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The conceptual content of Table 1 corresponds roughly with the content of the Molecular Conceptions in Table 2; the Macroscopic Conceptions were added as a result of the research. We discovered that many of the most important problems students encountered in trying to reach the goal of using the kinetic molecular theory to explain phenomena did not have to do with their understanding of molecules at all; instead their difficulties arose from incorrect or partially correct ideas about what substances were changing and how they were changing. Thus achieving the goal of the unit required recognition of learning that would have to occur at the macroscopic as well as the molecular level. This issue is discussed in more detail below. The form of Table 2 is also different from the form of Table 1, indicating that the two tables are based on different assumptions about the nature of student learning. Table 1 consists of a list of ideas to be learned, whereas Table 2 contrasts the ideas to be learned (scientific goal conceptions) with ideas that are common among students at the beginning of the unit (naive conceptions). Thus Table 2 depicts the learning of these ideas as a process of conceptual change, rather than simply as a process of adding new knowledge. There were also important differences in the way that the activities or behaviors that students would learn to engage in for the two units were described for the two units. In the Models of Matter unit these activities were described as science processes: Observing and describing, investigating and manipulating, organizing and quantifying, and generalizing and applying. In contrast, the activities students were to engage in for the Matter and Molecules unit were described primarily as applications of scientific knowledge: Describing, explaining, making predictions about, and controlling the world around us. These contrasts are discussed in detail by Berkheimer, Anderson, & Blakeslee (1988). Thus the research on student conceptions led us to a substantially different understanding of what students would have to learn in order to accomplish the main goal of the unit. This new understanding led, in turn, to substantial revisions in both the curriculum or content of the unit and methods of instruction. The instructional changes are discussed by Berkheimer, Anderson, & Blakeslee (1988). This paper focuses on the curricular issues that we encountered while revising the unit. Curricular IssuesAmong the curricular issues that we encountered, the most important concerned (a) the development of both macroscopic and molecular conceptions, (b) the epistemological status of molecules, (c) the physical characteristics of molecules, (d) the nature of scientific explanations, and (e) the need to balance scientific elegance and student comprehension. Each of these issues is discussed below. Macroscopic and Molecular ConceptionsAs described above, the main focus of the commercial unit was on using kinetic molecular theory (called "the small particle model") to explain physical changes in substances. Interviews with students revealed that there were often difficulties with their explanations that had nothing to do with their understanding of molecules per se. Students who believed that substances "shrivel up" when heated, for example, had trouble explaining thermal expansion. The idea that all matter is made of molecules was problematic for students who did not believe that gases such as air and helium are matter — or who believed that forms of energy such as heat and light are matter. More general problems were also apparent at the macroscopic level. For example, non-conserving explanations of physical changes were common: Many students believed that substances ceased to exist when they dissolved or evaporated, or that condensing water formed on the spot. Others believed that substances changed mass when they changed state. It was also often difficult for students to decide just what changes in what substances needed to be explained. They explained how sugar escapes from a tea bag immersed in water, for instance, by focusing on what happens to the tea bag when it gets wet rather than on what happens to the sugar when it gets wet. In general, we discovered that these sorts of difficulties at the macroscopic level were often important barriers to the development of successful molecular explanations. Thus teaching students to use the kinetic molecular theory generally involved also teaching them how to analyze a situation at the macroscopic level. These considerations led to two kinds of modifications in the unit content. First, we carefully considered our choices of examples and problems for the students and used only those that did not involve excessive ancillary teaching about macroscopic concepts. Second, we added a number of macroscopic conceptions to the unit content, as indicated by the organization of Table 2. In the revised unit almost as much time is devoted to teaching about macroscopic properties of substances and how they change as is devoted to teaching about molecules and their properties. As a result of this change of emphasis, students using the new unit were much more successful in making connections between ideas about molecules and observable events in the real world. The Epistemological Status of MoleculesThe commercial unit opened with a series of lessons in which students considered the effectiveness of various models — the "push model," the "shake model", the "sticky model" and the "small particle model" — in explaining phenomena involving mixing and dissolving. The purpose behind this approach was to help students understand two important points about how scientific theories are developed and tested. First, scientific knowledge is inherently uncertain; rather than discovering the "truth" about the world, scientists invent alternative models or hypotheses. Second, the best model is selected on an empirical basis; it is the one that is most successful in making predictions about phenomena. Although this seemed like observations of classroom teaching and our discussions with the collaborating teachers led us to question the appropriateness of the approach for both pragmatic and theoretical reasons. At a pragmatic level, our observations revealed that the alternative models seemed to create conceptual confusion without producing epistemological insight for most students. There was little evidence that they were ready to understand the first of the two intended points: The inherent uncertainty of scientific knowledge. They saw statements about the world as being true or false, fact or fiction. They could see that they might not know whether a given idea was true or false, but they were not ready to accept the existence of a class of statements about the world whose truth or falsehood might not be decidable. The students' ways of deciding which model was best revealed an even more troubling set of issues concerning the second intended point of the alternative models (that the best model is chosen on the basis of empirical evidence). Many students found the push model, which suggested that substances mix because the solute "pushes" its way into the solvent, more satisfying and easier to use than the more complicated small particle model, and they preferred to continue making adjustments to the push model rather than reject it for the small particle model. The remainder of the unit, however, was devoted to the development of the small particle model, so teachers who wished to continue with the unit as written were faced with the uncomfortable problem of convincing their students (or simply telling them) that they had drawn the wrong conclusions from the evidence. These practical difficulties led us to a number of questions about the validity of the epistemological points themselves. With regard to the first point (the uncertainty of scientific knowledge), for example, just how uncertain are we about the existence and nature of molecules? Are they merely useful theoretical constructs or are they actual little objects that we can describe in some detail? The answers to these questions are not as clear as the original unit's treatment of the alternative models suggests. While it is true that explanations in the physical sciences depend heavily on invented theoretical constructs such as force, energy, or velocity, explanations in the biological sciences often work quite differently. Biologists often explain the workings of a system in terms of the workings of subsystems that they believe to be quite real, not mere theoretical constructs. We explain the movement of a person's arm, for instance, in of the actions of her muscles, which we believe to be real even though we cannot see them. In turn, we explain the actions of the muscles in terms of actions of muscle cells, the actions of muscle cells in terms of the actions of their organelles, and the actions of the organelles in terms of the molecules of which they are composed. At what point in this chain of explanations have we crossed the line between real subsystems and theoretical constructs? We also came to consider the second epistemological point (that models are chosen on the basis of evidence) as problematic. The evidence considered in the first part of the unit had to do with mixing and dissolving. Generations of scientists before Dalton had observed those phenomena without being convinced of the validity of the small particle model, so why should that evidence be convincing to students? In fact, as historians of science such as Kuhn (1970) and Toulmin (1961, 1972) have pointed out, the process of selecting among competing models is far more complex than the treatment of the alternative models suggests. Theories are judged not on the basis of particular "key experiments," but on the basis of extensive bodies of evidence that accumulate over long periods of time and on the basis of the role that they play in the "intellectual ecology" of the scientific community. Thus we concluded that, in trying to represent certain aspects of the scientific enterprise, the alternative models presented a picture that was distorted in other respects. Ultimately, these considerations led us to drop the alternative models in the revised version of the unit. The revised unit presents molecules not as theoretical constructs but as real entities, the "pieces" of which matter is made. We did not emphasize either the uncertainty of our knowledge about molecules or the nature of the evidence on which our belief in their existence is based. The treatment in the new unit did retain another important epistemological message: That the kinetic molecular theory consists of not simply facts or propositions about the nature of substances, but intellectual tools that can be used do describe and explain the properties of substances and how they change. In as much as students appreciate the tool-like character of scientific theories, the issue of their truth becomes less salient. Our criteria for judging tools focus on usefulness more than truth: We keep a tool as long as it is useful and discard it when a more useful tool becomes available. This resolution of the problem has not proved to be entirely successful. Upon discovering how small we believe molecules to be, some students raise questions about how we can know that they exist at all. The unit in its present form does not answer those questions particularly well, so some students remain dissatisfied. Perhaps, though, this is not an altogether undesirable state of affairs. The students are raising important epistemological questions themselves rather than being confused by the unit's treatment of issues that they are not intellectually ready to consider. Physical Characteristics of MoleculesIn an effort to avoid unnecessary complexity, the commercial unit avoided the word "molecules" entirely. It referred instead to "small particles," and did not distinguish one kind of small particle from another. In the end, it appeared that this attempt at simplicity caused more problems than it resolved. Some students believed that there was a single generic type of "small particles," which floated in various substances like specks of dust in the air (in fact some students believed that specks of dust were the small particles that the text referred to). Other students thought that there was a single generic type of small particles of which all substances were made. The revised unit therefore includes more details about the nature and structure of molecules than its predecessor. Molecules are referred to by name, and the structures of a half dozen sample molecules are introduced (water, alcohol, sugar, oxygen, nitrogen, carbon dioxide), not so that they can be memorized but so that students can appreciate how and why molecules are different from one another. The small size of molecules is also emphasized; molecules are pictured next to other small objects with which students are familiar (cells and specks of dust) so that students can appreciate that molecules are much smaller. This additional information about the physical properties of molecules helped many students to resolve issues students using the old unit. They understood clearly that they could not see individual molecules, and that motion need not be visible in a substance for its molecules to be in motion. They were also more likely to propose explanations for physical changes involving the motion and arrangement of molecules, rather than their transmutation or destruction. The Nature of Scientific ExplanationsPrevious research by several investigators (e.g., Hesse & Anderson, 1988; Soloman, 1982) has revealed that students often have trouble understanding what constitutes an acceptable scientific explanation. When asked to explain something, they are likely to rely on analogies or descriptions or simple repeated phrases and definitions. In contrast, scientists consider an explanation to involve a detailed account of the relationship between the system or phenomenon to be explained and relevant scientific theories. Some students discovered, for example, that the phrase, "The particles move faster and farther apart," could be invoked in a variety of situations, such as those involving thermal expansion, melting, and boiling. What they did not appreciate was that simply invoking this phrase did not constitute an adequate explanation unless they specified which molecules they were talking about, why they were moving faster and farther apart, what else might be happening to them, and how the molecular process was connected with the observable phenomenon. In the end, these students did not come to appreciate the power or the importance of scientific explanations, seeing them instead as involving the trivial repetition of phrases or the use of their own common sense. In the new unit this difficulty was addressed by developing a heuristic to guide students in their attempts to produce coherent scientific explanations. The heuristic reminded students that good scientific explanations should (a) identify the substance changing and specify how it is changing, and (b) explain the change in terms of molecules and their motions. This heuristic helped students to decide for themselves when they had developed an adequate explanation and encouraged them to develop more complex and complete explanations. If also provided a basis for communication between teachers and students about the quality of various suggested explanations. Thus their understanding of scientific content was enhanced by the inclusion of more general information about the nature of scientific explanations. Balancing Scientific Elegance and Student ComprehensionScientific explanations tend to be elegant and parsimonious. Often the most elegant are the most scientifically advanced, the most abstract, and the most difficult for students to understand. Thus the design of teaching materials involves dealing with a constant tension between elegance and comprehensibility. Theories and explanations that appear simple and elegant can sometimes be very difficult for students to understand because the simplicity is embedded deep within the complex conceptual ecology of a scientific theory. In these situations it is sometimes necessary to settle for less elegant and satisfying explanations that are more accessible to students. For example, the idea that molecules attract each other played an important role in the original unit, and it was used to explain a variety of phenomena, including condensation, freezing, and surface tension. Unfortunately, understanding and using this idea turned out to be very difficult for most students. It answered a set of questions that they did not spontaneously ask and it was useful only to students who already understood many other complex ideas. It did not occur to most students, for example, to wonder why molecules would cling together when they started to move more slowly; the students were willing to accept that they did. Ultimately, the surface tension activities were dropped from the unit. While the idea of attraction was used in the text explanations of condensation and freezing, it was not emphasized. SummaryWe have described a series of curricular decisions that were made during the development of the Matter and Molecules unit. While the general goal of the unit remained the same as the general goal of its predecessor (to help students use the kinetic molecular theory to explain the nature of matter and physical changes in matter), there were many changes at a more detailed level. Some content was added, such as the macroscopic conceptions, the heuristic guiding the development of explanations, and information about physical characteristics of molecules. Other content was dropped, such as the alternate models and the activities involving surface tension. There were many other changes in approach or emphasis. Information about students' conceptions and their learning was not the sole determining factor in any of these decisions, but it played an important role in all of them. Because this information was available to us, the curricular decision-making process was better informed and more constrained than it otherwise would have been. ConclusionIn as much as there is a generally accepted science curriculum in the United States, it is a product of market research by publishers, tinkering and guesswork by curriculum developers, and decisions made by various organizations about what to include on achievement tests. As a result, most students experience science in schools as a mishmash of relatively unconnected topics and understand little of the content that is "covered." A fundamental rethinking of the science curriculum is clearly in order, and it seems that research on cognitive structure and conceptual change could play an important role in such a process. Yet so far this has not occurred, for a number of reasons. Conceptual change researchers have generally focused on individual topics that are accepted parts of the curriculum (e.g., heat, or photosynthesis, or genetics). Their investigations have almost always led to the conclusion that curriculum and instruction in those topics need to be drastically revised, but relatively little has been written that focuses on the relationship between conceptual change research and the curricular decision-making process, either within topics or on a larger scale. This paper is our attempt to make explicit some of our thinking about those relationships. Curricular decision-making is a difficult, time-consuming process involving many factors that cannot be addressed by research. In particular, research can do relatively little to help us decide what scientific knowledge is most valuable. No individual could ever master all scientific knowledge, so how can we decide which knowledge is most important for the students in our schools? There are no perfect answers to this question. What our students will most need to know when they are adults will depend on social and economic developments that we cannot fully foresee, on the particular roles that they will play in our society, and on values and beliefs that not all people share. To make wise judgments about content therefore requires judgments about social, political, and economic issues that cannot be fully analyzed by research. Nevertheless, we hope that the issues discussed in this paper illustrate how conceptual change research can inform curricular decisions. The research helped us to become aware of a number of curricular issues and to resolve them in productive ways. It helped us to address problems of curriculum using strategies that were informed by knowledge of students' cognition and development. These informed strategies made curriculum development more like engineering and less like the trial-and-error tinkering that has prevailed in the past. We hope that the approach to the curriculum development described in this paper will stimulate fruitful discussions among researchers, science educators, and curriculum specialists. Hopefully, such discussions can lead to the evolution of a science curriculum that is more coherent and more understandable by students than the current curriculum. REFERENCES[This list does not include Mas & Perez or most of the textbooks referred to in the introduction.] Anderson, C.W., Sheldon, T.H. , & Dubay, J. (in press). The effects of instruction on college non-majors' conceptions of respiration and photosynthesis. Journal of Research in Science Teaching. Berger, C.F. , Berkheimer, G.D. , Lewis, L.E. , & Neuberger, H.T. (1979). Houghton-Mifflin Science. Grades K-6. Boston: Houghton Mifflin Company. Berkheimer, G.D. , Anderson, C.W. , & Blakeslee, T.D. (1988, April). Using a new model of curriculum development to develop a matter and molecules unit. Paper presented at the annual meeting of the National Association for Research in Science Teaching, Lake Ozark, Missouri, April 5-8. Berkheimer, G.D. , Anderson, C.W. , & Blakeslee, T.D. (1988). Matter and molecules teacher's guide: Activity book (Occasional Paper No. 122). East Lansing: Michigan State University, Institute for Research on Teaching. Berkheimer, G.D. , Anderson, C.W. , Lee, 0., & Blakeslee, T.D. (1988). Matter and molecules teacher's guide: Science book (Occasional Paper No. 121). East Lansing: Michigan State University, Institute for Research on Teaching. Cohen, M.R. , DelGiorno, B.J. , Harlan, J.D. , & Staver, J.R. (1984). Scott, Foresman science. Glenview, Illinois: Scott, Foresman, and Company. Feynman, R.P. , Leighton, R.D., & Sands, M. (1963). The Feynman lectures on physics (Vol. 1). Menlo Park, CA: Addison-Wesley. Gabel, D.L., Samuel, K.V. , & Hunn, D. (1987). Understanding the particulate nature of matter. Journal of Chemical Education, 64, 695-697. Hesse, J.J. , & Anderson, C.W. (1988, April). Students' conceptions of chemical change. Paper presented at the annual meeting of the American Educational Research Association, New Orleans, LA. Hollon, R.E. , & Anderson, C.W. (1985). The curricular significance of students' conceptions of heat and temperature. Paper presented at the annual meeting of the American Education Research Association, Chicago, IL. Kromondy, E.J., & Essenfeld, B.E. (1988). Biology: A systems approach. Menlo Park, CA: Addison-Wesley. Kuhn, T. (1970). The Structure of scientific revolution, 2nd edition. Chicago: University of Chicago Press. Lee, 0. , Eichinger, D. , Anderson, C.W. , Berkheimer, G.D. , & Blakeslee, T.D. (1989, April). Changing middle school students' conceptions of matter and molecules. Paper presented at the annual meeting of the American Educational Research Association, San Fransisco. Leyden, M.B. , Johnson, G.P., Barr, B.B. (1988). Introduction to Physical science. Menlo Park, CA. Addison-Wesley. Mallinson, G.G., Mallinson, J.B. , Smallwood, W.L. , & Valentino, C. (1985). Silver Burdett science K-6 text series, grade 3. Morristown, NJ: Silver Burdett. Mas, C.J.F. , Perez, J.H., & Harris, H.H. (1987, July). Parallels between adolescents' conception of gases and the history of chemistry. Journal of Chemical Education, 64(7), 616-618. Metcalfe, H.C. , Williams, J.E., Castka, J.F. (1986). Modern chemistry. New York: Holt, Rinehart, and Winston. Novick, S. , & Nussbaum, J. (1981). Pupils' understanding of the particulate nature of matter: A cross-age study. Science Education, 65(2), 187-196. Osborne, R.J., & Cosgrove, M.M. (1983). Children's conceptions of changes in the state of water. Journal of Research in Science Teaching, 20(9), 825-38. Otto, J.H., & Towle, A. (1985). Modern biology. Holt, Rinehart, and Winston. Smith, E.L. , & Anderson, C.W. (1986, April). Middle school students' conceptions of matter cycling in ecosystems. Paper presented at the annual meeting of the National Association for Research in Science Teaching, San Francisco. Solomon, J. (1983). Thinking in two worlds of knowledge. Paper presented at the International Seminar of Misconceptions in Science and Mathematics. Cornell University, Ithaca, NY. Toulmin, S. (1961). Foresight and understanding. Great Britain: The Anchor Press, Ltd. Toulmin, S. (1972). Human understanding. Princeton, NJ: Princeton University Press. Papers And Teaching Materials
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- Nature of matter
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