What is Chemistry about?
Philosophers, like children, tend to ask plain questions such as: what is chemistry about? What is its specific subject matter that distinguishes chemistry from other sciences? Dictionaries tell us that chemistry is about substances, chemical reactions, molecules, and atoms - but what is a substance, a chemical reaction, a molecule, and an atom, and how do these concepts relate to each other? Unlike substances in philosophy, a chemical substance is a piece of matter of any size, form, and state of aggregation with clearly defined and unique chemical properties that are qualitatively different from the chemical properties of other substances.
A chemical property of a substance is its ability to change into other substances under certain conditions, and such changes from one substance to another are called chemical reactions. Because a substance is defined through its specific chemical reactions and a chemical reaction is defined through the specific substances involved, we end up in circular definitions: reactions define substances and substances define reactions. Can we escape the circle by giving priority to either substances or reactions?The seemingly innocent question of what chemistry is about prompts us to decide between two opposing metaphysical traditions: substance philosophy and process philosophy. Substance philosophers claim priority for entities, things, or substances and consider changes, like motion in space, to be only secondary attributes of entities. In chemistry, however, change is essential rather than secondary; and it is radical because through chemical reactions all properties radically change. This suggests that process philosophy would be more suitable here, because it gives priority to processes and considers entities only as temporary states. Moreover, process philosophers can point to the fact that in the natural world there are no fixed and isolated chemical substances but only permanent chemical change of matter.
However, in order to describe these changes precisely we need concepts that grasp the various states of change, for which the concept of chemical substances appears to be most suitable.Chemists have solved the puzzle in a way that sheds light on the manifold uses of experimentation in science. Because, as process philosophy correctly says, there are no fixed and isolated chemical substances in the natural world, chemists make them in the laboratory and put them in bottles, so that they are pure, isolated, and remain stable for further investigation. The material world is thus adjusted to the conceptual needs. However, the experimental trick works only through a quasi-operational definition of chemical substances, according to which a chemical substance is the result of perfect purification, which includes thermodynamic operations such as distillation. It happened that only the results of such purification procedures meet the definition of chemical substances, that only they have clearly defined and unique chemical properties that qualitatively differ from those of other substances.[95] The trick thus yields substances that are characterized through their chemical changeabilities, which combines aspects of both substance and process philosophies. Once such chemical substances are produced, they can also be characterized and later recognized by other properties, such as optical and thermodynamic properties.
Chemists have used the same experimental strategy to develop an operational hierarchy of matter that formally resembles the metaphysical hierarchy known since Aristotle. Every technique that takes materials apart defines a part-whole relationship between the end products and the starting material. Thus, a material that can be taken apart by purification is, by definition, a mixture and the resultant materials are its component substances; a material that cannot be taken apart is, by definition, a chemical substance.
There are two other sets of separation techniques that each define a part-whole relationship between materials.
A mixture that can be taken apart into different materials by mechanical means, such as sorting or cutting, is a heterogeneous mixture; otherwise it is a homogeneous mixture. A chemical substance that can be taken apart by chemical means, including electrochemical processes, is a compound; otherwise it is a chemical element. At the same time the chemical separation defines the elemental composition of a compound, which is an important chemical property.Overall, this results in an operationally defined four-level hierarchy, from chemical elements, to compounds, to homogeneous and heterogeneous mixtures. The hierarchy allows characterizing both a material and its changes through its composition on the lower levels. For instance, a compound is characterized by its elemental composition and a homogeneous mixture by its composition of substances.
Because chemistry is about radical change, it needs to deal with fundamental problems, as the following example illustrates. Assume you want to characterize something through its specific changes: as long as you do not perform the change, you have no certain idea about that change, but once you have performed the change, the thing you want to characterize no longer exists. Again, the logical puzzle is solved experimentally in chemistry. Because any material, from homogeneous mixtures down in the hierarchy to elements, cannot by definition be changed through mechanical separations, one can mechanically take small pieces from such a material and perform chemical test changes on these samples. The operational hierarchy guarantees that the chemical characteristics of all samples are exactly the same as those of the entire piece of material.
Thus far we have dealt only with substances and reactions. What about atoms and molecules? Because these are widely conceived as the true microscopic components of all materials, many argue that chemistry is ultimately about atoms and molecules rather than about substances.
Investigating substances and chemical reactions is only a means to develop a better understanding of atoms and molecules and their dynamical behavior and reconfigurations that we perceive as chemical change. One could also argue that all our knowledge about atoms and molecules is only a means to understand better and then explain and predict the chemical behavior of substances.While all chemical knowledge actually starts with the artificial creation of pure chemical substances and then continues with investigating them in the laboratory, the two positions differ only in what kind of knowledge they consider means and ends of chemistry.[96] The first position (which one might call theoreticism) takes the knowledge of substances as means for the knowledge of atoms and molecules to be considered an end in itself. For the second position (experimentalism) the knowledge of atoms and molecules is only a theoretical means for the proper end of understanding the behavior of substances. And because substances are artificially produced in the laboratory to suit our conceptual needs, one can also assume a third position, which one might call realism in the original sense because, unlike idealism, it acknowledges a fundamental difference between our concepts and the world. This position takes our knowledge of substances, whether reinforced by theoretical knowledge or not, only as a means to develop a better understanding of our messy material world, which includes both our natural environment and the chemical processes that happen in all kinds of industries.
Of course the three positions express different views about the end of science in general, and they usually come from different areas of science - here, theoretical, experimental, and applied science. However, in chemistry the difference between theoreticism and experimentalism is more complicated than an introductory textbook of chemistry might suggest. That is because there is no one-to-one relationship between substances and molecules, such that each substance would consist of a single kind of molecule.
Indeed, the concept of molecules works only for certain substances as a useful model approximation. If we assume that substances consist somehow of atoms, the molecular model singles out certain groups of atoms that on time average stick a bit closer together with each other than with other atoms. This model works quite well with many organic substances and gases, but fails, for instance, with simple substances like water, metals, or salts, for most purposes. In liquid water one can single out hundreds or thousands of different kinds of molecules, depending on one's accuracy and time average, such that pure water would be a complex molecular mixture. In metals and salts, all atoms stick together in the same way such that each piece would consist of a single molecule. Hence, rather than talking of molecules, a more generic concept is that of interatomic structures of substances.Interatomic structures of substances are dynamic entities, even if we disregard quantum mechanics for the sake of simplicity. To take water again as an example, the structure continuously changes on a time scale of less than a trillionth of a second. We might be able to identify some hundred kinds of preferred structures that recur on time average, but others appear if we only slightly change the temperature. Also, for those organic substances where the molecular model works quite well, interatomic distances and angles change with temperature. Theoreticism is thus confronted with severe conceptual problems because the classical chemical concepts no longer work. If, in theoretical terms, a chemical reaction is defined by a change of the interatomic structure, pure substances would be complex mixtures that undergo permanent chemical reactions; and changes of temperature that do not change the substance identity would induce radical chemical reactions on interatomic structure. The problem of theoreticism is that it lacks useful concepts of kinds, both for entities and processes. If such concepts are introduced by virtue of model approximations, theoreticism would have to concede that chemistry is ultimately about its own models about the world rather than about the material world itself, i.e., only about what theoreticians are doing.
Compare that with experimentalism, which cannot only acknowledge such models as useful intellectual tools but can also claim that its own concepts perfectly fit at least a part of the material world, even if that part is artificially produced in the laboratory.However, experimentalism also smacks of self-satisfaction because it creates and focuses on the laboratory systems that best fit its conceptual framework. If the goal of science is to understand the world that we all live in, then realism is the only viable position, such that theoretical and experimental laboratory investigations are only useful means to that end.[97] That is even more important, if chemistry, as many think, is about developing an understanding of our material world in order to improve it according to human needs.
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