Chemistry Reference
In-Depth Information
is a macroscopic concept, but systematically distinguished from the concept of
phase in what is currently accepted as the fundamental macroscopic theory, namely
thermodynamics. This is a theory dealing with features of the world lying beyond
the scope of mechanics that requires the division of the mass of a body into amounts
of different substances composing it. We must resort to thermodynamics to explain
how the mechanical phenomenon of pressure can arise by osmosis as well as
how purely chemical phenomena involving change of substance lead to reaction
equilibria. It is therefore natural to look for a general notion of substance governed
by principles of sameness and distinctness of substance implicit in the theory.
In particular, it is a natural place to look for systematic criteria based on the
distinctive effects of the presence of several substances and of the special case of
no more than one.
One such principle is the theorem of thermodynamics underlying Gibbs
'
discovery of the entropy mixing, which is independent of the nature of the sub-
stances, and in particular how similar they may be in any respect, but depends only
on the amounts of the substances involved. The notion presupposes that the
temperature and pressure, which would otherwise be sources of entropy change,
are maintained throughout. A so-called ideal solution in which the components
exhibit no interactive (attractive or repulsive) forces is one whose stability com-
pared with that of the isolated components is entirely due to the entropy of mixing.
The more similar the substances, the greater the difficulty in capitalising on this
entropy difference in order to separate them (Denbigh and Denbigh
1985
, Ch. 4).
But the theoretical difference remains: mixing two quantities of the same substance
doesn
t increase the entropy, but any difference is marked by an entropy of mixing.
A more practicably applicable theorem is the phase rule, the first applications of
which revealed many new substances where they were not suspected. The rule
relates an experimentally ascertainable magnitude to the number,
c
, of substances in
a quantity of matter and the number,
f
, of phases it exhibits, as follows:
'
Variance
¼ c f þ
2
0
:
The variance is the number of independent variables or degrees of freedom deter-
mining the state of the quantity. For a single substance,
f
3. Where there are three
phases, the variance is zero and we speak of the substance at its triple point, a
specific temperature and pressure at which the substance exhibits three phases.
The usual situation is where the phases in question are solid, liquid and gas, which
in the case of water occurs at 273.16 K (0.01
C) and 611.72 Pa pressure
(0.0060373 atm) and is used to fix a temperature above 0 K with which to calibrate
the absolute (Kelvin) scale of temperature. Water also exhibits a number of distinct
solid phases between which other triple points occur. Arbitrary small changes in
temperature or pressure will convert the entire quantity of water at a triple point to
one of the phases. The pressure of the ordinary triple point for water is the minimum
pressure at which water can be liquid, so that in the low pressures of outer space,
heating ice converts it directly to vapour. Here we see how the theoretical criterion
of being a single substance points to sorts of properties that can serve as charac-
teristic features of the substance.
