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Module 5
PURE SUBSTANCE
Substance can exist either as a solid, a liquid or a gas. But at a particular temperature and
pressure, a substance can exist at these three phases. Water is an example. At 0 0C or 273 K,
water can exist as vapor in the atmosphere, as a liquid in the ocean or as a solid like an
iceberg. This module deals with the study of the different phases of pure substances and the
phase change processes of pure substances. This includes also the concepts of saturation
temperature and saturation pressure, phase equilibrium and triple point.
Upon completion of this module, the students will be able to:
1. Define and understand the properties and conditions of a pure substance.
2. Determine and understand the processes of a pure substance.
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Lesson 1 – Properties of Pure Substance
Pure substance refers to the one that has a uniform chemical composition and can
exist in more than one phase but its chemical composition must be the same in each phase.
For example, a system of water and water vapor in a system is a pure substance since their
combination does not react and each phase contains the same composition. Also, some
mixture of gases that does not react with each other is considered to be a pure substance.
A substance that has a fixed chemical composition throughout is called a pure substance such
as water, air, and nitrogen.
A pure substance does not have to be of a single element or compound. 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.
Phases of a Pure Substance
Pure substance normally has three phases, solid, liquid and gas. The more specific phases of
the pure substances and the phase change processes are the following.
1. Solid. In this phase, the substance does not take the shape of a container.
2. Compressed Liquid or Subcooled Liquid. It is the phase at which the liquid is nonsaturated, means any liquid that it is not about to vaporize. Any addition of heat
increases only the temperature of the liquid but does not cause any change of its phase.
3. Saturated Liquid. In this phase, any addition of heat causes some liquid to vaporize
leading to a mixture of saturated liquid and vapor.
4. Saturated Liquid-Vapor Mixture. In this phase, further addition of heat causes more
liquid to evaporate to form more vapor.
5. Saturated Vapor. In this phase, the vapor has absorbed more heat than necessary
to vaporize it and convert all liquid into vapor.
6. Superheated Vapor. In this phase, all liquid had converted to vapor and any addition
of heat will lead only to hotter vapor.
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7. Ideal Gas. In this phase, a highly superheated vapor behaves in accordance with the
ideal gas law.
8. Real Gas. In this phase, the gas does not behave in accordance with the ideal gas
law.
9. Gas Mixtures. In this phase, two or more gases mixed together freely.
10. Vapor/Gas Mixture. In this phase, two or more gases mixed freely with water
vapor.
Property Tables
1. Compressed Liquid
A compressed liquid is one which has a pressure higher than the saturation pressure
corresponding to the existing temperature.
Example: Is liquid at 110 KPa and 1000C a compressed liquid?
From steam tables, Psat at 1000C = 101.325 KPa
Comparing the two values, the actual liquid water pressure of 1100C is
greater than Psat at 1000C. Therefore, it is a compressed liquid.
The properties of a liquid are relatively independent of pressure (incompressible). A
general approximation is to treat compressed liquid as saturated liquid at the given
saturation temperature.
The property most affected by pressure is enthalpy. For enthalpy use the following
approximation:
2. Subcooled Liquid
A subcooled liquid is one which has a temperature lower than saturation temperature
corresponding to the existing pressure.
Example: Liquid water at 600C and 101.325 KPa is a subcooled liquid.
Reason is that, from steam tables, the saturation temperature at 101.325 KPa is
1000C. Since the actual temperature of the liquid water of 600C is less than
1000C, therefore it is a subcooled liquid.
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3. Saturated Liquid
A saturated liquid is a liquid at the saturations (saturation temperature and saturation
pressure) which has temperature equal to the boiling point corresponding to the
existing pressure. It is a pure liquid, meaning it has no vapor content.
Examples:
 Liquid water at 1000C and 101.325 KPa
 Liquid water at 233.90C and 3 MPa
 Liquid water at 324.750C and 12 MPa
4. Saturated Liquid‐Vapor Mixture
Vapor is the name given to a gaseous phase that is in contact with the liquid
phase, or that is in the vicinity of a state where some of it might be condensed.
A wet vapor is a combination of saturated vapor and saturated liquid.
During vaporization, a mixture of part liquid part vapor exists. To analyze this mixture,
you need to know the proportions of the liquid and vapor in the mixture. The ratio of
the mass of vapor to the mass of the total mixture is called quality, x
The percent moisture, (y) of wet vapor is the percent by weight that is saturated
liquid.
y=
mliquid
mtotal
Following the definitions of quality (x) and percent moisture (y),
x=
𝑚𝑔
y=
𝑚𝑓
𝑚
𝑚
(100
(100)
For saturated liquid:
y = 100%
x=0
For saturated vapor:
x = 100%
y=0
For wet vapor
0 < x <
100
5
0
>
y
<
100
But x +y = 100 in percent form
x + y = 1 in decimal form
Saturated liquid‐vapor mixture is treated as a combination of two sub‐systems (two
phases). The properties of the “mixture” are the average properties of the saturated
liquid‐vapor mixture.
The relative amounts of liquid and vapor phases
(quality x) are used to calculate the mixture
properties.
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Similarly,
Note: pressure and temperature are dependent in the saturated mixture region.
The Quality Region, also referred to as the Saturated Liquid-Vapor Mixture Region, is
the area enclosed between the saturated liquid line and the saturated vapor line. At
any point within this region the quality of the mixture (sometimes referred to as the
dryness factor) is defined as the mass of vapor divided by the total mass of the fluid.
For most substances, the relationships among thermodynamic properties are too
complex to be expressed by simple equations. Thus, properties are frequently
presented in the form of tables. The subscript “f” is used to denote properties of a
saturated liquid and “g” for saturated vapor. Another subscript, “fg”, denotes the
difference between the saturated vapor and saturated liquid values of the same
property.
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For example:
vf = specific volume of saturated liquid
vg = specific volume of saturated vapor
vfg = difference between vg and vf ( vfg = vg – vf)
Enthalpy: is a property defined as H = U + PV (kJ) or h = u + Pv (kJ/kg) (per mass unit).
Enthalpy of vaporization (or latent heat): represents the amount of energy needed to
vaporize a unit mass of saturated liquid at a given temperature or pressure. It
decreases as the temperature or pressure increase, and becomes zero at the critical
point.
5. Saturated Vapor
A saturated vapor is a vapor at the saturation conditions (saturation temperature and
saturation pressure). It is 100% vapor, meaning it has no liquid or moisture content.
Examples:
 Steam at 212.42 0C and 2MPa
 Steam at 352.37 0C and 17 MPa
6. Superheated Vapor
A superheated vapor is a vapor having a temperature higher than the saturation
temperature corresponding to the existing pressure.
Examples:
 Steam at 2000C and 101.325 KPa
It is because 2000C > tsat at 101.325 KPa which is 1000C
 Steam at 3000C and 5 MPa
3000C > tsat at 5 MPa which is 263.990C
Superheated region is a single-phase region (vapor only), temperature and pressure
are no longer dependent. If T>> Tcritical or P≪ Pcritical, then the vapor can be
approximated as an “ideal gas”.
7. Degrees of Superheat, 0SH
The degrees of superheat is the difference between the actual temperature and
superheated vapor and saturation temperature for the existing pressure.
In equation form:
0
SH = Actual superheated temperature – tsat at existing pressure
Example:
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Determine the degrees of superheat of superheated steam at 200 0C and 101.325
KPa.
Solution:
From steam tables:
tsat at 101.325 KPa = 1000C
0SH
= 2000C – 1000C
0SH
= 1000C
8. Degrees Subcooled, 0SB
The degrees subcooled of a subcooled liquid is the difference between the saturation
temperature for the given pressure and the actual subcooled liquid temperature.
0SB
= tsat at given pressure – actual liquid temperature
Example:
Determine the degrees subcooled of liquid water at 900C and 101.325 KPa
Solution:
From steam tables:
tsat at 101.325 KPa = 1000C
0SB
= 1000C – 900C
0SB
= 100C
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Lesson 2 – Processes of Pure Substance
Consider a process where a pure substance starts as a solid and is heated up at constant
pressure until it all becomes gas. Depending on the prevailing pressure, the matter will pass
through various phase transformations.
1. Solid
2. Mixed phase of liquid and solid
3. Sub‐cooled or compressed liquid (means it is not about to vaporize)
4. Wet vapor or saturated liquid‐vapor mixture, the temperature will stop rising until
the liquid is completely vaporized.
5. Superheated vapor (a vapor that is not about to condense).
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 At a given pressure, the temperature at which a pure substance starts boiling is called
the saturation temperature, tsat.
Saturation temperature, (tsat)
This is the temperature at which liquid start to boil or the temperature at which
vapors begin to condense. The saturation temperature of a given substance depends
upon the existing pressure. It is directly proportional to the pressure, i.e., it increases
as the pressure is increased and decreases as the pressure is decreased.
Examples:
 Water boils at 100OC at atmospheric conditions (101.325 KPa)
 Water boils at 179.9 0C at a pressure of 1000 KPa
 Steam condenses at 311.06 0C at 10 MPa
 Steam condenses at 390C at 0.007 MPa
 Likewise, at a given temperature, the pressure at which a pure substance starts boiling
is called the saturation pressure, Psat.
 During a phase‐change process, pressure and temperature are dependent properties,
Tsat = f (Psat).
 Latent Heat of Vaporization is the amount of heat added to/remove from the
substance in order to convert it from saturated liquid/saturated vapor to saturated
vapor/liquid with the temperature remaining constant. It is inversely proportional to
the temperature or pressure of the substance.
Example:
Determine the latent heat of vaporization of water at
a) 1000C
b) 2000C
and c) 3000c
Solution:
From steam tables:
a) hfg at 1000C = 2257.0 KJ/kg
b) hfg at 2000C = 1940.7 KJ/kg
c) hfg at 3000C = 1404.9 KJ/kg
 The critical point is the point at which the liquid and vapor phases are not
distinguishable.
The critical point represents the highest pressure and highest temperature at which
liquid and vapor can coexist in equilibrium. The state of water at critical conditions
whether it is saturated liquid or saturated vapor is unknown. Hence, the latent heat of
vaporization of water at this condition is either zero or undefined.
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 The “triple point” is the point at which the liquid, solid, and vapor phases can exist
together. On P‐v or T‐v diagrams, these triple‐phase states form a line called the triple
line.
 Sensible Heat is the heat that causes change in temperature without a change in phase.
Examples:
a. Heat added in raising the temperature of steam from 1000C at 101.325
KPa to 1500C
b. Heat removed in lowering the temperature of water from 90 0C to 800C.
 Latent Heat is the heat that causes change in phase without change in temperature.
Example:
Heat added in converting 1 kg of water at 1000C and 101.325 KPa to 1 kg of
steam at 1000C and 101.325 KPa
Vapor Dome
The general shape of a P‐v diagram for a pure substance is very similar to that of a T‐
v diagram.
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The P‐T or Phase Change Diagram
This is called phase diagram since all three phases are separated from each other by
three lines. Most pure substances exhibit the same behavior.
 One exception is water. Water expands upon freezing
There are two ways that a substance can pass from solid phase to vapor phase 1) it
melts first into a liquid and subsequently evaporates, 2) it evaporates directly without
melting (sublimation).




the sublimation line separates the solid and the vapor.
the vaporization line separates the liquid and vapor regions
the melting or fusion line separates the solid and liquid.
these three lines meet at the triple point.