2.1 CONCEPTS
Physical chemists call the part of the universe that they want to consider the
systemand the remainder the surroundings. The system may be a collection
of water molecules, an emulsion droplet, a beaker containing a solution, a
loaf of bread, a yeast cell, etc. Anopensystem can exchange mass and
energy with its surroundings, aclosedsystem can exchange no mass, and an
isolatedsystem neither mass nor energy. If the system is large enough, it has
measurable properties, which are conveniently separated in two classes.
Intensiveparameters are independent of the amount of matter and thus
include temperature, pressure, refractive index, mass density, dielectric
constant, heat conductivity, pH, and other compositional properties,
viscosity and so on. Extensive parameters depend on (and often are
proportional to) the amount of matter and thus include mass, volume,
energy, electric charge, heat capacity, etc.
Most systems that food scientists consider are heterogeneous, and this
is further discussed in Chapter 9. As mentioned, thermodynamics is model
independent, but it is necessary to consider the existence of more than one
phase. Aphase is defined as a (part of a) system that is (a) uniform
throughout and (b) bounded by a closed surface, at which surface at least
some of the intensive parameters change abruptly. For instance, density,
refractive index, and viscosity change, whereas composition does not, as
between water and ice; in many cases compositional parameters change as
well, as between water and oil. In general, temperature does not alter
abruptly, and pressure may or may not. Since most of the changes
mentioned occur over a distance of several molecular layers, the criterion of
abruptness implies that a very small region of material can never constitute a
phase: the change has to occur over a distance that is small compared to the
size (in every direction) of the region considered. That is why elements like a
soap micelle or a layer of protein adsorbed onto a surface cannot be
considered to constitute a phase. Another criterion is that the boundary or
interface between the two phases contain energy, and that enlargement of
the interfacial area thus costs energy, the amount of which can in many
cases be measured (Section 10.1).
Thermodynamics is primarily concerned with energy and entropy.
Energy, also called internal energy (U), comprises heatandwork;itis
measured in joules (J). Work may be mechanical, electrical, chemical,
interfacial, etc. It may be recalled that work generally equals force times
distance (in N?m¼J) and that force equals mass times acceleration (in
kg?m?s^2 ¼N). According to the first law of thermodynamics, the
quantity of energy, i.e., heat þ work þ potential energy, is always
preserved.