CHANGE OF PHASE
CHANGE OF PHASE
The phase change of materials in a system is very important to thermodynamics.
It is possible to design systems to take advantage of the phase changes between
solid and liquid or between liquid and vapor to enhance the performance of the
DISTINGUISH between intensive and extensive
DEFINE the following terms:
Vapor pressure curve
DESCRIBE the processes of sublimation, vaporization,
condensation, and fusion.
Classification of Properties
As discussed earlier in this module, properties are classified as either intensive or extensive.
Properties are intensive if independent of the amount of mass present and extensive if a function
of the amount of mass present. Properties such as pressure, temperature, and density are
intensive, whereas volume and mass are extensive. An extensive property may be made intensive
by dividing the particular property by the total mass. Total volume (V), which is an extensive
property, can be changed to specific volume, which is an intensive property, by dividing by the
mass of the system, n = V/m. Any specific property (specific volume, specific enthalpy, specific
entropy), is an intensive property, as indicated in Figure 3.
The use of intensive and extensive properties is demonstrated in the following discussion.
Consider as a system 1 lbm of water contained in the piston-cylinder arrangement of Figure 4.
Suppose that the piston and weight maintain a pressure of 14.7 psia in the cylinder and that the
initial temperature is 60°F, part (a) of Figure 4. As heat is transferred to the water, the
temperature increases. The specific volume increases slightly, and the pressure remains constant.
When the temperature reaches 212°F, additional heat transfer results in a change in phase
(boiling), as indicated in part (b).