Modern Engineering Thermodynamics

3.7 Phase Diagrams 69
WHAT IS A “PHASE”?
A material “phase” is a physically distinct region of space that is chemically uniform with homogenous physical properties.
For example, imagine a system consisting of ice cubes and liquid water in drinking glass. The ice cubes are one phase
(solid), the water is a second phase (liquid). The glass itself is a different material in a solid phase.
Material phases are different states of matter, such as solid, liquid, gas, or plasma. It is possible for a material to have more
than one liquid or solid phase. For example, depending on the cooling process, metals can solidify into several distinct
crystal phases.
The liquid to vapor phase transformation is called vaporization (Figure 3.9), and the vapor to liquid phase transformation is
called condensation. Similarly, the solid to liquid phase transformation is known as melting, and the liquid to solid phase
transformation is called freezing or solidification. Finally, the solid to vapor phase transformation is known as sublimation,
and the vapor to solid phase transformation is deposition (or frost in the case of water).
Gas
or
Vapor
Vaporization
Condensation
Sublimation
Liquid
Deposition
Melting
Freezing
Solid
FIGURE 3.9
What is a phase?
FROM WHENCE COMETH THE GAS?
The term gas was coined by the Belgian chemist Jan Bapist Van Helmont (1577–1644), derived from the Greek word κєøσ,
meaning “gaping void.”
Critical point
Pressure, p
P c
Solid
Triple point
Sublimation curve
Fusion curve
Liquid
B
Vaporization curve
Vapor A
Supercritical fluid
(a gas)
Gas
T c
Temperature, T
FIGURE 3.10
The definitions of a gas and a vapor.