PURE SUBSTANCE
PURE SUBSTANCE
Pure substance: A substance that has a fixed chemical
composition throughout.
Air is a mixture of several gases, but it is considered to be a
pure substance.
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Pure Substance
A substance that has a fixed chemical composition throughout is called a pure
substance.
Example include Water, Nitrogen, Helium, and Carbon dioxide etc.
A pure substance can be a single chemical element/compound, or a homogeneous
mixture of several elements.
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
.
A mixture of ice and liquid water is pure substance but a mixture of liquid
air and gaseous air is not.
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Phases of a Pure Substance
There are three principal phases of matter; solid, liquid , gases.
However a substance may have several phases within a principal phase,
each with a different molecular structure.
For example existence of Carbon in form of diamond or graphite, two
liquid phases of Helium and so on.
Intermolecular bonds are strongest in solids and weakest in gases.
The molecules in a solid are arranged in a three-dimensional pattern
(lattice) that is repeated throughout.
.
Molecules are kept at fixed position due to large intermolecular forces.
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PHASES OF A PURE SUBSTANCE
The molecules
in a solid are
kept at their
positions by the
large springlike
inter-molecular
forces.
In a solid, the
attractive and
repulsive forces
between the
molecules tend
to maintain them
at relatively
constant
distances from
each other.
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Introduction General Energy Analysis Properties of pure substance Ener gy analysis
Pure substance
PhasesEnergy
Phaseanalysis
ChangeofProcess
Saturation
Phases of a Pure Substance: 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.
Heat is now transferred to the water until its temperature rises.
As the temperature rises, the liquid water expands which result in upward
motion of piston.
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Introduction General Energy Analysis Properties of pure substance Ener gy analysis
Pure substance
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Phaseanalysis
ChangeofProcess
Saturation
Phases of a Pure Substance: Compressed Liquid and Saturated Liquid
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.
A liquid that is about to vaporize is called a saturated liquid. Properties
at the saturated liquid state are noted by the subscript f and v2 = v f .
In the compressed liquid region, the properties of the liquid are
approximately equal to the properties of the saturated liquid state at the
temperature.
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Introduction General Energy Analysis Properties of pure substance Ener gy analysis
Pure substance
PhasesEnergy
Phaseanalysis
ChangeofProcess
Saturation
Phases of a Pure Substance: Saturated Vapor and Superheated Vapor
Once boiling starts, the temperature stops rising until the liquid is
completely vaporized.
The temperature will remain constant during the entire phase-change
process if the pressure is held constant.
Midway about the vaporization line, the cylinder contains equal amounts
of liquid and vapor
.
The mixture of steam and water is called saturated steam-water mixture.
As heating is continued, the fraction of steam increase in the mixture.
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Phases of a Pure Substance: Saturated Vapor and Superheated Vapor
As heating is continued, the vaporization process continues until the last
.
drop of liquid is vaporized.
At this point, the entire cylinder is filled with vapor that is on the borderline of
the liquid phase.
Any heat loss from this vapor will cause some of the vapor to condense (phase
change from vapor to liquid).
A vapor that is about to condense is called a saturated vapor.
The subscript g will always denote a saturated vapor state. Note v4 = vg.
Thermodynamic properties at the saturated liquid state and saturated vapor
state are given in Table A-4 as the saturated temperature table and Table A-5
as the saturated pressure table.
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Phases of a Pure Substance: Saturated Vapor and Superheated
Vapor After the completion of phase change process, further transfer of
heat results in an increase in both. the temperature and the specific
volume of
the vapor.
If some heat is take away at this point (300◦C and 1 atm), the
condensation of vapor would not start until temperature reach 100◦C. A
vapor that is not about to condense (i.e., not a saturated vapor) is called
a superheated vapor.
State 5 is called a superheated state because T5 is greater than the
saturation temperature for the pressure.
Thermodynamic properties for water in the superheated region are found
in the superheated steam tables, Table A-6.
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PHASE-CHANGE PROCESSES OF PURE
SUBSTANCES
• Compressed liquid (subcooled liquid): A substance that it is not
about to vaporize.
• Saturated liquid: A liquid that is about to vaporize.
At 1 atm and 20°C,
water exists in the
liquid phase
(compressed liquid).
At 1 atm pressure
and 100°C, water
exists as a liquid
that is ready to
vaporize
(saturated liquid).
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• Saturated vapor: A vapor that is about to condense.
• Saturated liquid–vapor mixture: The state at which the liquid and
vapor phases coexist in equilibrium.
• Superheated vapor: A vapor that is not about to condense (i.e., not a
saturated vapor).
At 1 atm pressure, the
As more heat is transferred,
temperature remains
part of the saturated liquid
constant at 100°C until the
vaporizes (saturated liquid– last drop of liquid is vaporized
vapor mixture).
(saturated vapor).
As more heat is
transferred, the
temperature of the
vapor starts to rise
(superheated vapor).
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Phases of a Pure Substance
This constant-pressure phase-change process is illustrated on a T −υ
diagram: .
If the entire process described in the previous slides is reversed by cooling
the water while maintaining the pressure at the same value, the water will
go back to state 1.
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Introduction General Energy Analysis Properties of pure substance Ener gy analysis
Pure substance
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Phaseanalysis
ChangeofProcess
Saturation
Saturation Temperature and Saturation Pressure
Anything wrong with the statement ”water boils at 100◦C”?
The complete statement would be “water boils at 100◦C at 1 atm
pressure”.
The temperature at which water starts boiling depends on the pressure;
therefore, if the pressure is fixed, so is the boiling temperature.
Saturation temperature, Tsat : the temperature at which a pure substance
changes phase at a given pressure.
Saturation pressure, Psat : the pressure at which a pure substance changes
phase at a given temperature.
At a pressure of 101.325 kPa, Tsat is 99.97◦C. Conversely, at a temperature
of 99.97◦C, Psat is 101.325 kPa.
Latent Heat: The amount of energy absorbed or released during a phasechange process is called the latent heat.
Latent heat of fusion: The amount of energy absorbed during melting is
called the latent heat of fusion.
Latent heat of vaporization The amount of energy absorbed during
vaporization is called latent heat of vaporization.
Reading homework: Some consequences of Tsat and Psat dependence
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Property Diagrams for Phase-Change. Processes: The T −υ
diagram
Water starts boiling at a much higher temperature (179.9◦C) at 1 Mpa. At
higher pressures, the specific volume of the saturated liquid is larger
and the specific volume of the saturated vapor is smaller than the
corresponding values at 1 atm pressure.
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Property Diagrams for Phase-Change Processes: The T −υ
diagram
The saturation line shrinks with
incre.ase in pressure and becomes a point
at pressure 22.06 MPa.
Critical Point: is the point at which the saturated liquid and saturated
vapor states are identical.
At pressures above the critical pressure, there is not a distinct phasechange process.
The temperature, pressure, and specific volume of a substance at the
critical point are called, respectively, the critical temperature Tcr , critical
pressure Pcr , and critical specific volume υcr .
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Property Diagrams for Phase-Change Processes: The T −υ
diagram
The critical-point properties of water are P = 22.06 MPa, T = 373.95◦C,
m3/kg.
cr
cr
and υcr = 0.003106
Above critical point, there is only one phase present which is of vapor
state.
Above the critical state, there is no line that separates the compressed
liquid region and the superheated vapor region.
By connecting the saturated liquid an.d saturated vapor points by a line:
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Property Diagrams for Phase-Change Processes: The P−υ
diagram
The constant temperature lines
can be sketched on P−υ, for a liquid water
contained in a piston cylinder arrangement at 1MPa and 150◦C, by
removing weights and adding heat to the system.
.
The constant temperature lines have a downward trend as the specific
volume increases by decrease in pressure at a constant temperature.
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Property Tables for Phase-Change Processes
Due to complexity of relationships among thermodynamic properties,
properties are frequently presented in the form of tables.
In addition to the temperature, pressure, and volume data, Tables A-4
through A-8, contain the data for the specific internal energy, u, the
specific enthalpy, h, and the specific entropy, s for steam.
A separate table is prepared for each region of interest such as the
superheated vapor, compressed liquid, and saturated (mixture) regions.
Entropy is a property associated with the second law of thermodynamics
and measure the order of disorderness of a system on a molecular level.
The enthalpy is a convenient grouping of the internal energy, pressure, and
volume and is given by
.
The enthalpy per unit mass, h, in KJ/. kg:
The enthalpy, h, is quite useful in calculating the energy of mass streams
flowing into and out of control volumes.
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Saturated Liquid and Saturated Vapor States
The properties of saturated liquid and saturated vapor for water are listed
in Tables A-4 and A-5.
Both tables give the same information.
In Table A-4, properties are listed as .a function of temperature.
The subscript f is used to denote properties of a saturated liquid, and the
subscript g to denote the properties of saturated vapor.
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Saturated Liquid and Saturated Vapor States
Another subscript commonly used is f g, which denotes the difference
between the saturated vapor and saturated liquid values of the same
property.
For example:
.
The quantity h f g 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
In Table A-5, properties are listed as a function of pressure.
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gy analysis of closed systems Energy analysis of open systems The Second La
Pure substance and Phases Phase Change Process Saturation Properties
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Saturated Liquid and Saturated Vapor
States
Introduction General Energy Analysis Properties of pure substance Ener
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Pure substance and Phases Phase Change Process Saturation Properties
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Introduction General Energy Analysis Properties of pure substance Ener
Saturated Liquid and Saturated Vapor States
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Question
Is iced water a pure substance? Why?