CHAPTER 2 SYSTEMS AND THEIR PROPERTIES
2.2 PHASES ANDPHYSICALSTATES OFMATTER 31
Figure 2.1 Experimental procedure for producing shear stress in a phase (shaded).
Blocks at the upper and lower surfaces of the phase are pushed in opposite directions,
dragging the adjacent portions of the phase with them.degree of deformation that depends on the magnitude of the stress and maintains this
deformation without further change as long as the shear stress continues to be applied.
On the microscopic level, deformation requires relative movement of adjacent layers of
particles (atoms, molecules, or ions). The shape of an unstressed solid is determined by
the attractive and repulsive forces between the particles; these forces make it difficult
for adjacent layers to slide past one another, so that the solid resists deformation.Afluidresponds to shear stress differently, by undergoing continuous relative motion (flow)
of its parts. The flow continues as long as there is any shear stress, no matter how small,
and stops only when the shear stress is removed.
Thus, a constant applied shear stress causes a fixed deformation in a solid and contin-
uous flow in a fluid. We say that a phase under constant shear stress is a solid if, after the
initial deformation, we are unable to detect a further change in shape during the period we
observe the phase.
Usually this criterion allows us to unambiguously classify a phase as either a solid or
a fluid. Over a sufficiently long time period, however, detectable flow is likely to occur
inanymaterial under shear stress ofanymagnitude. Thus, the distinction between solid
and fluid actually depends on the time scale of observation. This fact is obvious when
we observe the behavior of certain materials (such as Silly Putty, or a paste of water and
cornstarch) that exhibit solid-like behavior over a short time period and fluid-like behavior
over a longer period. Such materials, that resist deformation by a suddenly-applied shear
stress but undergo flow over a longer time period, are calledviscoelastic solids.
2.2.2 Phase coexistence and phase transitions
This section considers some general characteristics of systems containing more than one
phase.
Suppose we bring two uniform phases containing the same constituents into physical
contact at an interface surface. If we find that the phases have no tendency to change
over time while both have the same temperature and the same pressure, but differ in other
intensive properties such as density and composition, we say that theycoexistin equilibrium
with one another. The conditions for such phase coexistence are the subject of later sections
in this book, but they tend to be quite restricted. For instance, the liquid and gas phases of
pure H 2 O at a pressure of 1 bar can coexist at only one temperature,99:61C.
Aphase transitionof a pure substance is a change over time in which there is a con-
tinuous transfer of the substance from one phase to another. Eventually one phase can