pure substance

1. Chapter 3
Properties of a Pure Substance
 Three familiar properties of a
substance in the previous
chapter —
– specific volume,
– pressure, and
– temperature.

2. 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. )

3.  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.
Simple Compressible Substances
(system)

4. 3.2 VAPOR–LIQUID–SOLID-PHASE
EQUILIBRIUM IN A PURE SUBSTANCE
Fig.3.1
0.1MPa
20 0
C,1kg
Heat, ν
99.6 0
C
Heat ,ν

5.  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.

6. Fig. 3.2 A vapor-pressure curve
for a pure substance
Sub-cooled liquid
Compressed liquid

7.  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).

8.  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.

9.  Saturated vapor
– A substance exists as vapor at the
saturation temperature.
 The quality of dry saturated vapor is
100%.

10.  Superheated vapor
is the vapor at a temperature greater
than the saturation temperature.
 Actually, the substances we call gases
are highly superheated vapors.

11. Fig. 3.3 Temperature–volume diagram for water showing liquid and
vapor phases.
20
o
C
Supercritical
fluid

12. Table 3.1

13. FIGURE 3.4 T –v diagram for the two-phase liquid–vapor
region to show the quality specific volume relation.

14. To Derivative the Quality, x
 V =Vliq +Vvap = mliq v f+mvap v g
then divide the above equation by total
mass m,

16. Table 3.2

17. FIGURE 3.5 Pressure temperature diagram
for a substance such as water.

18. FIGURE 3.6 Carbon dioxide phase diagram.

19. Fig. 3.7 Water phase
diagram.

20. 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.

21.  Consider the saturated-liquid and saturated-
vapor 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 exception, in a saturation state, should
be noted.

22.  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.

23. 3.4 TABLES OF THERMODYNAMIC
PROPERTIES
FIGURE 3.8 Listing of the steam tables.
200
Pg=1.554
Pg=1.0
o
C
Pg=5.0

24. • Example
Let us calculate the specific volume of saturated
steam at 200o
C 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

25. Example. 3.1

26. Example 3.2

27. continued

28. Example 3.3

29. Example 3.4
(p.412)

30. 3.5 THERMODYNAMIC SURFACES

31. 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.

32. +

33.  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.

34. Example 3.5

35. Example 3.6

36.  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?

37.  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).

38. FIGURE 3.14 Temperature-specific volume diagram for water
that indicates the error in assuming ideal gas for saturated
vapor and for superheated vapor.

39.  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.
Compressibility factor, Z

40. Fig.3.15 Compressibility of nitrogen

41. 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.

43. Example 3.7

44. Example 3.8