1. BITS Pilani
Pilani Campus
Properties of a pure substance

2. Pure Substance
The pure substance is one that has
p
a homogeneous and invariable
chemical composition.
p
A pure substance may exist in many
phases, but the chemical composition
is same in all the phases
phases.
BITS Pilani, Pilani Campus

3. BITS Pilani
Pilani Campus
Vapor-Liquid-
Vapor-Liquid-Solid Phase
Equilibrium in a Pure Substance

4. Phase Equilibrium in a Pure
Substance
Saturation temperature means the temperature at
which change of phase takes place at a given pressure.
Saturation Pressure means the pressure at which
change of phase takes place at a given temperature.
BITS Pilani, Pilani Campus

5. Phase Equilibrium in a Pure
Substance
Latent heat: the amount of energy absorbed or
released during a phase-change process.
Latent heat of fusion: the amount of energy
absorbed during melting
melting.
Latent heat of vaporization: the amount of energy
absorbed during vaporization.
BITS Pilani, Pilani Campus

6. Constant pressure change from solid to
vapor phase for pure substance
(substance that contracts on freezing)
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7. Phase Equilibrium in a Pure Substance
q
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8. Phase Equilibrium in a Pure
Substance
S b t
Sub-cooled Solid (1)( )
Saturated Solid (2)
Sub-cooled
Sub cooled / compressed liquid (between
3 & 4)
Saturated liquid (3 - w.r.t. S L equilibrium,
S t t d li id t S-L ilib i
4 - w.r.t. L - G equilibrium)
BITS Pilani, Pilani Campus

9. Phase Equilibrium in a Pure
Substance
S b t
Saturated vapour (5)
p ( )
Saturation temp(2 - S; 3 - L w.r.t. S-L
equilibrium;
q ;
4 – L, 5 - G w.r.t. L - G equilibrium)
Liquid vapour mixture (between 4 and
5)
Superheated vapour (beyond 5)
Saturation pressure
BITS Pilani, Pilani Campus

10. Phase Equilibrium in a Pure Substance
q
Superheated vapour
Saturated liquid(4)
Saturation temp
p Liquid vapour mixture(4-5)
Saturated Solid (2)
Saturated vapour(5)
Sub-cooled
Sub cooled / compressed liquid(3 4)
liquid(3-4)
Saturated liquid(3)
Sub-cooled Solid Solid –Liquid Mixture(2-3)
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11. Saturated
Liquid line
Critical Point
Saturated
Vapour line
Triple Line
Saturated
Solid line

12. Saturated
Solid line
Saturated
Liquid line
P-v-T surface
of a
substance
that contracts
on freezing.
Saturated
Vapour line Triple Line

15. Constant pressure change from solid to vapor
phase for pure substance
h f b t
(substance that expands on freezing eg.water)
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

16. Constant pressure change from solid to
vapor phase for pure substance (water)
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

17. Constant pressure change from solid to
vapor phase for pure substance (water)
BITS Pilani, Deemed to be University under Section 3 of UGC Act, 1956

18. P-v-T surface
su ace
of a
substa ce
substance
that expands
g
on freezing.

19. Saturated Saturated
Solid line Liquid line
P-v-T surface
of a
substance
that contracts
on freezing.
Saturated
Vapour line Triple Line

21. The Triple Point …
p
The three lines that met at the triple point, they
represent the conditions of:
h d f
Fusion l
line - Solid-liquid equilibrium
ldl d lb
Vaporization line – Liquid-vapor equilibrium
Sublimation line – Solid-vapor equilibrium
Where all three lines meet, we have a unique
combination of temperature and pressure where all
three phases are in equilibrium
equilibrium.
BITS Pilani, Pilani Campus

22. The Triple Point …
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23. Unusual behavior of water …
The phase diagram shows that water
would fi
ld first ffreeze to f form i
ice as i
it
crosses into the solid area.
With further decreasing pressure, ice
would th
ld them sublime t give vapor.
bli to i
So i h decreasing pressure,
S with d i
Liquid Solid Vapor
BITS Pilani, Pilani Campus

24. Unusual behavior of water …
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25. The Critical Point
The liquid vapor equilibrium curve has an upper
limit,
limit labeled as C This is known as the Critical
C.
Point.
Above the critical temperature, it is impossible to
condense a gas into a liquid just by increasing
pressure.
pressure
The critical temperature varies from substance to
p
substance.
BITS Pilani, Pilani Campus

26. Phase Equilibrium in a Pure Substance
If for given pressure,
The temperature of liquid is lower than
saturation temperature, it is called a
subcooled liquid (T < TS) or a compressed
liquid (P > PS).
A liquid that is not about to vaporize.
If for given pressure, the temperature of vapor
is greater than saturation temperature it is
temperature,
called a superheated vapor.
A vapor that is not about to condense
BITS Pilani, Pilani Campus

27. Phase Equilibrium in a Pure Substance …
At saturation temperature and
pressure, if a substance exists as
liquid,
liquid it is called saturated liquid
liquid.
At saturation temperature and
pressure, if a substance exists as
vapor, it is called saturated vapor.
BITS Pilani, Pilani Campus

28. Liquid + vapor region
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29. Compressed liquid region
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30. Superheated vapor region
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31. Supercritical fluid region
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32. Gibbs Phase Rule
The Phase Rule describes the possible
number of degrees of freedom in a
(closed) system at equilibrium, in terms
of the number of separate phases and
the number of chemical constituents in
the system.
The Degrees of Freedom [F] is the
number of independent intensive
variables that need to be specified in
value to fully determine the state of the
system.
BITS Pilani, Pilani Campus

33. Gibbs Phase Rule
Gibbs Phase Rule:
F=C-P+2
Where P: The number of phases
C: The Chemical Constituents
For Example: A system with one
component and one phase has two
degrees of freedom: temperature and
pressure,
pressure say,
say can be varied
independently.
BITS Pilani, Pilani Campus

34. Gibbs Phase Rule
For pure substance (
p (C=1) )
F=1+2–P=3–P
F=3–P
For P = 1 F = 2 ( P & T independent)
For P = 2, F = 1( P & T dependent)
For P = 3, F = 0 ( Triple Point)
, p )
BITS Pilani, Pilani Campus

35. Example 1
p
Determine whether water at each of the following g
states is a compressed liquid, a superheated
vapor, or a mixture of saturated liquid and
vapor.
a. 10 MPa, 0.003 m3/kg
b. 200 OC, 0.1 m3/kg
c. 1 MPa, 190 OC
d. 10 kPa, 10 OC
We consult Table B.1.1 if T is given, and Table
B.1.2 if P is given.
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36. Example 1: Solution
a. 10 MPa, 0.003 m3/kg g
vf = 0.001452, vg = 0.01803 m3/kg. So it is a
mixture of liquid and vapor.
b. 200 OC, 0.1 m3/kg
v < vg = 0.12736 m3/kg, so it is two phase
g p
mixture
c. 1 MPa, 190 OC
T > Tsat = 179.91 OC, so it is superheated vapor.
d. 10 kPa, 10 OC
P > Pg = 1.2276 kPa, so it is compressed liquid.
BITS Pilani, Pilani Campus

37. Example 2
Give the phase and specific volume for following
states:
a. Water at T = 275 OC, P = 5 MPa
b. Water at T = -2 OC, P = 100 kPa
c. Ammonia at
T = 170 OC, P = 600 kPa
,
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38. Example 2
Give the phase and specific volume for
following states:
a. Water at T = 275 OC, P = 5 MPa
,
Consult Table B.1.1 or B.1.2
Psat = 5.94 MPa, so we have superheated
t
vapor.
v = 0.04141 m3/kg
00 3/ g
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39. Example 2
b Water at T = -2 OC P = 100 kP
b. W t t 2 C, kPa
Consult Table B.1.5
Psat = 0.518 kPa, so we have compressed
solid.
v = vl = 0.0010904 m3/kg
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40. Example 2
Ammonia at T = 170 OC P = 600 kP
c. A i t C, kPa
Consult Table B.2.2
T > Tc and P<Pc, so we have superheated
p
vapor.
v = (0.34699 + 0.36389)/2 = 0.3554 m3/kg
BITS Pilani, Pilani Campus

41. Example 3
b. Ammonia at T = 170 OC, P = 600
kPaa
Consult Table B.2.2
T > Tc and P<Pc, so we have
superheated apor
s perheated vapor.
v = (0.34699 + 0.36389)/2 = 0.3554
( )
m3/kg