Gibbs’s phase rule treats mixtures by relating the number of independent variables governing their state to the numbers of phases and independent substances. For the case of a single substance, it provides a criterion of purity. But where more substances are involved, the notion of independent substance is less readily understood. Textbook writers sometimes use algebraic terminology in ways that are suggestive but cannot be taken as literally accurate. I suggest that a mereological interpretation applies to these cases, as it (...) captures more concisely the insights underlying the use of algebraic ter- minology and illuminates the general notion of substance. (shrink)

‘Water is H 2 O’ is naturally construed as an equivalence. What are the things to which the two predicates ‘is water’ and ‘is H 2 O’ apply? The equivalence presupposes that substance properties are distinguished from phase properties. A substance like water (H 2 O) exhibits various phases (solid, liquid, gas) under appropriate conditions, and a given (say liquid) phase may comprise several substances. What general features distinguish substance from phase properties? I tackle these questions on the basis of (...) an interpretation of a theorem of thermodynamics known as Gibbs' phase rule which systematically relates these two kinds of feature of matter. The interpretation develops the idea that the things substance and phase predicates apply to are quantities of matter which sustain mereological relations and operations and exploits these mereological features in distinguishing the two kinds of property. Gibbs' phase rule is a macroscopic principle applicable for macroscopic intervals of time. Bringing intervals of time into the picture calls for a more detailed consideration of the relation between macroscopic equilibria and the corresponding dynamic equilibria at the microlevel and throws into question the simple idea that quantities can always be regarded as collections of molecules. The account provides some insight into how the continuous, macroscopic conception of matter (‘gunk’) is reconciled with the discrete microscopic conception and illuminates the interpretation of substances present in mixtures. (shrink)

According to microessentialism, it is necessary to resort to microstructure in order to adequately characterise chemical substances such as water. But the thesis has never been properly supported by argument. Kripke and Putnam, who originally proposed the thesis, suggest that a so-called stereotypical characterisation is not possible, whereas one in terms of microstructure is. However, the sketchy outlines given of stereotypical descriptions hardly support the impossibility claim. On the other hand, what naturally stands in contrast to microscopic description is description (...) in macroscopic terms, and macroscopic characterisations of water are certainly possible. This suffices to counter the claim that microdescriptions are necessary. Whether it counters the impossibility claim depends on whether all macroscopic descriptions are stereotypical (stereotypical descriptions presumably being macroscopic). In so far as systematic import of “stereotypical” can be determined, it would seem not. But some macroscopic characterisations have definite affinity with everyday knowledge, which presumably stands in conflict with the spirit of the impossibility claim. Since what is characterised are properties expressed by predicates like “is water”, the necessity of identity has no bearing here, and matters of interpretation pose problems for claims to the effect that science fixes the extension of “water” as ordinarily understood. (shrink)

Two inter-linked theses are defended in this paper. One is the Duhemian theme that a rigid distinction between physical and chemical properties cannot be upheld. Duhem maintained this view not because the latter are reducible to the former, but because if physics is to remain consistent with chemistry it must prove possible to expand it to accommodate new features, and a rigid distinction would be a barrier to this process. The second theme is that naturally occurring isotopic variants of water (...) are in fact distinct substances, and naturally occurring samples of water are mixtures of these substances. For most practical purposes it is convenient to treat protium oxide, deuterium oxide, and so on, as the same chemical substance, but to insist on this as a matter of principle would stand in conflict with the first thesis. (shrink)

It is only when mixing two or more pure substances along a reversible path that the entropy of the mixing can be made physically manifest. It is not, in this case, a mere mathematical artifact. This mixing requires a process of successive stages. In any finite number of stages, the external manifestation of the entropy change, as a definite and measurable quantity of heat, isa fully continuous function of the relevant variables. It is only at an infinite and unattainable limit (...) thata non-uniform convergence occurs. And this occurs when considered in terms of the number of stages together with a distinguishability parameter appropriate to the particular device which is used to achieve reversibility. These considerations, which are of technological interest to chemical engineers, resolve a paradox derived in chemical theory called Gibbs'' Paradox. (shrink)

In this article I assess the problems and prospects of a microstructural approach to chemical substances. Saul Kripke and Hilary Putnam famously claimed that to be gold is to have atomic number 79 and to be water is to be H2O. I relate the first claim to the concept of element in the history of chemistry, arguing that the reference of element names is determined by atomic number. Compounds are more difficult: water is so complex and heterogeneous at the molecular (...) level that `water is H2O' seems false under some interpretations. I sketch a response to this problem. (shrink)

What are the criteria determining the individuation of chemical kinds? Recent philosophical discussion, which puts too much emphasis on microstructure, seems to presuppose a reductionist conception not motivated by the scientific facts. The present article traces the development of the traditional notion of a substance with the rise of modern chemistry from the end of the 18th century with a view to correcting this speculative distortion.