PROPERTIES OF PURE SUBSTANCES

• PURE SUBSTANCE
A substance that has a fixed chemical composition
throughout is called a pure substance. Water,
nitrogen, helium, and carbon dioxide, for example,
are all pure substances.
• A mixture of various chemical elements or
compounds also qualifies as a pure substance as long
as the mixture is homogeneous.
• However, a mixture of oil and water is not a pure
substance. Since oil is not soluble in water
• A mixture of two or more phases of a pure substance
is still a pure substance as long as the chemical
composition of all phases is the same.
 PHASE-CHANGE PROCESSES
OF PURE SUBSTANCES

• There are three principal phases—solid, liquid,

and gas.
• Water exists as a mixture of liquid and vapor
in the boiler and the condenser of a steam
power plant.
• In this section is focused on the liquid and
vapor phases and their mixture.
Compressed Liquid and Saturated Liquid
• Consider a piston–cylinder device containing liquid
water at 20°C and 1 atm pressure. Under these
conditions, water exists in the liquid phase, and it is
called a compressed liquid, or a subcooled liquid,
meaning that it is not about to vaporize.
•Heat is now transferred to the water
until its temperature rises to, say, 40°C.
As the temperature rises, the liquid
water expands slightly, and so its specific
volume increases.

compressed liquid
 Saturated Vapor and Superheated Vapor
• As more heat is transferred, the temperature keeps
rising until it reaches 100°C. At this point water is still
a liquid, but any heat addition will cause some of the
liquid to vaporize. That is, a phase-change process
from liquid to vapor is about to take place.

A liquid that is about to vaporize

is called a saturated liquid.
Therefore, state 2 is a saturated
liquid state.

Saturated liquid.
 Saturated Vapor and Superheated Vapor
• Once boiling starts, the temperature stops rising until
the liquid is completely vaporized. That is, the
temperature will remain constant during the entire
phase-change process if the pressure is held
constant.

Saturated liquid–vapor mixture. saturated vapor.

 Superheated Vapor
• Once the phase-change process is completed,
we are back to a single phase region again and
further transfer of heat results in an increase in both
the temperature and the specific volume. At state 5,
the temperature of the vapor is, let us say, 300°C.

Superheated Vapor
T-v diagram for the heating process of
water at constant pressure.
Saturation Temperature and Saturation Pressure
• At a given pressure, the temperature at which a pure
substance changes phase is called the saturation
temperature Tsat.
• If the pressure inside the cylinder were raised to 500 kPa
by adding weights on top of the piston, water would start
boiling at 151.8°C.

The liquid–vapor saturation curve of a

pure substance (water).
Variation of the standard atmospheric pressure
and the boiling (saturation) temperature of
water with altitude
The T-v Diagram
 Critical Point
• The critical point, is defined as
the point at which the saturated
liquid and saturated vapor states
are identical.
• At pressures above the critical
pressure, there is not a distinct
phase change process
• Above the critical state, there is no
line that separates the compressed
liquid region and the superheated
vapor region. However, it is
customary to refer to the substance
as superheated vapor at
temperatures above the critical
temperature and as compressed
liquid at temperatures below the
critical temperature.
The P-v Diagram
 The P-T Diagram
For water, the triple-
point temperature and
pressure are 0.01°C and
0.6117kPa, respectively.
• At triple point , all
three phases of water
coexist in equilibrium
only if the temperature
and pressure have
precisely these values.
• No substance can exist
in the liquid phase in
stable equilibrium at
pressures below the
triple-point pressure.
 PROPERTY TABLES
Enthalpy—A Combination Property

Saturated Liquid and Saturated Vapor States

The quantity hfg is called the enthalpy

of vaporization (or latent heat of
vaporization). It represents the amount
of energy needed to vaporize a unit
mass of saturated liquid at a given
temperature or pressure.
EXAMPLE 1
Schematic and T-v diagram for Example 1.
EXAMPLE 2
Schematic and P-v diagram for Example 2
 Saturated Liquid–Vapor Mixture
During a vaporization process, a
substance exists as part liquid and
part vapor. That is, it is a mixture
of saturated liquid and saturated
vapor. To analyze this mixture
properly, we need to know the
proportions of the liquid and vapor
phases in the mixture. This is done
by defining a new property called
the quality x as the ratio of the
mass of vapor to the total mass of
the mixture:
Consider a tank that contains a
saturated liquid–vapor mixture.
The volume occupied by saturated
liquid is Vf and the volume
occupied by saturated vapor is Vg.
The total volume V is the sum of
the two:
EXAMPLE 3
 Schematic and T-v diagram for Example 3
• Another way is to first
determine the quality x, then
the average specific volume v,
and finally the total volume:

mg 2
x   0.2
mt 10
v v v
f fg
 0.001036  (0.2)(2.3593  0.001036)
 0.473 m3 / kg
V  10(0.473)  4.73m3
 Superheated Vapor

Compressed Liquid
EXAMPLE 4
Temperature of Superheated Vapor
 EXAMPLE 5
Determine the missing properties and the phase
descriptions in the following table for water:

Solution : (a) The quality is given to be x 0.6,Therefore,

we have saturated liquid–vapor mixture at a pressure of
200 kPa. Then the temperature must be the saturation
temperature at the given pressure:
• (b) This time the temperature and the internal energy
are given. To determine the region we are in, we first
go to the saturation table and determine the uf and ug
values at the given temperature. At 125°C, we read uf
524.83 kJ/kg and ug 2534.3 kJ/kg. Next we compare
the given u value to these uf and ug values, keeping in
mind that

In our case the given u value is 1600, which falls between

the uf and ug values at 125°C. Therefore, we have saturated
liquid–vapor mixture. Then the pressure must be the
saturation pressure at the given temperature:
 The quality is determined from

(c) This is similar to case (b), except pressure is given

instead of temperature. Following the argument given
above, we read the uf and ug values at the specified
pressure. At 1 MPa, we have uf 761.39 kJ/kg and ug
2582.8 kJ/kg. The specified u value is 2950 kJ/kg, which
is greater than the ug value at 1 MPa. Therefore, we
have superheated vapor, and the temperature at this
state is determined from the superheated vapor table by
interpolation to be
• (d) In this case the temperature and pressure are
given. To determine the region we are in, we go to
the saturation table and determine the saturation
temperature value at the given pressure. At 500 kPa,
we have Tsat= 151.83°C. We then compare the given T
value to this Tsat value, keeping in mind that
In our case, the given T value is
75°C, which is less than the Tsat
value at the specified pressure.
Therefore, we have compressed
liquid.

(e) The quality is given to be x = 0, and thus we have

saturated liquid at the specified pressure of 850 kPa. Then
the temperature must be the saturation temperature at
the given pressure, and the internal energy must have the
saturated liquid value:
 THE IDEAL-GAS EQUATION OF STATE
• Any equation that relates the pressure, temperature,
and specific volume of a substance is called an
equation of state.
• The vapor phase of a substance is customarily called
a gas when it is above the critical temperature. Vapor
usually implies a gas that is not far from a state of
condensation.
• Charles and J. Gay-Lussac, Frenchmen,
experimentally determined that at low pressures the
volume of a gas is proportional to its temperature.
That is,
(1)
 • R is called the gas constant. Equation (1) is called the
ideal-gas equation of state, or simply the ideal-gas
relation.
• A gas that obeys this relation is called an ideal gas.
• In this equation, P is the absolute pressure, T is the
absolute temperature, and v is the specific volume.

where Ru is the universal gas constant and M is the molar

mass
The molar mass M can simply be defined as the mass
of one mole
 COMPRESSIBILITY FACTOR
• Gases deviate from
ideal-gas behavior
significantly at states
near the saturation
region and the critical
point
• Percentage of error
involved in assuming
steam to be an ideal
gas, and the region
where steam can be
treated as an ideal gas
with less than 1 percent
error.
• The compressibility factor Z defined as

• It can also be expressed as

• Obviously, Z 1 for ideal gases. For real gases Z can be
greater than or less than unity.
• We have said that gases follow the ideal-gas equation
closely at low pressures and high temperatures.
• The Z factor for all gases is approximately the same at
the same reduced pressure (PR) and reduced
temperature (TR ) .
Comparison of Z factors for various gases.
Properties of Saturated Water (Liquid–Vapor): Temperature Table
Properties of Saturated Water (Liquid–Vapor): Pressure Table
Properties of Superheated Water Vapor
Properties of Compressed Liquid Water