There is nothing so strange in a strange land
than the stranger that visits it.
Even though thermodynamics is a subject that is very easy to learn,
some students ﬁnd it diﬃcult because they get confused with the meaning
of the terms used in thermodynamics. In order to avoid such confusion,
the thermodynamic meaning of the frequently used terms are clearly stated
in this chapter. One can use this chapter as one uses a dictionary, coming
back to it to refer to the thermodynamic meaning of a term, whenever
there is a need for doing so.
2.1 System, Surroundings & Boundary
System In thermodynamics, a system is either a quantity of
matter or a region of space selected for study.
Surroundings Everything that is outside the system is known as the
Boundary The closed surface that separates the system from its
surroundings is known as the system boundary. Mat-
ter and/or energy are exchanged between the system
and its surroundings across the boundary of the sys-
Let us say we want to study the behaviour of air contained in a piston-
cylinder device, the cross-section of which is shown in Figure 2.1. We
consider the air within the piston-cylinder device as the system. Surround-
ings consists of the piston, the cylinder, and the environment outside the
piston-cylinder device. The cross-section of the boundary is shown by the
dashed line in Figure 2.1, and it is an imaginary surface that separates the
air from the inner surfaces of the piston and the cylinder. Note that the
shape and size of the boundary of the system are changed when the piston
is moved by applying a force on the piston. Such a boundary is known as
Figure 2.1 Cross-section of a piston-cylinder device containing air.
Another example of a system is the gas trapped in a closed tank shown
in Figure 2.2. Its surroundings consists of the tank and the environment
outside the tank. The boundary is shown by the dashed line in Figure 2.2.
Thermodynamic Terminology 9
If the tank is made up of rigid walls then the shape and size of the boundary
cannot be changed when a force is applied on it. Such a boundary is known
as a rigid boundary.
Figure 2.2 Cross-section of a closed tank containing gas.
Yet another example of a system is shown in Figure 2.3. We want to
investigate the behaviour of the water contained in the tank. The problem
here is that the mass of water within the tank does not remain the same
throughout the investigation since water enters and leaves the tank through
the inlet and outlet, respectively. Therefore, all what we could investigate
is the behaviour of the mass of water within the tank at any given time. In
such a situation, we choose the region in space that contains the mass of
water within the tank as the system. The boundary of the system is marked
by the dashed line shown in Figure 2.3. Everything outside the boundary
is the surroundings.
Figure 2.3 Cross-section of a tank containing water.
10 Chapter 2
2.2 Closed, Open & Isolated Systems
A system can be a closed system, an open system or an isolated system
depending on what can pass through its boundaries.
Closed System A closed system contains the same matter within
the system throughout the investigation. There-
fore, no matter crosses the boundary of a closed
system. However, energy can cross the boundary
of a closed system.
Open System An open system allows both matter and energy to
enter or leave the system.
Isolated System In an isolated system, neither matter nor energy
enters or leaves the system.
Air trapped in the piston-cylinder device shown in Figure 2.1 is an ex-
ample of a closed system since no air enters or leaves the system crossing
the boundary. Another example of closed system is the gas contained in the
tank shown in Figure 2.2 since the mass of gas remains the same throughout
The system of Figure 2.1 has a movable boundary, and therefore work
can be delivered to air by pushing the piston so as to decrease the volume
of air. The system of Figure 2.2 has a rigid boundary, and therefore no
work can be delivered to the gas contained within the rigid tank by moving
the boundary. Heat could enter or leave the closed systems of Figure 2.1
and Figure 2.2 through the boundaries, provided the walls are made up of
heat conducting materials.
The example of Figure 2.3 is an open system since matter, that is water,
enters and leaves the system during the investigation. Heat can be supplied
to the system through the walls of the container. Work, for example, can
be provided to the system using a stirrer to stir water. An open system
is also known as control volume, and its boundary known as control
It should be borne in mind that one very seldom comes across an iso-
lated system in real life. Nevertheless, in some cases, certain real systems
are approximated to isolated systems when carrying out thermodynamic
Thermodynamic Terminology 11
Aproperty is any characteristic of a system which can be measured or
calculated. Temperature and pressure are examples of properties that can
be measured. Internal energy, enthalpy (see Chapter 4) and entropy (see
Chapter 11) are examples of properties that can be calculated. Properties
can be either intensive properties or extensive properties, the deﬁnitions of
which are given below.
Extensive Property Any property that relates to the quantity of all
matter present in the system is an extensive
Extensive properties can be added up. Volume and energy of the entire
system are examples of extensive properties. We, in general, use upper case
symbols to denote extensive properties, such as Vfor the total volume of
the system, Efor total energy, U for internal energy, Hfor enthalpy and
Intensive Property Any property that is deﬁnable at a point in the
system is an intensive property, and its value
may change from one point of a system to an-
Intensive properties cannot be added up. Temperature is an example of
an intensive property, and we know that a thermometer inserted at diﬀerent
points in a system may register diﬀerent temperatures. Pressure and density
are also examples of intensive properties. We, in general, use lower case
symbols to denote intensive properties. There are exceptions however, like
the symbols Tand Pused for the intensive properties temperature and
Dividing an extensive property of a system by the total mass of the
system, we get a property known as speciﬁc property. An example of
speciﬁc property is speciﬁc volume, which is the total volume divided by
the mass, and it takes the unit m3/kg. Speciﬁc internal energy, speciﬁc
enthalpy and speciﬁc entropy are all speciﬁc properties.
Dividing an extensive property of a system by the amount of substance
present in a system, we get a property known as molar property.Molar
volume, which is the total volume divided by the amount of substance, is
12 Chapter 2
an example of molar property. In this textbook, we use the unit m3/kmol
for molar volume. Molar internal energy, molar enthalpy and molar entropy
are all molar properties.
Both the speciﬁc and molar properties are intensive properties. In this
textbook, we choose to denote them by the same lower case symbol. For
example, vis used to denote both the speciﬁc and molar volumes, ufor
speciﬁc and molar internal energies, hfor speciﬁc and molar enthalpies,
and sfor speciﬁc and molar entropies. Such a usage is acceptable since the
respective units of v,u,hand swill clarify what the symbols represent.
Student: Teacher, what do you mean by amount of substance?
Teacher: You know that a quantity of matter can be measured by its mass.
What you may not know is that the quantity of matter can also be mea-
suredintermsoftheamount of substance. The unit of amount of
substance is the mole, abbreviated ‘mol’. The number of elementary en-
tities in one mole of any substance is equal to the Avogadro’s number,
which is 6.022 ×1023. Here elementary entities mean atoms, molecules,
ions, electrons, etc.
Student: Teacher, you have not used the unit mol for the amount of substance.
You have used kmol. What is kmol then?
Teacher: One kilomole is equivalent to 1000 moles, and therefore the number
of elementary entities in one kilomole is equal to 6.022 ×1026.When
using kg as the unit for mass, it is convenient to use kilomole, abbreviated
‘kmol’, as the unit for the amount of substance.
Student: Teacher, now I know that the amount of substance is a way to quan-
tify matter, and its unit is mole or kilomole. I also know that one kmol
of matter contains 6.022 ×1026 elementary entities. In some textbooks,
however, I have seen the unit kgmol. Is kgmol the same as kmol?
Teacher: Yes, it is. Consider a substance with molar mass M.Weknow
that Mgrams of this substance is equivalent to one mole. If we take M
kilograms of this substance then we have one kilogram-mole, abbreviated
kgmol. Using these facts, let us work out the following:
1kgmol =Mkg = 1000 Mg= 1000 mol =1kmol
Student: Okay. I see that one kgmol is the same as one kmol. Teacher, I want
to know one more thing. What is molar mass?
Thermodynamic Terminology 13
Teacher: The ratio between the mass of a substance mand the amount of
substance nis known as the molar mass, denoted by M. Therefore, M
=m/n. For example, the molar mass of carbon12, which is a particular
form of carbon, is exactly 12 kg/kmol. The common unit for molar mass
is g/mol, which is equivalent to kg/kmol, which is the unit used in this
Student: Thank you, Teacher. I have one more question. Is molar mass the
same as molecular weight?
Teacher: The molecular weight is numerically equal to the molar mass taken
in the unit g/mol or kg/kmol. However, unlike the molar mass, molecular
weight is dimensionless. By the way, molecular weight is also known as
relative molecular mass.
State The condition at which a system exists is called the state of
a system. The state of a system is identiﬁed or described by
It is straight forward to describe the state of a system by its extensive
properties such as the total volume of the systems. Describing a state
of a system by its intensive properties, however, is a diﬃcult task since
intensive properties of a state may change from one point in the system
to another. If we are to describe the state of a system by the intensive
property temperature, for example, we need to measure the temperature at
numerous points across the entire system using a very, very large number
of thermometers. That would, of course, be an extremely diﬃcult task to
2.5 Equilibrium State
Equilibrium State An equilibrium state is a state at which all in-
tensive properties remain uniform throughout
14 Chapter 2
When an intensive property remains uniform throughout the system, it
has one and the same value at each and every point in the system.
For example, a thermometer would register identical values of tem-
peratures at all points in a system at an equilibrium state. Therefore, a
single value of temperature is enough to describe the entire system at an
Similarly, pressure gauge would register the same value of pressure at
any point in a system at an equilibrium state. Therefore, a single value of
pressure is enough to describe the entire system at an equilibrium state.
In a system containing more than one component, chemical composi-
tion would be the same at any point in a system at an equilibrium state.
Therefore, a single value of chemical composition is enough to describe the
entire system at an equilibrium state.
It is the same with any other intensive property, such as speciﬁc volume,
speciﬁc enthalpy or any other speciﬁc property, of a system at an equilibrium
state. It is therefore an equilibrium state is a very convenient state to work
ALL INTENSIVE PROPERTIES
REMAIN UNIFORM IN A SYSTEM
AT AN EQUILIBRIUM STATE.
Let us now consider the ideal gas equation of state PV =nRT,where
Pis the pressure, Vis the volume, nin the amount of substance, Ris the
universal gas constant, and Tis the temperature. It is interesting to note
that when we use the ideal gas equation of state, we use a single value of P
and a single value of Tto represent the state, which is justiﬁed only if the
given state is an equilibrium state. It is important to note that properties
of a system can be interrelated using the ideal gas equation of state, only
if the state concerned is an equilibrium state.
Thermodynamic Terminology 15
When a system changes from one state to another, it is said to execute
aprocess. The continuous series of states that a system passes through
during a process is called the path of the process. The following are a
few processes that one frequently comes across in thermodynamics:
Constant Pressure Process: It is a process during which the pres-
sure remains constant, while the other properties of the system may
Constant Volume Process: During a constant volume process, vol-
ume remains constant while the other properties may change from
one state to another.
Isothermal Process: It is a constant temperature process. Even
though the temperature remains constant in an isothermal process,
heat may be transferred between the system and its surroundings. It
is common to supply heat to a system to maintain its temperature
Adiabatic Process: It is a process taking place while the system re-
mains thermally insulated from its surroundings. That is, no heat is
transferred between the system and its surroundings.
Student: Teacher, isn’t an adiabatic process the same as an isothermal process.
Teacher: No, it is not. What makes you think that an adiabatic process is the
same as an isothermal process?
Student: You said that no heat is transferred between the system and the
surroundings of an adiabatic process. That means the temperature of the
system could not change during the process. You also said that a process
taking place at constant temperature is an isothermal process. Therefore,
these two processes are the same. Aren’t they?
Teacher: No, dear Student, they are not the same. I see that you are thinking
that a temperature of a system must remain constant if the system does
not exchange heat with its surroundings.
Student: Yes, Teacher. I think that.
16 Chapter 2
Teacher: Dear Student, it is wrong to think that a temperature of a system
does not change if the system does not exchange heat with its surround-
Student: Is it? Umm... Teacher, forgive me. I can’t think of a system under-
going an adiabatic process where the temperature of the system changes.
Could you please help me with an example of such a process?
Teacher: Yes, I could. Consider a gas contained in a piston-cylinder device.
Let us take the gas as the system. Assume that the piston and the cylinder
are made up of heat resistant material. As a result, no heat is exchanged
between the system and the surroundings. Therefore, any process that this
system undergoes is an adiabatic process. Could you agree with that?
Student: Yes, I could.
Teacher: Now, let us imagine that a force is applied on the outer face of the
piston, as shown in Figure 2.4.
initial state ﬁnal state
Figure 2.4 No heat is supplied, but a force is applied on the piston.
If the force applied is large enough for the piston to overcome friction, it
would move such that the volume of the gas would be decreased. That is,
the gas is compressed by applying a force. As you know, compressing a gas
increases its pressure. Increasing the pressure increases its temperature.
Therefore, we have just seen an example where the work done to compress
the gas is responsible for increasing the temperature of the gas. So you
see the temperature of the gas could be increased by compressing the gas
during an adiabatic process.
Student: Teacher, I think that I am beginning to see the diﬀerence between
an adiabatic process and an isothermal process.
Teacher: That is good. Let us then continue with the thermodynamic termi-
Thermodynamic Terminology 17
2.7 Simple Compressible System
As absolute beginners in thermodynamics, let us restrict ourselves to
studying the behaviour of simple compressible systems.Theseare
systems comprising pure substances uninﬂuenced by surface tension eﬀects,
motion, and gravitational, electrical and magnetic ﬁelds.
Student: Teacher, what do you mean by pure substance?
Teacher: A pure substance is a substance with uniform chemical composition.
Water is a pure substance since its chemical composition is the same
Student: Water is one single chemical compound, and therefore it has uniform
chemical composition. Air, which is a mixture of more than one chemical
compound, is not a pure substance. Am I correct?
Teacher: Remember that uniform chemical composition is the keyword
in deﬁning a pure substance. Thus, air can be treated as a pure substance
as far as the chemical composition of air remains uniform. For that matter,
any mixture with a uniform chemical composition can be treated as a pure
Student: I think I have got it. Any substance can be treated as a pure sub-
stance as far as its chemical composition remains uniform.
As we have seen above simple compressible systems are uninﬂuenced
by surface tension eﬀects and external force ﬁelds, and therefore work of
changing surface area, electrical work, magnetic work, and the likes are
absent in such systems. Thus, properties, such as surface tension, electrical
charge, and magnetic dipole moment, have no signiﬁcance when dealing
with simple compressible systems.
The only form of work that we come across when working with simple
compressible systems is boundary work, that is the work done to com-
press (or expand) the volume of the system. And, the properties used to
characterize a state of a simple compressible system are pressure, volume,
temperature, internal energy, enthalpy and entropy. We already have some
18 Chapter 2
idea about pressure, volume and temperature. Internal energy and enthalpy
are introduced in Chapter 4, and entropy is introduced in Chapter 11.
It is common in the thermodynamic literature to refer to the simple
compressible system as simple system.
2.8 State Postulate
Even though a state of a system can be characterized by a number of
properties, repeated observations and experiments have shown that only two
intensive properties of a simple compressible system can be independently
chosen. That is to say the following:
Two independent, intensive properties are adequate
to completely specify an equilibrium state of a sim-
ple compressible system,
which is a simpliﬁed version of what is known as the state postulate.
The above version of the state postulate is sometimes referred to as a
Once the two independent, intensive properties that specify the equi-
librium state are chosen, any other intensive property at that equilibrium
state is uniquely determined by these two properties.
Take, for example, air at 1 bar pressure and 27◦C temperature. As-
suming that air behaves as an ideal gas, the molar volume of air shall be
calculated using the ideal gas equation of state as follows:
P=(8.314 kJ/kmol ·K) (300 K)
In this case, we take Pand Tas the independent intensive properties.
Then, the third intensive property, molar volume v, is automatically ﬁxed.
Thermodynamic Terminology 19
2.9 Property Diagram
Let us choose the independent, intensive properties as pressure Pand
speciﬁc volume v, and make a diagram of properties as in Figure 2.5.
Consider point Iin the property diagram, it has coordinates (Po,vo). Since,
according to the state postulate, two independent, intensive properties are
adequate to ﬁx an equilibrium state of a simple compressible system, point I
in the P-vdiagram represents an equilibrium state of a simple compressible
Figure 2.5 Equilibrium states Iand Fare shown on a property diagram.
Once (Po,vo) is speciﬁed, all the other intensive properties, such as T,
u,hand s, at that equilibrium state are automatically ﬁxed as functions of
Poand vo. Therefore, the state represented by point Ion a P-vdiagram
can as well be represented by a point on a h-sdiagram, etc. That is to
say an equilibrium state of a simple system is ﬁxed by two of its intensive
properties just in the same way as a point in space is ﬁxed by its coordinates.
Let us now consider the equilibrium state represented by point Fhaving
the coordinate (Pf,vf) in Figure 2.5. We know that all the other intensive
properties at that equilibrium state are functions of Pfand vfalone.
Let us consider a gas contained in a piston-cylinder device, say, at
the equilibrium state Ishown in Figure 2.5. Let us imagine that the gas
undergoes a process, during which the gas expands and its pressure falls,
until it reaches the equilibrium state Fshown in Figure 2.5. It is important
to note that whatever path is followed by the process to reach point F,the
properties at Fremain the same since they are functions only of Pfand
20 Chapter 2
Suppose we need to determine the total change in the pressure of the
system during the process considered. Since pressure is a property which
depends on the state, and not on how the system reaches that state, the
total change in pressure between the two equilibrium states Iand Fis
where Pfand Porepresent the respective values of the property Pat states
In general, for any one of the properties P,v,T,u,hor s, represented
by the notation χ, say, we could write
where χfand χorepresent the respective values of the property χat states
“don’t run away........., come back”