Pure Substance
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A substance that has a fixed
chemical
composition
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throughout is called
a pure
substance, e.g.: H2O,
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 A pure substance does not have to be of a single
chemical element or compound
 A mixture of chemical elements/compounds also
qualifies as a pure substance as long as the mixture
is homogeneous, e.g. Air
 However, a mixture of oil and water is
oil
not a pure substance - oil is not
homogeneous/soluble in water, forming
water
two chemically dissimilar regions

Phases Of Pure Substances
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o having a distinct molecular
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 A phase is identified
as
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arrangement
that
throughout and
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separated
the others by easily identifiable
Pag
Prev from
boundary surfaces. Eg: Two phases of water in iced
water
 Three principal phases: solid, liquid and gas
 A substance may have several phases within a
principal phase, each with a different molecular
structure. For eg:
 Helium has two liquid phases;
 Iron has three solid phases.
PROCESS 4-5
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P
Tsat
Properties of water
in the superheated
region
listed
in
Superheated Tables
v2=vf
v4=vg
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• Above critical temperature and pressure  no significant
process of phase change occurs.
• The volume continues to increase and only one phase that
similar to vapour phase exists.
Quality and saturated
region
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 During vaporization, a substance
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and vapour) that consists
of af mixture
of saturated liquid and
m
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f The 2ratio
o of vapor to the total mass of
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saturated vapour.
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mixture is quality.P
x
 x=0
mvapour
mtotal

mvapour
mliquid  mvapour

mg
m f  mg
(or 0%) for saturated liquid and x=1 (or 100%) for
saturated vapour, so the range for quality is 0  x  1.
 Quality has significance for saturated mixtures only. It has no
meaning in the compressed liquid or superheated vapor
regions.
 The properties of the saturated liquid are the same whether it
exists alone or in a mixture with saturated vapor. During the
vaporization process, only the amount of saturated liquid
changes, not its properties. Similarly to a saturated vapor.
Example of compressed liquid table
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The formatre
ofvi
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P
Table 1–7 is
similar to the
superheated
vapor tables,
except the
saturated data
represent the
saturated liquid
properties
Tsat
Example 2.1
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o this table
Using Property Tables, complete
for H2O:
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om 3 of 5
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PrP (MPa) Pva(m3/kg) T (oC) Phase and Quality
(a)
1.725
(b)
0.85
(c)
15
(d)
(e)
(f)
(g)
0.227
400
0.001124
0.75
180
0.221
0.3879
0.25
100
0.095
150
The Compressibility
Factor,
Z
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
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From the Generalized
Compressibility
Chart,
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o can be made :
the following
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P pressures (P << 1), gases
1.) At very low
R
behave as an Ideal Gas regardless of
Temperature.
2.) At high temperatures (TR > 2), Ideal Gas
behaviour can be assumed with good accuracy
regardless of Pressure except when PR>>1.
3.) The deviation of a gas from Ideal Gas
behaviour is greatest in the vicinity of the Critical
Point.
INTERNAL ENERGY, ENTHALPY AND
SPECIFIC
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HEATS OF SOLIDS AND
LIQUIDS
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 A substance whose
specific fvolume
(or density) is constant
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o - the specific volumes of solids
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 incompressible
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P
and liquids essentially
remain constant during a process.
 The
constant-volume and constant-pressure specific heats
are identical for incompressible substances:
CV  CP  C
kJ
kg.K 
 Specific
heat values for several common liquids and solids
are given in Table A–3.
 The specific heats of incompressible substances depend on
T only. Thus, internal energy of the incompressible
substances:
du  CdT  u  Cave T2  T1 
integratio n
Enthalpy??
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