Laurence E. Strong Earlham College Richmond, Indiana 47374
Differentiating Physical and Chemical Changes
It is customary for the author of a textbook to begin his writing by a statement of the subject matter with which he plans to deal. Presumably such a statement is important as an aid to the student in beginning the process of differentiating between the essential, the incidental, and the irrelevant parts of what follows. Chemistry is hardly an exception to this kind of organization. Above all, in a book that presents an introduction to chemistrv or a general view-of the subject, the author owes h$ readers some initial delineation of the subject. But what are the characteristics of the subiect we call chemistrv? I n earlier, and possibly simpler, times it became fashionable for chemistry textbooks to distinguish between chemical changes and physical changes. Gradually in recent years doubt has been cast on the propriety of this seemingly straightforward distinction. Professor Walter Genslerl makes a forceful argument for abandoning the distinction between the physical and the chemical on the grounds that it is empty of content. A few years ago, I would have agreed with him, but more recently I have become less confident that there is no valid ex~erimental,intellectual, or pedagogica.1 distinction t o L b e made.' Indeed it now appears clear that chemistry does include a class of phenomena that can be clearly distinguished on experimental grounds from other types of changes. What constitutes appropriate grounds for the distinction is the crux of the issue. Professor Gensler bases his argument on grounds that are essentially theoretical. For each change that he considers he asks whether any bonds can be considered to have been made or broken and if there are he argues that the change is then a reaction. However. the theorv now in use bv chemists is a theory that is I;resumabl; applicable t o matter quite generally. I t is to be expected then that classification based on theory will be extremely broad. Indeed, a major burden of the strategy of a good theory is to provide a set of logical linkages between otherwise diverse phenomena. So the modern theory of matter really attempts to show that phase changes of all kinds can be logically linked both to one another and to mechanical and electrical changes. While the attempt is not wholly successful, it is the most promising one yet conceived. Surely the subject matter of concern to chemists derives from natural phenomena. Biologists, chemists, geologists, and physicists all deal with the same theory, but they are distinguished in their work by the phenomena which engage their attention. So the distinctions, if any, between physical and chemical changes ought to be experimental distinctions. I t is my contention that there are distinctions worth recognizing. Although one need not conclude that a chemist rigidly excludes all but chemical changes from his concern at least one can recognize a class of phenomena which
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GENSLER,W., J. CHEM.EDUC., 47, 154 (1970).
generates the central problem of understanding for the chemist and therefore seems to define chemistry. I n what follows, a few selected systems will be considered, and it will be proposed that these systems possess in common a set of central characteristics. The problem will be to see if these central characteristics can define a unique class of systems and then whether such a demonstrated uniqueness is useful to chemists and the teachers of chemists. When hydrogen gas and oxygen gas are mixed together a t a temperature around 400°C or above, a change takes place in which some hydrogen and oxygen disappear with the formation of a new gaseous product which can be easily separated in solid form from the rest of the system by dropping the temperature below O°C, provided the pressure is greater than a few millimeters of mercury. The conditions under which the change takes place can be altered in various ways so that pressure and/or temperature can be shifted over a wide range. Also, the ratio of the amounts of hydrogen and oxygen initially mixed together can be chosen quite arbitrarily. These choices of conditions have no effect on the properties of the product formed, although they will, to some extent, determine how much product is formed. When various properties of the product are compared with those of the initial materials, many of the latter properties are found to be distinctly different from the properties of the product material. A short summary of central characteristics of the hydrogen-oxygen type of system can be made in the followirig words (1) Identity of product determined by identity of initial materials ( 2 ) Mixing of initial materials is essential when more than one reagent is involved (3) Discontinuity between properties of initial materials and final product ( 4 ) Invariance of product properties when temperature, pressure, and initial composition are varied
The scope of these central characteristics may be clarified by considering several other systems that exemplify the characteristics arid then several systems for which one or another characteristic is missing. Liquid mercury arid liquid bromine when mixed and either vigorously shaken or heated to above 200°C undergo a dramatic change. The product material forms as white needles or a fairly easily melted yellow solid or a separable mixture of the two. Which product is formed depends on the ratio of the amounts of mercury and bromine initially mixed. Even so, each product is formed characteristically in a precisely defined range of initial ratios. Thus, although two distinct products are formed, the central characteristics of the mercury-bromine system are identical with those developed for the hydrogen-oxygen system. Solid lead nitrate and potassium iodide can be mixed together, and when some water is added and then removed a change takes place that is partially dependent Volume
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on the relative amount of water added. I n general, a t least three different products are formed. With a relatively large amount of water (e.g., one liter or more per mole of reagents) added, a yellow, insoluble solid and a white soluble solid are formed together in a fixed mass ratio and these two solids can be differentiated from the reagents by their crystal form, among other properties. With a relatively small amount of water (e.g., one liter or less per three moles of reagents) a different white solid is formed with distinctive needlelike crystals in place of the yellow solid when the ratio of potassium iodide to lead nitrate is sufficiently high. SO, although the number of products produced in this system is greater than in the first two examples described and although two products are produced together, nonetheless the same central characteristics are applicable. Many other examples can be cited. Thus the central characteristics of identity, mixing, discontinuity, and invariance apply to a wide variety of material systems and their changes in state. Systems that exhibit these central characteristics can be referred to as chemical systems and the changes as chemical reactions. Of course, the choice of the proper reference words is a matter of opinion whereas the nature of the central characteristics is a matter of observation. Along with the description of examples of systems that conform to the central characteristics, it may be helpful to discuss a few examples of systems commonly dealt with by chemists but which do not exhibit a11 the features that have been described as central to chemical systems. For example, water and sucrose can be mixed to form a liquid product. Although this product is neither water nor sucrose, its properties can be made to approach the properties of water as closely as desired by increasing the initial amount of water per unit of sucrose. Therefore, the water-sucrose system exhibits two of the central characteristics, identity and mixing, but not the other two, discontinuity and invariance. A wide variety of other examples can be cited and the general term solution is usually applied to the product formed while the change itself is called dissolving. There are certain solid systems with the same two characteristics but in which the product has a fairly well-defined crystal structure. Some chemists choose to refer to these latter as non-stoichiometric compounds and to their formation as a reaction, but solution terminology is equally applicable and probably less confusing for students. One other comment about the water-sucrose system is in order. There is some evidence to suggest that water and sucrose form a loose complex in the solution. However, no well-defined ratio of amounts characterizes the complex. I t can only be concluded that the ratio of water and sucrose that combine can be continuously varied over a considerable range. The process might be referred to as interaction rather than a reaction. No well-defined entity nor even a series of discrete entities adequately describes the state of the system insofar as a combination of sucrose and water is concerned. Another solution system presents a special feature. When gaseous hydrogen chloride and water are mixed, i t is possible to arrive a t a state in which the entire system is a homogeneous liquid-a solution. However, the properties of the solution lead to the inter690
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pretation that two products are present in the solution, namely hydrogen ions and chloride ions in a precisely fixed invariant ratio by mass or mole. Thus in this system we conclude that solution formation is also accompanied by a chemical reaction. There are a considerable variety of examples similar to the one described here. Solution formation represents one example of a phase change. Another one that should be mentioned is the transfer of a substance from one phase to another, such as the transfer of water from solid form to liquid or liquid form to gas. This type of system does exhibit discontinuity and identity but not invariance. When iodine is transferred from an aqueous phase to a benzene phase, three of the central characteristics, identity, mixing, and discontinuity are exhibited but not invariance. Of course there are transfers between uhases in which invariance is exhibited and in those cases it is appropriate to say that a chemical reaction accompanies the transfer. Of the four central characteristics the one most crucial to demonstrating chemical change is that of invariance. Invariance in a chemical system always includes one or more mass ratios for reagents or products and the relation between a reagent and its product. Chemists often refer to these invariant ratios as stoichiometric ratios or the stoichiometry of the reaction. Essential to the recognition of a reaction then is the demonstration of its stoichiometry. Invariance also has another role in assigning structural units appropriate to the theory of a given p h e n ~ m e n a . ~ Energy transfer and symmetry operations generally exemplify invariance. What has been described thus far is an experimental basis for recognizing and describing a chemical reaction. There is also a theory of chemical reactions. This is based on assumptions about nuclei and electrons and their electrostatic interactions. However, the theory is assumed to be applicable to matter in all its aspects. All the changes described here, as well as many others, are susceptible to theoretical treatment. The assumption that electrons and nuclei or bonds or electrostatic forces or energy levels are involved is not enough in itself to define a change as chemical. But chemical reactions pose special problems for the theory of matter a t the point of interpreting invariance. It is the thesis of this paper that a chemical reaction represents a special kind of material change. The central characteristics of a chemical reaction system are identity, mixing, discontinuity, and invariance. Of these characteristics, invariance is the most dramatic and in some ways the most puzzling. If one wishes, all those changes that do not possess the four central characteristics can be referred to as physical changes. The pedagogical problem raised by Professor Gensler has to do with what sort of analysis of different kinds of changes should be presented to a student in an elementary chemistry course. The analysis that is presented here is surely not one to burden the first chapter of an elementary textbook. However, it might well underlie the strategy of the course as a whole and be developed somewhat slowly so the student would begin to understand the nature of the fundamental problem of chemistry and begin to see the extent and limitations of contemporary interpretations. STRONG, L. E., J. CHEM.EDUC., 45, 51 (1968).
