Chemistry Reference
In-Depth Information
7.4 Concluding Comments
Like temperature, the notion of a chemical substance is a macroscopic notion which
resists reduction to purely microscopic notions. The familiar idea that gas temper-
ature is the average kinetic energy of the constitutive molecules does not amount to
a reduction because temperature presupposes equilibrium, the microscopic corre-
late of which is the Boltzmann distribution and this in turn presupposes the
macroscopic notion of temperature (Needham
2009
,
2010a
). Similarly, any quan-
tity of matter characterised as being a particular kind of substance will have
microscopic features, yet it doesn
t follow that the macroscopic notion of substance
is reducible to microstructure. The reason is less straightforward than in the case of
temperature, but of essentially the same kind. If having particular microscopic
features, such as comprising a particular kind molecule, provided an adequate
general criterion of being a single substance, where the microscopic features can
be independently specified (i.e. without reference to the substance in question),
matters would be different. But as matters actually stand, substances are not in
general of a microscopic kind such as being molecular, i.e. comprised of numerous
microscopic entities of a single kind. Molecules are microscopic entities themselves
composed of entities of several kinds (electrons, protons and neutrons) combined in
such a way that they move as a single, integral unit, and not merely minimal units
corresponding to a compositional formula like SiO
2
or NaCl which do not comprise
internally bound parts not so bound to other immediate neighbours. (“The smallest
part of a chemical compound that can take part in a chemical reaction” sometimes
offered as a definition of a molecule calls for immediate qualification (Daintith
1990
, p. 195) because it encompasses much more, capturing instead this broader
notion of a minimal unit.) Although there are many molecular substances, com-
prised of millions of molecules—in this sense—of the same kind, the fact that many
substances are not molecular entails that comprising a particular kind of molecule is
not an adequate general criterion of being a single substance.
The concept of a chemical substance is more complicated, then, than a simple
molecular structure thesis would have it. The assumption that there is a single kind
of molecule collections of which constitute samples of a corresponding substance
must give way to an
explicit specification
of any microstructural description that is
held to correspond to a particular substance. It is almost a tautology that a substance
has that substance
'
s microstructure, not quite because of the existential presuppo-
sition, which received its empirical justification with Perrin
'
s investigations into
Brownian motion and the general recognition of a discrete microstructure underly-
ing an apparently continuous macrostructure. But this is such commonplace knowl-
edge that it goes without saying, and just as Perrin
'
s investigations told us almost
nothing about the chemical nature of microstructure, so “water has the microstruc-
ture it has” and essentially equivalent variants say practically nothing. A micro-
structural account of a substance, i.e. what being the same substance as the
particular substance amounts to, can only be a detailed explicit specification of
the microstructure. The question is whether such specifications are independent of
'
