Philosophy of Chemistry (Stanford Encyclopedia of Philosophy)

Our contemporary understanding of chemical substances is elemental and atomic: All substances are composed of atoms of elements such as hydrogen and oxygen. These atoms are the building blocks of the microstructures of compounds and hence are the fundamental units of chemical analysis. However, the reality of chemical atoms was controversial until the beginning of the 20th century and the phrase fundamental building blocks has always been ambiguous. So even today, the claim that all substances are composed of elements does not give us sufficient guidance about the ontological status of elements and how the elements are to be individuated.

In this section, we will begin with the issue of elements. Historically, chemists have offered two answers to the question What is it for something to be an element?

These two theses describe elements in different ways. In the first, elements are explicitly identified by a procedure. Elements are simply the ingredients in a mixture that can be separated no further. The second conception is more theoretical, positing elements as constituents of composite bodies. In the pre-modern Aristotelian system, the end of analysis thesis was the favored option. Aristotle believed that elements were the building blocks of chemical substances, only potentially present in these substances. The modern conception of elements asserts that they are actual components, although, as we will see, aspects of the end of analysis thesis linger. This section will explain the conceptual background behind chemistry's progression from one conception to the other. Along the way, we will discuss the persistence of elements in chemical combination, the connection between element individuation and classification, and criteria for determining pure substances.

The earliest conceptual analyses concerning matter and its transformations come in the Aristotelian tradition. As in modern chemistry, the focus of Aristotle's theories was the nature of substances and their transformations. He offered the first systematic treatises of chemical theory: On Generation and Corruption, Meteorology, and parts of Physics and On the Heavens.

Aristotle recognized that most ordinary, material things are composed of multiple substances, although he thought that some of them could be composed of a single, pure substance. Thus, he needed to give a criterion of purity that would individuate pure substances. His criterion was that pure substances are homoeomerous: they are composed of like parts at every level. [I]f combination has taken place, the compound must be uniformany part of such a compound is the same as the whole, just as any part of water is water (DG I.10, 328a10f.).[1] So when we encounter diamond in rock, oil in water, or smoke in air, Aristotelian chemistry tells us that there is more than one substance present.

Like some of his predecessors, Aristotle held that the elements Fire, Water, Air, and Earth were the building blocks of all substances. But unlike his predecessors, Aristotle established this list from fundamental principles. He argued that it is impossible for the same thing to be hot and cold, or moist and dry Fire is hot and dry, whereas Air is hot and moist ; and Water is cold and moist, while Earth is cold and dry (DG II.3, 330a30330b5). In this way, Aristotle characterized the elements in terms of maximal degrees of heat and humidity. Non-elemental substances are characterized by intermediate degrees of the primary qualities of warmth and humidity.

Aristotle used this elemental theory to account for many properties of substances. For example he distinguished between liquids and solids by noting the different properties imposed by two characteristic properties of elements, moist and dry. [M]oist is that which, being readily adaptable in shape, is not determinable by any limit of its own; while dry is that which is readily determinable by its own limit, but not readily adaptable in shape (DG II.2, 329b30f.). Solid bodies have a shape and volume of their own, liquids only have a volume of their own. He further distinguished liquids from gases, which don't even have their own volume. He reasoned that while water and air are both fluid because they are moist, cold renders water liquid and hot makes air gas. Similar arguments were given about solidity and fluidity. Dry together with cold makes earth solid, but together with hot we get fire.

Chemistry focuses on more than just the building blocks of substances: It attempts to account for the transformations that change substances into other kinds of substances. Aristotle also contributed the first important analyses of this process, distinguishing between transmutation, where one substance overwhelms and eliminates another and proper mixing. The former is closest to what we would now call change of phase and the latter to what we would now call chemical combination.

Aristotle thought that proper mixing could occur when substances of comparable amounts are brought together to yield other substances called compounds.[2] Accordingly, the substances we typically encounter are compounds, and all compounds have the feature that there are some ingredients from which they could be made.

What happens to the original ingredients when they are mixed together to form a compound? Like modern chemists, Aristotle argued that the original ingredients can, at least in principle, be obtained by further transformations. He presumably knew that salt and water can be obtained from sea water and metals can be obtained from alloys. But he explains this with a conceptual argument, not a detailed list of observations.

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Philosophy of Chemistry (Stanford Encyclopedia of Philosophy)