Modern Engineering Thermodynamics

2.3 Phases of Matter 35
System boundary
System boundary
System boundary
(a) Isolated System: Neither
mass nor energy can cross
the system boundary.
FIGURE 2.1
The three types of thermodynamic systems.
(b) Closed System: Mass
cannot cross the system
boundary, but energy can.
(c) Open System: Both
mass and energy can cross
the system boundary.
TYPES OF THERMODYNAMIC SYSTEMS
Isolated system. Any system in which neither mass nor energy crosses the system boundary.
Closed system. Any system in which mass does not cross the system boundary, but energy may cross the system boundary.
Open system. Any system in which both mass and energy may cross the system boundary.
CRITICAL THINKING
If we select your body as a thermodynamic system, is it an open or closed system? What happens to you if we force you to
be a closed system?
to the water treatment plant, and the water supply for the plant. But such a system would be too large to analyze
properly, since we must be able to find all the energy that crosses its boundary, at any point along the boundary.
Therefore, it is much easier to view Figure 2.1c as an open system with a small, well-defined system boundary.
The choices of the proper system, along with the proper form of the thermodynamic laws, always are decisions
that you, the analyst, must make whenever beginning to solve a thermodynamics problem. Making a sketch
of the system that shows the system boundary is a useful aid in making these decisions. The system sketch in
thermodynamics is equivalent to the free body diagram sketch in mechanics. Its value cannot be overstated.
2.3 PHASES OF MATTER
The physical phase of a substance is defined by the molecular structure of the substance. For example, water
can be described chemically as H 2 O, but it may exist in a number of molecular configurations. At low
temperatures, water takes on a rigid crystalline molecular structure, ice, but at higher temperatures its molecular
structure becomes amorphous as it becomes a liquid and random as it becomes a vapor. We can easily
identify three common structural phases of matter: solid, liquid, and vapor (or gas). But, whereas only
one liquid phase or one vapor phase may be possible, many different solid molecular configurations of a
substance may exist.
The term homogeneous can be used to describe either physical or chemical uniformity. Here we use the term pure
substance to describe substances that are chemically uniform (Figure 2.2), and reserve the term homogeneous to
describe substances that are physically uniform (i.e., have a single physical phase). Hence, we define a pure
substance as anything that contains the same uniform chemical composition in all its physical phases. For example,
a mixture of water vapor and liquid water is a pure substance. On the other hand, air is not really a pure
substance, because when it is cooled sufficiently, some of its components condense into their liquid state, thus
changing the composition of the remaining gases.