A Coherent Conceptual Structure of the Chemistry Curriculum .
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Wobbe de Berrv van Berkel. and Adri H. Verdonk -~ Vos. - Centre of Science and Mathematics Education (CSMEU), Utrecht University, Princetonplein 5, 3584 CC Utrecht, the Netherlands
I n most countries where chemistry is part of the secondary school curriculum the topics and concepts to be taught are laid down, either by the state or by another authority, in a syllabus. If there is not a n oficial document, there is a t least a n implicit consensus that is reflected in textbooks and examination papers. Every chemistry syllabus contains a list of concepts-chemical reaction, element, compound, atom, molecule, etc. There is a hard core in these course outlines that does not differ much from country to country and that has not changed essentially over a long period of time (13). Does a list of concepts determine the content of a chemistry course? The problem is that chemical concepts are strongly interrelated. The meaning of a specific concept is determined largely by the way it is related to other coucepts, either explicitly or implicitly. Acid and base are interrelated concepts, as are substance and chemical reaction. Relations between concepts are a s important a s the concepts themselves because they provide much of the context in which each concept acquires a specific meaning. Yet most course outlines in secondary education confine themselves to listing concepts, mentioning few if any explicit relations between them, let alone providing the reader with a coherent conceptual structure of the curriculum a s a whole. The same is true for textbooks and even for curriculum studies (4). The need for coherence often is a neglected aspect in curriculum discussions.
sists of interrelated elements. The elements in our structure are chemical concepts. We define a curriculum structure a s coherent if it is, in its entirety, in agreement with a specified objective. In designing our structure we decided to limit it to the chemical content of t h e curriculum. leavina teachine strategies and theories of learning, important as they ma; be for actual implementation of a curriculum, aside. This design allows the structure to be combined &th various teaching or learning- theories. - strategies In designing and describing our conceptual structure we chose a chemical point of view in looking a t the chemistry curriculum. We do not, however, claim that this structure also applies to the scientific discipline called "chemistry". In constructing additional relations between concepts we made use of the following criteria: 1. the structure must be chemically correct;
2. it must include all essential chemical concepts that ap-
pear in a standard secondary school syllabus: 3. it must include essential rglations alreadv described in standard textbooks and syllabuses; 4. it must present secondary school chemistry as a coherent
and complete unity. With these criteria in mind, we were able to design a coherent conceptual structure only after accepting two conditions that appeared to be unavoidable. Even then a few ~roblemsremained, some of which will be mentioned be-
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The "Missing Framework
The content of general chemistry .courses is a regular topic of discussions in this Journal (5-9).Maybe the issue of coherence is a different matter for courses in general chemistry a t university level, but it is surprising to find such a cumculum discussion also focus on what specific concents should or should not be taught. while much less attenkon is being paid to how these c&cepts could be embedded i n a n overall concentual structure (1&12). We agree with Spencer (7)that curriculum discussions also should look into a cumculum as a whole and attempt to find and assess a "missing framework" in it. I n a PhD research project a t the CSMEU's Chemical Education D e ~ a r t m e n ta t the University of Utrecht, the Netherlands, we aim a t explicating a con~eptualstructure of chemistry curricula. We focus on chemical concepts rather than- topics. Textbooks and course outlines from various countries have been analyzed a s part of the project, yielding a number of essential concepts and relations between them, which add up to fragments of a conceptual structure. We found no textbook or other document offering a coherent description of the essential concepts of the secondary school cumculum a s well a s their mutual relations. This article outlines a curriculum based on a conceptual structure that was desimed. - . startine from available fraements, by constructing additional relations that consolidate these f r a w e n t s into a coherent whole. "Structure" in this article refers to a more or less limited entity that con-
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The first condition was that. contrarv to criterion 4. the structure had to cover not on$secondary school chemistry hut also general chemistrv a t the level of tertiarv education in o;der to become a coherent whole. This Buggests that secondary school chemistry is not a complete subject in its own right but that it is inseparably linked to further education in chemistry. If this is correct, our work also may have relevance for the discussion on the general chemistry curriculum in this Journal. The second condition we had to accept was that chemistry must be taught within a strictly scientific context, in which students are being treated a s if they were future chemical researchers receiving the necessary education. Moreover, this context is assoc&ted with a sp&c view on science and science education that seems to stem from the 19th century and that sees science a s providing descriptions, explanations, and predictions of natural phenomena. I n this view chemistry, like other scientific disciplines, is characterized by a demarcated field of research. a set of research methods far investigatingthis field, and an ever increasing amount of knowledge in the form of true
facts and theories obtained by applying these methods. The general objective of the curriculum is, therefore, that students learn to explain and to predict chemical phenomena by studying the facts, theories, and methods pros chemical knowlduced bv ~redecessors(3).A ~ d i c a t i o n of edge insociety admittedly a; important, but they appear Volume 71
Number 9 Sentember 1994
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in the marein ., of the curriculum. i.e.. usuallv at the end of a chapter. Specialization is possible only after one has covered the whole field of introductorv and general chemistry The section below outlines a chemistry curriculum based on our conceptual structure. The scope of this article does not permit too many details; more information is available on request. This curriculum may be seen as an extended version of the traditional curriculum, not in the sense that more concepts are added but that it offers more coherence between concepts. Introducing a Mystery
The curriculum begins by stating that chemistry is a science, assuming implicitly that students accept their role as would-be scientists. The next step is to define chemical Drocesses. as o ~ ~ o s to e d~hvsical~rocesses.as the subiect bf chemistry. chemicafp;ocess,'or chemkal reactiog, is then defined as a conversion of substances into other substances. Chemical reaction and substance are, therefore, the key concepts in our structure. The chemical substance concept, necessary to define the chemical reaction, is a very specific one. It is limited to pure substances and as such it differentiates between, for instance, white and red phosphorus but not between water and ice. The definition of a pure substance, which is left implicit in most textbooks, in its turn, presupposes the reaction concept (13). The definition of a chemical reaction requires that substances be distinguished from one another. This is possible because each Dure substance is thought to be characterized by its own unique set of physical and chemical properties. Therefore substance DroDertv is another i m ~ o r t a n cont cept that is related toiudstaice as well as to Eeaction. The chemical reaction is introduced with a few examples such as the preparation of iron sulfide from iron and sulfur, the combustion of magnesium ribbon, the electrolysis of water or the thermolysis of ammonium dichromate. In these examples the reaction products clearly differ from the reactants. At this early stage of the curriculum students can handle only substance properties that are directly observable or measurable. Most of these properties are bulk properties of the pure substance-olor, melting point, boiling point, density, combustibility, etc. Isolation of pure substances from (reaction) mixtures is, therefore, an important technique ifone wants to be able to recomize a chemical reaction. This is why a chapter on separation techniques is included early in the curriculum. Some of the techniques introduced are distillation, filtration, extraction, and chromatography.They enable students to distinguish between chemical substances, which is a prerequisite for deciding whether a certain process is a chemical reaction or not. This is the first goal of the curriculum. Although students learn to recognize a chemical reaction, the reaction itself is still a mvsterious phenomenon. If two substances are brought together it is impossible for students at this stage of the curriculum to explain or predict the reaction ~roductsor even to tell whethkr a reaction will take place a t all. Because chemical change has been made the ohiect of investigation in chemistrv. students and teachers no& face the challenge of solving &is mystery by making chemical reactions explicable, predictable, and manageable. Closely linked to solving the reaction mystery is the ~roblemof getting a hold on the enormous diversitv of chemical substances and substance properties. The curriculum is built on the two key concepts, i.e., chemical substance and chemical reaction. We build in coherence into our structure by presenting the cumculum as one large auest for the hidden factors that determine chemical change and the creation of new substances.
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Every concept must get its place somewhere within this quest. Every chapter must lift a part of the veil. Solving the Mystery Step by Step
We distinpuish in our auest between a descri~tiveand a theoretical Tor physicochkmical) approach. In the descriptive approach teacher and students map out the territory of chemistry by traveling through it and registering reactions and substances encountered. In the theoretical approach they do not focus on specific reactions or substances but instead trv to formulate in general terms the couditions that a reaction must fulfil order to occur and that a suhstance must fulfil in order to exist. The descriotive approach will be discussed briefly below, takingLsubstances and reactions tocether. As for the theoretical approach, conditions for sucstances and reactions will be discussed separately. I n each of these approaches a distinction may be made between a level of phenomena and a level of particles. Relations between these macro and micro levels are part of our structure but elaborating on them would require at least a separate article and, therefore, will not be attempted here. The descriptive and the theoretical approach are, of course, interrelated. Descriptions of unexpected phenomena will lead to a need for theoretical explanations. On the other hand, as theory grows, descriptions of a higher theoretical level become available ( e . ~ .acid-base . reactions in terms of Arrhenius supersededUbyBr$nsted or Lewis). Evew classification of substances or reactions is. in fact. based on a theoretical viewpoint. In the descriptive approach, which is dominant in textbooks before 1960 (14. 15). both reactions and substances are described indikd;all; and, as theory becomes available, also in a systematic way in categories. In inorganic chemistry the descriptions usually are arranged according to the positions of the participating elements in the periodic table, while organic chemistry has its own taxonomy of homologous series. A systematic presentation includes generalizations in the form of rules, e.g., for precipitation of salts or trends. such as the relation between molecular mass and boiling point of alkanes. Aqualitative discussion of acids and bases and of oxidizing and reducing agents is also a part of this approach. However, only a small portion of the enormous and ever-increasing amount of substances and chemical reactions can be covered in the time available. Besides, dealing with facts and rules without much explanation becomes increasingly unsatisfactory for teachers and maybe also for students. This is an important reason why the emphasis has shifted from descriptive to theoretical chemistry in the 60's and 70's (16). The theoretical approach addresses the question of how to explain and predict the existence of substances and the occurrence of reactions on the basis of chemical theory. A substance is characterized on the macro level by its properties and on the micro level by its molecular structure. Bond formation creates structural units (either molecules or unit cells) with a certain energy and a specific spatial arrangement of atoms as their main features. A pure substance in the chemical sense can he redefined at the micro level in terms of the mutual identicalness of these units (13). Restrictions on Substances
Although millions of substances are known, their number is highly limited by a set of restrictions which, together, purport to explain why there is a set of distinct, individual substances instead of a continuum, i.e., why there is no element between sodium and magnesium, no alcohol between methanol and ethanol, etc.
The first of these restrictions is that all substances are derived from not more than about 90 distinct chemical elements and, in practice, most are derived from an even smaller number. This k e e ~ sthe number of elements and symbols to be learned Githin reasonable proportions. (Imaeine havine to learn chemistw in a world with a thonsand'br a millio> elements!) Secondly, with few exceptions, the number of elements in a compound is two, three, or four, at most. This h i t s the number of symbols that appear in a formula. (Again, imagine a world with dozens of elements in each compound.) In the third place, each combination of elements usually forms few compounds, e.g., there are only three iron oxide formulas (FeO, FenOs, and Fes04) and there is only one chloride of sodium, NaCl (there might have been dozens). Organic carbon compounds are an-exception to t h ~ srule and consequently more than 90 percent of all known compounds are carbbn compounds; secondary school chemistry, however, restricts itself to a limited number of these compounds. In the fourth ulace. numbers in a molecular formula are usually s m a l i i n t e g e r s , with t h e exception of nonstoichiometric compounds and, again, many organic carhon compounds. Fortunately, these exceptions do not dominate the curriculum A fifth restriction that further limits the number of conceivable substances. is the ~rinciuleof ueriodicitv in the periodic table. This &ncipleenabied ~ e n d e l e e v to predict urouerties of eallium. scandium, and eermanium and some bf their com&unds, &cluding the f&ulas of these com~ o u n d slone before such uredictions could be based on a theory of a&mic structure. The Periodic Table is an open system and purely on chemical grounds the number of elements could be much lareer than it is. (Nuclear Dhvsics Drovides restrictions. of most bemist& courses.) The othe; bucthis is not restrictions mentioned above are related to the concept of valency, defined originally in terms of combining proportions of elements and later in terms of valencv electrons and the octet rule. In most textbooks attempis to make these restrictions understandable are based on atomic theory. Concepts associated with structure and properties of atoms, such as bonding (including bond types, bond energies, bond lengths, and bond angles), electronegativity etc. are introduced. The Aufbau principle helps to understand the periodicity in the per~odictable. 1n more advanced courses orbitals and the I'auli principle play an important role. The soecial onsition of carbon in oreanic chemistrv is related to [he ten'dency of carbon atoms to form chains &d rings. This also provides an opportunity for introducing and explaining the various forms of isomerism. The restric$ons are important because they exclude a laree number of conceivable formulas snch as NaCL (171, bu
