Transcript Chapter 3 Properties of a Pure Substance Three familiar properties of a

Chapter 3
Properties of a Pure Substance

Three familiar properties of a
substance in the previous
chapter —
– specific volume,
– pressure, and
– temperature.
3.1 THE PURE SUBSTANCE

has a homogeneous and invariable
chemical composition,
 exist in more than one phase, and
 exist with no change of phase.
 Examples :
– liquid water,
– a mixture of ice and liquid water,
– a mixture of gases, such as air

A mixture of liquid air and gaseous air –
(X)
– Because the chemical composition of the liquid
phase is different from that of the vapor phase. )
Simple Compressible Substances
(system)
Those whose surface effects, magnetic
effects, and electrical effects are
insignificant when dealing with the
substances.
 But changes in volume, such as those
associated with the expansion of a gas
in a cylinder, are very important.

3.2 VAPOR–LIQUID–SOLID-PHASE
EQUILIBRIUM IN A PURE SUBSTANCE
0.1MPa
20 0C,1kg
Fig.3.1
Heat, ν
99.6 0C
Heat ,ν

Saturation Temperature
– The temperature at which vaporization
takes place at a given pressure.

And this given pressure is called the
Saturation Pressure for the given
temperature.
Sub-cooled liquid
Compressed liquid
Fig. 3.2 A vapor-pressure curve
for a pure substance

Saturated liquid (state)
– A substance exists as liquid (state) at the
saturation temperature and pressure.

Subcooled liquid (Compressed liquid)
– If the temperature of the liquid is lower than the
saturation temperature for the existing pressure, it
is called either a subcooled liquid (implying that
the temperature is lower than the saturation
temperature for the given pressure) or a
compressed liquid (implying that the pressure is
greater than the saturation pressure for the given
temperature).

Quality of substance
– When a substance exists as part liquid and
part vapor at the saturation temperature,its
quality is defined as the ratio of the mass
of vapor to the total mass.

Quality has meaning only when the
substance is in a saturated state.

Saturated vapor
– A substance exists as vapor at the
saturation temperature.

The quality of dry saturated vapor is
100%.
Superheated vapor
is the vapor at a temperature greater
than the saturation temperature.
 Actually, the substances we call gases
are highly superheated vapors.

oC
Supercritical
fluid
20
Fig. 3.3 Temperature–volume diagram for water showing liquid and
vapor
phases.
Table 3.1
FIGURE 3.4 T –v diagram for the two-phase liquid–vapor
region to show the quality specific volume relation.
To Derivative the Quality, x

V =Vliq +Vvap = mliq v f+mvap v g
then divide the above equation by total
mass m,
Table 3.2
FIGURE 3.5 Pressure temperature diagram
for a substance such as water.
FIGURE 3.6 Carbon dioxide phase diagram.
Fig. 3.7 Water phase
diagram.
3.3 INDEPENDENT PROPERTIES
OF A PURE SUBSTANCE
•The state of a simple compressible pure
substance is defined by two independent
properties.
• For example, if the specific volume and
temperature of superheated steam are
specified, the state of the steam is
determined.
A exception, in a saturation state, should
be noted.
 Consider the saturated-liquid and saturatedvapor states of a pure substance. These two
states have the same pressure and the same
temperature, but they are definitely not the
same state. Therefore, in a saturation state,
pressure and temperature are not independent
properties.
 Two independent properties such as pressure
and specific volume or pressure and quality are
required to specify a saturation state of a pure
substance.
A mixture of gases, such as air, has the
same characteristics as a pure substance
as long as only one phase is present,
concerns precisely this point.
 The state of air, which is a mixture of gases
of definite composition, is determined by
specifying two properties as long as it
remains in the gaseous phase.

3.4 TABLES OF THERMODYNAMIC
PROPERTIES
oC
Pg=1.554
Pg=5.0
Pg=1.0
200
FIGURE 3.8 Listing of the steam tables.
• Example
Let us calculate the specific volume of saturated
steam at 200oC having a quality of 70%.
•
<Solution>
Using Eq. 3.1, and looking up Table B.1.3 gives
v = 0.3 (0.001 156) +0.7 (0.127 36) = 0.0895 m 3 /kg
Example. 3.1
Example 3.2
continued
Example 3.3
Example 3.4
(p.412)
3.5 THERMODYNAMIC SURFACES
3.6 THE P–V–T BEHAVIOR OF LOW- AND
MODERATE-DENSITY GASES
•At very low densities the average distances
between molecules is so large that the
intermolecular ( IM ) potential energy may
effectively be neglected.
• In such a case, the particles would be
independent
of one another, and the situation is referred to
as an
ideal gas.
•Therefore, a very low density gas behaves
according to the
ideal gas equation of state.
+

R is a different constant for each particular
gas. The value of R for a number of
substances is given in Table A.5 of
Appendix A.
Example 3.5
Example 3.6
Over what range of density will the ideal
gas equation of state hold with accuracy?
 How much does an actual gas at a
given pressure and temperature
deviate from
ideal gas behavior?


As would be expected, at very low
pressure or high temperature the error is
small and the gas behavior becomes
closer to the ideal gas model.
 But this error becomes severe as the
density increases (specific volume
decreases).
FIGURE 3.14 Temperature-specific volume diagram for water
that indicates the error in assuming ideal gas for saturated
vapor and for superheated vapor.
Compressibility factor, Z

A more quantitative study of the question of
the ideal-gas approximation

Z =1, for an ideal gas

The deviation of Z from unity is a measure of
the deviation of the actual relation from the
ideal-gas equation of state.
Fig.3.15 Compressibility of nitrogen
Is there a way in which we can put all of the substances on a common
basis? To do so, we “reduce” the properties with respect to the values
at the critical point.
Example 3.7
Example 3.8