Chemistry Reference
In-Depth Information
short on a human time scale down to a microsecond and less; more precise
specification of a lower bound depends on the circumstances.
Another kind of restriction is imposed by the range of conditions over which the
substance is stable. The determination of melting and boiling points which had
become such a well-established method of characterising substances in nineteenth
century organic chemistry was not applicable to biologically active substances such
as proteins, which decompose on heating before changing phase. But by the time
proteins were recognised as comprising macromolecules—much larger than any-
thing envisaged by chemists in the first decades of the twentieth century (Zandvoort
1988
; Furukawa
1998
)—other methods of identification had been developed.
Throughout much of the twentieth century, substances have been identified by
their distinctive spectra. Although spectra have been subjected to thorough analysis
concerning the origin of their specific features in aspects of molecular structure,
they have also served as unanalysed characteristic marks in much the same way that
colour, smell, consistency, melting point and boiling point were used in the past.
Characteristics criteria of this sort can serve to classify distinct quantities of matter
as the same substance at the same time or one or more quantities as the same
substance at different times.
7.3 Comparison and Conflict Between
Macro- and Microscopic Criteria
Other criteria of being a single substance and sameness of substance are called
upon in circumstances like those mentioned when the thermodynamic criteria are
inapplicable. Though motivated by different considerations, these may sometimes
be applicable under the same conditions that thermodynamic criteria are. A natural
expectation might be that such overlapping criteria would agree where both are
applicable, so that they may be taken to complement one another in the cases where
one is not applicable. Unfortunately, the situation is not so rosy.
An example is the isotopic variants of substances. According to the IUPAC
ruling, atomic number determines sameness of element kind (Aston et al.
1923
,
p. 868). Previously, atomic weight had been taken to be characteristic of an
element. But the discovery of isotopes led to a situation in which elements with
different atomic weights were assigned the same position in the periodic table on
the strength of their “chemical” properties. Differences in the “physical” properties
of isotopes assigned the same position in the periodic table were usually small.
These are the mass-dependent properties such as solubility (expressed in grams per
unit volume rather than moles), specific gravity and properties depending on
intramolecular vibrations such as specific heat and the elastic properties of solids.
The glaring exception was radioactivity. Among groups of isotopes assigned the
same position in the periodic table, some were radioactive and others were not.
Faced with the collapse of the systematisation of the elements introduced by the
