitive model of learning chemistry. Then I discuss students' chemical misconceptions (their inappropriate understandings) in terms of a fundamental wncept-the particulate, kinetic nature of matter. Finally, the implications of these findings for instruction are considered.

A Cognitive Model of Learning Research in students'conceptual knowledge of chemistry

is based on a model of learning in which students construct their own concepts (2.3). According to the cognitive model of learning, during instruction learners generate their own meaning based on their background, attitudes, abilities, and experience.

The Learning Cycle

Learners selectively attend to the flow of information presented, and their prewnceptions determine the information to which they pay attention. Then the brain actively interprets this selected information and draws infer- ences based on i t s stored information. The newly generated meanings are then actively linked to the learner's prior knowledge base.

Thus, learning is viewed as a cyclical process. First, the new information is compared to prior knowledge. Then it is fed back into that same knowledge base.

Cognitive Structures

Thus, acwrding to the wgnitive model, students build sensible and coherent understandings of the events and phenomena in their world from their own point of view (3). In this paper, these coherent understandings are referred to as cognitive structures (4). Since these coherent understandings are in place, words such as "atom" and "nentral- ization" are actually labels that stand for elaborated cognitive structures stored in the brain (3).

Concepts and Propositions

These elaborated cognitive structures are themselves composed of interrelated wncepts. Each concept itself is formed by a linked set of simple, declarative statements called propositions that represent the body of knowledge the student possesses about that concept (4). An example of a proposition is the statement "An atom contains a nucleus."

Concepts, therefore, are considered to be the set of propositions that a person uses to infer meaning for a particu- lar topic, such as the nucleus of an atom. These wncepts are then linked with the students' other concepts to form

integrated cognitive structures of chemical knowledge. The information students use to wnstruct their concepts comes from two sources: public knowledge, as presented in texts and lectures; and informal prior knowledge from ev- ewday experiences, parents, peers, commercial products, and the common meanings of scientific terms (41.

Misconceptions

Since students do build their own wnce~ts. their con- structions of a chemical concept sometimes h&r from the one that the instructor holds and has trled to oresent. Gar- nett et al. (5) state that these different wncepts have been variously described by different researchers as precoucep- tions (6), misconceptions (6), alternative frameworks (71, children's science (a), and students' descriptive and ex- planatory systems (9).

In this paper the term "misconception" means any concept that differs from the commonly accepted scientific understanding of the term. Once integrated into a student's cognitive structure, these misconceptions interfere with subsequent learning. The student is then left to connect new information into a cognitive structure that already holds inappropriate knowledge. Thus, the new information .. . cannot be connected appropriately totheir cognitive structure, and weak understandinm or misunderstandings of - . the concept will occur.

Current Work on Chemical Misconceptions

Most of the work that has been done on misconceptions in chemistry was done relatively recently-in the 1980's. Misconceptions in physics and biology have been more in- tensively studied. Thus, misconceptions in chemistry rep- resents a fertile field for investigation.

This article synthesizes recent findings about the chemical misconceptions of students from the elementary and middle school level through the undergraduate level. Most of the misconceptions that have been identified reveal a weak understanding of the currently accepted model of matter. In this model, matter is composed of small, mobile particles such as atoms, molecules, and ions. Thus, the particulate and kinetic aspects of the current model of matter are used as a framework for presenting the findings of the studies.

Although this description of the cognitive model of learn- ingis brief, it can be seen that this model is a powerful tool that can aid in developing and understanding cognitive structures. This model is a part of Bodner's theory of con- structivism that is dealt with in more detail in ref 10 than is possible in this article.

Student Conceptions of the Particulate Nature of Matter Students of all ages seem to have trouble understanding

and using the scientifically accepted model that matter is made of discrete particles that are in constant motion and have empty space between them (11, 12). Indeed, an acceptable concept of the particulate nature of matter lays the foundation for understanding many chemical concepts:

Volume 69 Number3 March 1992 191

chemical reactions; the effects of pressure, volume, and temperature on gases; changes &state; dissolving; and equilibrium (13). Unfortunately, many students from all age groups appear to view matter as being made of a continuous medium that is static and space-filling.

Misconceptions of Matter as a Continuous Medium

In one of the earliest studies on student understanding of the particulate nature of matter (11, 121, students from elementary school to the university level were tested concerning their acceptance of the particle model as it applies to gases. The results revealed that over half ofthe students from junior high to senior high to university level held concepts that were consonant with a perception of matter as a continuous medium, rather than as an aggregation of particles.

Differential Acceptance

The authors also present evidence that the components of the particulate model of matter were differentially accepted:~he most readily accepted parts of the model were those closest to observable phenomena.

For example, the representation of the liquefaction of gases as a coalescing of particles was accepted by a t least 70% of the students at the junior high level and beyond. Here the particle explanation does not conflict with observable bulk phenomena. However, only 40% of the students in the same group accepted the concept that particles in the gaseous phase have empty space between them. This concept is not obvious from observable bulk phenomena.

Grade 9

Krajcik (14) interviewed grade 9 students and asked them to draw how the air in a flask would appear if they could see it throueh a verv wwerful maenifvim elass. He found that 14 of &e 17 s s e n t s held aconknio& model of matter. These students did not draw air as comvosed of tiny particles. Instead, they simply drew wavy lines to represent the air in the flask.

Grade 10

Ben-Zvi, Eylon, and Silberstein (15) used a questionnaire to investigate the beliefs about matter held by 300 grade 10 students who had been studying chemistry for half of the academic year. The questionnaire asked students to comoare the orooerties of two atoms: one taken from a piece if copper kr;, and one that had been isolated from the eas that formed when the comer wire vaoorized.

~ e a r l y h a l f of the students believed 'that the buik properties of the substance-such as electrical conductance, color, and malleability-were also properties of a single atom. Apparently, although the students could use the terms "atom" and "molecule", they could not relate these terms to the particulate model of matter. This indicates that the students still held their older, continuous model of matter. They had merely added the particulate model to their continuous one.

Grade 11

Nakhleh (16) interviewed grade 11 chemistry students who were in the last quarter of the academic year. These students had recently completed a unit on acids and bases. In this study, it appeared that 20% of the students still held a simplistic, undifferentiated view of matter.

When asked how a solution of an acid or a base would appear under a very powerful magnifying glass, these students drew waves, bubbles, or shiny patches. Figure 1 is a revresentation of a solution as viewed from this continu-

Figure 1. Arepresentation of students'concept of the microscopic nature of a solution of HCI.

Misconceptions of Atoms and Molecules

Grade 12

Griffiths and Preston (171 interviewed grade 12 Cana- dian students to investigate their understanding of the concepts of a molecule and an atom. The students were di- vided into three groups-"academic science", "academic nonscience", and "nonacademic nonscience". Griffiths and Preston identified 52 misconceptions. Among these misconceptions, the five listed below were held by half the students in the sample.

That molecules are much larger than they probably are. That molecules of the same substance may vary in size. That molecules of the same substance can change shapes in

different phases. That molecules have different weights in different phases. That atoms are alive.

Figure 2 is a representation of the common misconception that molecules expand when they are heated.

Figure 2. A representation of students'concept that molecules expand when heated.

In addition, the "academic science" group exhibited another set of misconceptions to a far greater degree than the other groups. Specifically, 30-70% of the academic science group held the following five misconceptions.

That water molecules were composed of solid spheres. That pressure affects the shape of a molecule. That molecules expand when heated. That the size of an atom depends on the number of protons it

has. That collisions between atoms alter atomic sizes.

GriEths and Preston argue that these misconceptions could have risen as a result of instruction.

University Level

At the universitv level. Cros et al. (18) interviewed first- year undergraduates regarding their conceptions ol'atoms. Thev found that students were aenerallv auite successful

ous perspective.

192 Journal of Chemical Education

in naming the parts of an atom or a nucieui. However, the

students were much less successful when they attempted and Harris (23) found that many Spanish students, rang- to describe the interactions of these particles. The students tended to invoke a simplistic Bohr model of the atom in their explanations.

Cros et al. interpret these fmdings to mean that the students' knowledge tended to be formal and qualitative, "with a worrying lack of wnnection with everyday life". A followup study (19) found that students'ability to explain the interactions of subatomic particles had improved only slightly despite a year of university study

Misconceptions of Molecules and Intermolecular Forces

Grade 12

Students apparently have similar difficulties with com- prehending the bonding and structure of covalent molecules (20). Peterson and Treagust used a paper and pencil test to study the understanding attained by grade 12 chemistry students concerning simple covalent molecules, such as HF. They identified eight misconceptions that dealt with bond ~olaritv. molecular shaoe. molecular DO- larity, intermolechar forces, and the octet rule.

Within these categories, 2&34% of the students held at least one misconception. The data indicate that 74% of the students could not correctly apply valence-shell electron- pair repulsion theory to identify structures of molecules. For examole. 25% considered only the re~ulsion of bonding electron &I&, and another 22% eonsidered only the effect of nonbonding electron pairs. Another 27% decided that bond polarity determined the shape of a molecule. How- ever, 78% ofthe students could correctly answer a test item designed to test their understanding of the principles of this theory.

Also, the students tended to identify intermolecular forces with the covalent bond within the molecule. They did not seem to be aware of the general difference in mag- nitude that exists between the strength of a covalent bond and the strength of an intermolecular force. A number of students also believed that the number of electrons in the valence shell of a nonmetal atom equals the number of w- valent bonds formed by that atom.

Misconceptions of Phase Changes

Consistent with their hazy ideas about atoms and molecules. students also aooear to have difficult^ exdainina phase changes. ~ s b o k e a n d Cosgrove (21) fo&d ihat stul dents, rangingin age from 8 to 17 years, described the bubbles formed by boiling water as being made of air, oxygen, or hydrogen. Many also had great difficulty in explaining how a saucer held over the boiling water became wet and why it dried off when it was removed from the steam. In- terkstingly, Osborne and Cosgrove report that the students could ~enerally use the wrms "condensation" and "evapo- ration? .owever, under further questioning, the students could not explain what these terms meant.

Bodner (22) administered a conceptual knowledge test to entering chemistry graduate students over a three-year period. His data indicate that even some graduate students, who majored in chemistry, may still have difficulty understanding some concepts. For example, one of the questions told students to assume that a beaker of water has been boiling for one hour. The students were then asked to state tce composition of the bubbles rising to the surface. Out of 120 students. 25% reported that the bubbles were made of air or oxygen or hydrogen.

Misconceptions of Gases

Work on students'conceptions of gases also supports the assertion that many students, across a wide range of ages, hold an inappropriate model of matter. Furio Mas, Perez,

ing in age from 12 to 18 years, hild an Aristotelian view of gases as weightless substances. Therefore, they wuld not correctlv oredict the weicht of a sealed container in which a liquidwas evaporated."students believed that the gases had lost mass and weight and that this was the reason they rose. The authors concluded that one of two explanations was oossible: Either these students could not com- prehend .j&etic theory, or they understood the theory but could not apply it to explain the behavior of gases.

Stavy (24) corroborated these findings and determined that students acquire the full particulate, kinetic model of a gas slowly-usually one to two years after the subject has been taught during formal instruction. Students in grades 4-7 phmari~y explained gases in terms of exam- ples. Students in grades 7 and 8 often referred to gases as a form of matter, even though they had been taught the particulate theory of matter in grade 7. However, in grade 9, students began to explain gases in terms of the particulate theory of matter after a two-year time lag.

Stavy also notes that the students did not apply the particulate model consistently. Students apparently found it dimcult to explain solids and liquids in terms of the particulate model, but they could do so for gases. She suggests that the particulate model for gases is less counter-intu- itive, and thus more understandable, than the particulate model for solids or liquids.

A series of studies have investigated concept learning as it oertains to eases (25-27). These studies involved univer- siiy freshmanufrom three universities from the East Coast, the Midwest. and the West Coast. In eachcase. the number of students kho could solve traditional gas law or stoichi- ometry questions was much higher than the number who could answer the conceptual questions. The differences in performances were generalls large. - For example, on one stoichioietry problem 66% of 323 students could answer a traditional question, but only 11% could answer the conceptual question. Students were not able to move from their algebraic knowledge of gas laws to - . a particulate model of gas&.

Students' Conceptions of the Kinetic Aspects of the Particulate Model of Matter

Research is also beginning to show that many students also hold a static, rather than kinetic, conception of the particulate model of matter. The evidence for this assertion is that students have been shown to encounter difficulty in the following three areas.

Students a h n are unable to state that balanced chemical equations represent the rearrangement of atoms (28,291.

Students have difficulty in recognizing and describing in- stances of obvsical or chemical chance (2932). . "

Students envision chemical equilibria and steady state as essentially static conditions (33, 34).

Misconceptions of Chemical Equations

Many students perceive the balancing of equations as a strictlv algorithmicexercise. Yarroch ,281 interviewed high schooich&stry students on how they balanced the xi&- ple equations used to describe reactions such as

N, + H, + NH,

These students were ranked by their teachers as A and B students, and they were interviewed during the last quarter of the school sear.

All of the students succe~sfully balanced the equations. However, half of them wuld not draw a correct molecular diagram to explain the equations in the microscopic system. Although the unsuccessful students were able to draw diagrams with the correct number of particles, they

Volume 69 Number 3 March 1992 193

seemed unable to use the information contained in the coefficients and subscripts to construct the individual molecules. These students represented 3Hz as

000000

rather than as

00 00 00

Figure 3 illustrates students'lack of understanding of the DurDose of coefficients and suhscri~ts in formulas and bal- ;need equations.

Figure 3. A representation of students'concept of the microscopic na. ture of the reaction between nitrogen and oxygen.

Ben-Zvi, Eylon, and Silberstein (29) agree that balancing and interpreting equations is a formidable task. As an example, they performed a task analysis on the combustion of hydrogen, as represented by the equation

2HzW + Oz@) + 2HzO(g)

Thev areue that an a m r o ~ r i a t e internretation of this " - A. . equation requires that a learner understand many things: the structure and physical state of the reactants and products, the dynamic nature of the particle interactions, the auantitative relationshins amone the ~articles. and the iarge numbers of particl& involved.

A

Misconceptions of Chemical Change

Static us. Dynamic Models

Many students also invoke static models to explain chemical chanees. Andersson (30) studied students. rang- - .. mg in age from 12 to 15 years, from Sweden where chem- i s t ~ instruction starts in made 7 or 8. At least 90ci ofthe stuients had studied oxid&ion.

He asked the students to explain the appearance and disappearance of substances in a chemical change. As an example, he asked students

Why doshiny copper waterpipes tarn dull and tarnished?

What happens when a nail ~ U S ~ S ?

He found that the students'answers tended to fall into the following five categories.

1 . It's just that way. I n this raw, studcntr a n simply unin- kre9tt.d in the change. lt'sjust something that they nouce happens.

2. Displacement h m one physieal loeation to another occurs. In this category students envisioned that a coating simply materializes, either from the air, as with rust on a nail, or from the water inside the pipes.

3. The material is modified. In this view, students argue that what appears to be a new substanee is actually the original substance-just in a modified form. An example of this would be when a student thinks that the wpper pipe simply turns dark due to heat. They think that it continues to be the same substance, although it does look different.

4. 'Pansmutation ocrure Students in this category would explarn that steel wuol gam* weight as it burns becawe the steel wool is changed into carbon, which is heavier. In this view, atoms simply change into a new kind of atom.


5. Chemieal interaction occurs. This is a category where acceptable answers would be found. Typically the student would say that oxygen in the air reacted with the copper pipe to form a wpper oxide coating on the pipe. For the other question, they think that the steel wool burned because oxygen wmbined with the inn. At best, only 15% of the students in the study could answer the last problem correctly.

All of the above categories except the last one represent responses that show that the student lacks an understand- ingof the following underlying conceptions.

That matter is composed of particles. That these particles are in constant motion. That these particles can react with each other by breaking or

forming bonds.

A static representation of chemical change was also found bv Ben-Zvi. Evlon. and Silberstein (29). Thev asked grade i0 students, &ho bad been studying chemikry for half a vear. to draw what thev thoueht the followine elec-

They found that 58% of the students drew static represen- tations. Only 38% drew any kind of dynamic representation. In fact, one student specifically noted on the drawing that the "2" in front of the "K" didn't mean anything molec- ularly because it was used for balancing purposes only!

Additive Changes

Ben-Zvi, Eylon, and Silberstein (29, 35) also note that some students seem to have .an additive model of reaction: Compounds are viewed as being formed by simply sticking fragments together, rather than as being created by the breaking and reforming of bonds.

For example, when asked if NO could be formed by a reaction between 0 2 and Nz, a student explained that they could not because neither O2 nor Nz could be decomposed. This type of answer is consistent with a static model of matter. Figure 3 also illustrates students' misconception that chemical reactions are simply additive.

Chemical us. Physical Changes

Stavridou and Solomonidou (32) studied Greek students, ranging in age from 8 to 17 years, as they attempted to classifv events as nhvsical chanees or cbemical chanees. - Their data indicati that over half of their students incur- rectlv classified a chemical chanec as "no chanw."The au- thor; note that these students seem to use :very static model for these events.

They also report that these students seemed to focus on the "external manifestationd nfthr nhenomena. which led them to incorrect conclusions in thii case. An interesting finding, which has not been reported elsewhere, is that some of the students who did have a concept of change nonetheless seemed to think that only physical changes were reversible. Thus, to them, chemical changes were al- ways seen as irreversible.

Misconceptions Concerning Equilibrium

Sidedness and Dynamism

Gussarsky and Gorodetsky (34) used word associations to probe the understandings that grade 12 Israeli students held of chemical equilibrium. They found that students tended not to perceive the equilibrium mixture as an en- tity; rather, they manipulated each side of the cbemical equation independently, as if it were a balance. These au-

thors soeculate that the method used to teach LeChate- lier's principle, if it is applied by mte, may even strengthen this inabilitv to treat the eauilibrium mixture as a whole.

The studekts also failed understand the dynamic nature of equilibrium. They assumed that reaching a balanced condition, as described in their text, meant that no further reaction was occurring. The authors note that students confused everyday meanings for equilibrium with chemical equilibrium. To the students, "equilibrium" meant physical balance like riding a bicycle, or mental balance, or balance in the sense of weighing. In any of these everyday uses of the word, the state of equilibrium is char- acterized by a static, balanced condition.

They also note that equilibrium problems are often hiehlv abstract. and the algebraic manioulations can be pekokaed by rote. heref fore, students i o not automati- callv understand what mani~ulatine aleebraic svmbols or - - other symbols really means in relation to the actual chemical svstem heine studied. Furthermore. the mi sconce^- tions Eegarding sLdeduess and dynamismseem to be re&- tant to instruction. The authors recommend directly confronting these misconceptions in instruction.

Reaction Rates and Concentrations

Australian high school chemistry students have also exhibited miscouceptions of equilibrium, even aRer instruction (36). In an interview, students were required to ex- pla~n and paph the changes that can occur in the reaction rates and the roncentrations during the following reaction betweennitric oxide and chlorine to form nitrosyl chloride:

2NO(g) + C12(g) $ 2NOCl(g) + heat

The students revealed misconceptions that relate to both the articulate nature of matter and to the dvnamic nature of cAemical reactions. Fully 50% of the students held that the concentrations of reactants and products were gov- erned by a simple arithmetic relationship. Most often they thought that the concentrations of the products equal the conc&trations of the reactants a t eqklihrium. The authors arme that this misconception was based on the fact that they do not understand how the coefficients in a chemical equation are used in the equilibrium expression.

&a&, this finding offers additional evidence that students do not have extensive or securely based knowledge concerning how to regard and apply the symbolism of a chemical equation. Also, over half of the students ex- pressed the belief that when an equilibrium was disturbed, the initial result was that the rate of the favored reaction would he increased and that the rate of the competing re- verse reaction would be decreased. This also implies that students have a poor understanding of the dynamics of an equilibrium system.

Approaching LeChatelier Problems

Finally, Kozma et al. (37) studied the understanding of equilibrium that college freshmen had attained. They gave students from introductorv chemistrv courses a written. constructed-response test chat probedqtheir understanding of equilibrium concepts. Students were also required to verbalize their thoughts as they worked through the test. Kozma et al. used the students' verbal commentary and their written answers to identify two groups of students whose conce~tions of eouilibrium were inconsistent vnth the scientificconceptio~

One group had an acceptable understanding that equilibrium involves a dynamic exchange among the components of the system, while the concentrations are held con- s tant . However, these students could not use tha t knowledge to solve LeChatelier problems.

The other group could manipulate the symbols to solve LeChatelier problems but incorrectly thought that equilibrium meant that there was no dynamic interchange between the components of the system. They maintained that dvnamic interchange occurred onlv when a svstem was stressed and that t6e interchanges-ceased when the new equilibrium point was reached.

Implications of These Misconceptions Creating a cognitive structure of a &mplex body of

knowledge such as chemistry is not easy, and it is small wonder that students from middle school to colleee level find chemistry difficult. Obviously, no amount of Fnstruc- tion will help a student who is not determined to work, but the research presented in this article does have several implications for instruction on anv level.

First. a ~ ~ a r e n t l v there are orofound misconceotions in the mind; bfmany students frbm a wide range oicultures concernine the articulate and kinetic nature of matter. Some of tgese n&conceptions persist even up to the graduate level.

Therefore, educators should help students begin to understand the differences between atoms, molecules, and ions. Students also need help discerning the conditions under which each term is appropriate. Students should be re- minded that if they can't explain a concept in molecular terms, then they really don't understand it.

Second, apparently students do not spontaneously visu- alize chemical events as dynamic interactions. Without an understanding of the kinetic behavior of particles, many topics in chemistry do not make conceptual sense and are learned by rote.

Therefore, students must be helped to realize that certain tonics relate to an underlvine assumotion of a kinetic model . " of matter: the hehawor ofgssrr, phase chances, solution chemistry, equrlibrium, el~ctrwhernistry, and general chemical reactions.

Third, the cognitive model of learning implies that misconceotions can occur when students come for instruction holdiig meanings for everyday words that differ from the scientific meanine. For example. "heat" and "temoerature" - & . are commonly used scientific terms for which students hold persistent misconceptions (3843).

.Therefore, educators should intrbduce scientific terms by emphasizing the differences between the everyday meaning and the more precise scientific meaning.

Fourth. leamine is much more difficult if students must - master different definitions for the same phenomenon. For example, students who take both physics and chemistry might become confused over the opposing views of electrical flow through a circuit (5). The same authors also note that reduction and oxidation can be defined in various terms: as a chame in oxidation number: as a gain or loss of - oxygen; or as thegain or loss of electrons.

Therefore. educators need to be esoeciallv orecise when ex- - . plamnp topm that have multrple definuians. Krajrik 113) has rrwewrd srvrml srudics of traehmg conceptual change that illustrate these points.

A helpful course of action would be to include questions on examinations that specifically probe for misconceptions. This would accomplish two goals. Educators would have a more accurate estimate of students'actual cognitive structures, and students might give more serious thought to understanding the conce~ts. Students would then have a better chance i f becoming meaningful learners of chemistry

Volume 69 Number 3 March 1992 195

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nakhleh 1992 why some students don't learn chemistry

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