Short Notes: Properties of Pure Substances | Short Notes for Mechanical Engineering PDF Download

 Page 1


Properties of pure substances 
Properties of Pure Substance
• A pure substance is a material with homogeneous and invariable 
composition.
• Pure substances can have multiple phases, an ice water mixture is still a pure 
substance but an air-steam is not a pure substance.
Pure Substance:
• A substance that has a fixed chemical composition throughout is called a 
pure substance i.e., water, nitrogen, helium, and CO2.
• Substances which are mixture of various elements or compounds also 
qualifies as pure substances as long as mixture is homogeneous.
Vapour Vapour
Liquid Liquid
(a) H2 0 (b) Air
Saturation Temperature and Saturation Pressure:
• At given pressure, the temperature at which a pure substance changes phase 
is called the saturation temperature Tsat.
• Likewise at a given temperature, the pressure at which a pure substance 
changes phase is called the saturation pressure psat.
Page 2


Properties of pure substances 
Properties of Pure Substance
• A pure substance is a material with homogeneous and invariable 
composition.
• Pure substances can have multiple phases, an ice water mixture is still a pure 
substance but an air-steam is not a pure substance.
Pure Substance:
• A substance that has a fixed chemical composition throughout is called a 
pure substance i.e., water, nitrogen, helium, and CO2.
• Substances which are mixture of various elements or compounds also 
qualifies as pure substances as long as mixture is homogeneous.
Vapour Vapour
Liquid Liquid
(a) H2 0 (b) Air
Saturation Temperature and Saturation Pressure:
• At given pressure, the temperature at which a pure substance changes phase 
is called the saturation temperature Tsat.
• Likewise at a given temperature, the pressure at which a pure substance 
changes phase is called the saturation pressure psat.
Example: For water at a pressure of 101.325 kPa, Tsat is 100°C, conversely at a 
temperature of 100°C, psat is 101.325 kPa.
Latent Heat:
• The amount of energy absorbed or released during a phase change process is 
called the latent heat.
• The amount of energy absorbed during melting is called the latent heat of 
vaporization.
• Similarly, the amount of energy absorbed during vaporization is called latent 
heat of vaporization and is equivalent to the energy released during 
condensation.
Liquid-Vapour Saturation Curve:
From the following figure, it is clear that Tsat increases with psat. Thus, a substance 
at higher pressure will boil at higher temperatures.
Tsat = f(Psat)
^sat •
• In the kitchen, higher boiling temperature means shorter cooking time and 
energy saving.
• The atmospheric pressure, and thus the boiling temperature of water, 
decreases with elevation. Therefore, it takes longer time to cook at higher 
altitudes than it does at sea level.
Property Diagrams for Phase-change Process 
The T-V Diagram:
• Consider piston cylinder device containing liquid water at 20°C and 1 atm.
• Water will start boiling at a much higher temperature (179.9°C) at inside 
pressure of the cylinder reaches at 1 MP.
• The specific volume of the saturated liquid is larger and the specific volume of 
the saturated vapour is smaller than the corresponding values at 1 atm 
pressure. That is, the horizontal line that connects the saturated liquid and 
saturated vapour states is much shorter.
• As the pressure is increased further, this saturation line will continue to get 
shorter as shown in figure and it will become a point when the pressure 
reaches 22.09 MPa for the case of water. This point is called the critical point
Page 3


Properties of pure substances 
Properties of Pure Substance
• A pure substance is a material with homogeneous and invariable 
composition.
• Pure substances can have multiple phases, an ice water mixture is still a pure 
substance but an air-steam is not a pure substance.
Pure Substance:
• A substance that has a fixed chemical composition throughout is called a 
pure substance i.e., water, nitrogen, helium, and CO2.
• Substances which are mixture of various elements or compounds also 
qualifies as pure substances as long as mixture is homogeneous.
Vapour Vapour
Liquid Liquid
(a) H2 0 (b) Air
Saturation Temperature and Saturation Pressure:
• At given pressure, the temperature at which a pure substance changes phase 
is called the saturation temperature Tsat.
• Likewise at a given temperature, the pressure at which a pure substance 
changes phase is called the saturation pressure psat.
Example: For water at a pressure of 101.325 kPa, Tsat is 100°C, conversely at a 
temperature of 100°C, psat is 101.325 kPa.
Latent Heat:
• The amount of energy absorbed or released during a phase change process is 
called the latent heat.
• The amount of energy absorbed during melting is called the latent heat of 
vaporization.
• Similarly, the amount of energy absorbed during vaporization is called latent 
heat of vaporization and is equivalent to the energy released during 
condensation.
Liquid-Vapour Saturation Curve:
From the following figure, it is clear that Tsat increases with psat. Thus, a substance 
at higher pressure will boil at higher temperatures.
Tsat = f(Psat)
^sat •
• In the kitchen, higher boiling temperature means shorter cooking time and 
energy saving.
• The atmospheric pressure, and thus the boiling temperature of water, 
decreases with elevation. Therefore, it takes longer time to cook at higher 
altitudes than it does at sea level.
Property Diagrams for Phase-change Process 
The T-V Diagram:
• Consider piston cylinder device containing liquid water at 20°C and 1 atm.
• Water will start boiling at a much higher temperature (179.9°C) at inside 
pressure of the cylinder reaches at 1 MP.
• The specific volume of the saturated liquid is larger and the specific volume of 
the saturated vapour is smaller than the corresponding values at 1 atm 
pressure. That is, the horizontal line that connects the saturated liquid and 
saturated vapour states is much shorter.
• As the pressure is increased further, this saturation line will continue to get 
shorter as shown in figure and it will become a point when the pressure 
reaches 22.09 MPa for the case of water. This point is called the critical point
and it is defined as the point at which the saturated liquid and saturated 
vapour state are identical.
• At pressure above the critical pressure, there will not be a distinct phase 
change. Instead, the specific volume of the substance will continually 
increase and at all times there will be only one phase present. It is customary 
to refer to the substance as superheated vapour at temperature above the 
critical temperature and as compressed liquid at temperatures below the 
critical temperature.
T V diagram of constant pressure phase-change process of a 
pure substances at various pressures.
The p-V Diagram:
• The general shape of the p-V diagram of a pure substance is very much like 
the T-V diagram but the T = constant lines on this diagram have a downward 
trend.
• Consider again a piston cylinder device that contains liquid water at 1 MPa 
and 150°C, water at this state exists as a compressed liquid. Now, the weights 
on top of the piston are removed one by one so that the pressure inside the 
cylinder decreases gradually.
• The water is allowed to exchange heat with the surroundings so its 
temperature remains constant.
• As the pressure decreases, the volume of the water will increase slightly, 
when the pressure reaches the saturation pressure volume at the specific 
temperature, the water will start to boil.
• During this vaporisation process, both the temperature and the pressure 
remain constant but the specific volume increases. Once the last drop of 
liquid is vaporised further reduction in pressure results in a further increase in 
specific volume.
• If the process is repeated for other temperatures similar paths will be 
obtained for the phase change processes.
Triple Phase:
• When all three phases of a pure substance co-exist in equilibrium. It is called 
triple phase.
• Triple phase states form a line called the triple line.
• The triple line appears as a point on the p-T diagram and therefore is often 
called the triple point.
Page 4


Properties of pure substances 
Properties of Pure Substance
• A pure substance is a material with homogeneous and invariable 
composition.
• Pure substances can have multiple phases, an ice water mixture is still a pure 
substance but an air-steam is not a pure substance.
Pure Substance:
• A substance that has a fixed chemical composition throughout is called a 
pure substance i.e., water, nitrogen, helium, and CO2.
• Substances which are mixture of various elements or compounds also 
qualifies as pure substances as long as mixture is homogeneous.
Vapour Vapour
Liquid Liquid
(a) H2 0 (b) Air
Saturation Temperature and Saturation Pressure:
• At given pressure, the temperature at which a pure substance changes phase 
is called the saturation temperature Tsat.
• Likewise at a given temperature, the pressure at which a pure substance 
changes phase is called the saturation pressure psat.
Example: For water at a pressure of 101.325 kPa, Tsat is 100°C, conversely at a 
temperature of 100°C, psat is 101.325 kPa.
Latent Heat:
• The amount of energy absorbed or released during a phase change process is 
called the latent heat.
• The amount of energy absorbed during melting is called the latent heat of 
vaporization.
• Similarly, the amount of energy absorbed during vaporization is called latent 
heat of vaporization and is equivalent to the energy released during 
condensation.
Liquid-Vapour Saturation Curve:
From the following figure, it is clear that Tsat increases with psat. Thus, a substance 
at higher pressure will boil at higher temperatures.
Tsat = f(Psat)
^sat •
• In the kitchen, higher boiling temperature means shorter cooking time and 
energy saving.
• The atmospheric pressure, and thus the boiling temperature of water, 
decreases with elevation. Therefore, it takes longer time to cook at higher 
altitudes than it does at sea level.
Property Diagrams for Phase-change Process 
The T-V Diagram:
• Consider piston cylinder device containing liquid water at 20°C and 1 atm.
• Water will start boiling at a much higher temperature (179.9°C) at inside 
pressure of the cylinder reaches at 1 MP.
• The specific volume of the saturated liquid is larger and the specific volume of 
the saturated vapour is smaller than the corresponding values at 1 atm 
pressure. That is, the horizontal line that connects the saturated liquid and 
saturated vapour states is much shorter.
• As the pressure is increased further, this saturation line will continue to get 
shorter as shown in figure and it will become a point when the pressure 
reaches 22.09 MPa for the case of water. This point is called the critical point
and it is defined as the point at which the saturated liquid and saturated 
vapour state are identical.
• At pressure above the critical pressure, there will not be a distinct phase 
change. Instead, the specific volume of the substance will continually 
increase and at all times there will be only one phase present. It is customary 
to refer to the substance as superheated vapour at temperature above the 
critical temperature and as compressed liquid at temperatures below the 
critical temperature.
T V diagram of constant pressure phase-change process of a 
pure substances at various pressures.
The p-V Diagram:
• The general shape of the p-V diagram of a pure substance is very much like 
the T-V diagram but the T = constant lines on this diagram have a downward 
trend.
• Consider again a piston cylinder device that contains liquid water at 1 MPa 
and 150°C, water at this state exists as a compressed liquid. Now, the weights 
on top of the piston are removed one by one so that the pressure inside the 
cylinder decreases gradually.
• The water is allowed to exchange heat with the surroundings so its 
temperature remains constant.
• As the pressure decreases, the volume of the water will increase slightly, 
when the pressure reaches the saturation pressure volume at the specific 
temperature, the water will start to boil.
• During this vaporisation process, both the temperature and the pressure 
remain constant but the specific volume increases. Once the last drop of 
liquid is vaporised further reduction in pressure results in a further increase in 
specific volume.
• If the process is repeated for other temperatures similar paths will be 
obtained for the phase change processes.
Triple Phase:
• When all three phases of a pure substance co-exist in equilibrium. It is called 
triple phase.
• Triple phase states form a line called the triple line.
• The triple line appears as a point on the p-T diagram and therefore is often 
called the triple point.
p V diagram of pure substance
• No substance can exist in the liquid phase in stable equilibrium at pressure 
below the triple point pressure.
• The same can be said for temperature for substance that contract on 
freezing.
• Substances at high pressure can exist in the liquid phase at temperatures 
below the triple point temperature.
The p-T Diagram:
• Solid - Liquid = Fusion
• Liquid - Vapour = Vaporisation
• Solid - Vapour = Sublimation
Substances that Substances that 
expand on contract on
Enthalpy
• Enthalpy is a measure of the total energy of a thermodynamic system.
• It includes energy required to create a system and the amount of energy 
required to make room for it by displacing its environment and establishing its 
volume and pressure.
• For certain type of processes particularly in power generation and 
refrigeration.
o Enthalpy (H )= U + pV(kJ) Or
Page 5


Properties of pure substances 
Properties of Pure Substance
• A pure substance is a material with homogeneous and invariable 
composition.
• Pure substances can have multiple phases, an ice water mixture is still a pure 
substance but an air-steam is not a pure substance.
Pure Substance:
• A substance that has a fixed chemical composition throughout is called a 
pure substance i.e., water, nitrogen, helium, and CO2.
• Substances which are mixture of various elements or compounds also 
qualifies as pure substances as long as mixture is homogeneous.
Vapour Vapour
Liquid Liquid
(a) H2 0 (b) Air
Saturation Temperature and Saturation Pressure:
• At given pressure, the temperature at which a pure substance changes phase 
is called the saturation temperature Tsat.
• Likewise at a given temperature, the pressure at which a pure substance 
changes phase is called the saturation pressure psat.
Example: For water at a pressure of 101.325 kPa, Tsat is 100°C, conversely at a 
temperature of 100°C, psat is 101.325 kPa.
Latent Heat:
• The amount of energy absorbed or released during a phase change process is 
called the latent heat.
• The amount of energy absorbed during melting is called the latent heat of 
vaporization.
• Similarly, the amount of energy absorbed during vaporization is called latent 
heat of vaporization and is equivalent to the energy released during 
condensation.
Liquid-Vapour Saturation Curve:
From the following figure, it is clear that Tsat increases with psat. Thus, a substance 
at higher pressure will boil at higher temperatures.
Tsat = f(Psat)
^sat •
• In the kitchen, higher boiling temperature means shorter cooking time and 
energy saving.
• The atmospheric pressure, and thus the boiling temperature of water, 
decreases with elevation. Therefore, it takes longer time to cook at higher 
altitudes than it does at sea level.
Property Diagrams for Phase-change Process 
The T-V Diagram:
• Consider piston cylinder device containing liquid water at 20°C and 1 atm.
• Water will start boiling at a much higher temperature (179.9°C) at inside 
pressure of the cylinder reaches at 1 MP.
• The specific volume of the saturated liquid is larger and the specific volume of 
the saturated vapour is smaller than the corresponding values at 1 atm 
pressure. That is, the horizontal line that connects the saturated liquid and 
saturated vapour states is much shorter.
• As the pressure is increased further, this saturation line will continue to get 
shorter as shown in figure and it will become a point when the pressure 
reaches 22.09 MPa for the case of water. This point is called the critical point
and it is defined as the point at which the saturated liquid and saturated 
vapour state are identical.
• At pressure above the critical pressure, there will not be a distinct phase 
change. Instead, the specific volume of the substance will continually 
increase and at all times there will be only one phase present. It is customary 
to refer to the substance as superheated vapour at temperature above the 
critical temperature and as compressed liquid at temperatures below the 
critical temperature.
T V diagram of constant pressure phase-change process of a 
pure substances at various pressures.
The p-V Diagram:
• The general shape of the p-V diagram of a pure substance is very much like 
the T-V diagram but the T = constant lines on this diagram have a downward 
trend.
• Consider again a piston cylinder device that contains liquid water at 1 MPa 
and 150°C, water at this state exists as a compressed liquid. Now, the weights 
on top of the piston are removed one by one so that the pressure inside the 
cylinder decreases gradually.
• The water is allowed to exchange heat with the surroundings so its 
temperature remains constant.
• As the pressure decreases, the volume of the water will increase slightly, 
when the pressure reaches the saturation pressure volume at the specific 
temperature, the water will start to boil.
• During this vaporisation process, both the temperature and the pressure 
remain constant but the specific volume increases. Once the last drop of 
liquid is vaporised further reduction in pressure results in a further increase in 
specific volume.
• If the process is repeated for other temperatures similar paths will be 
obtained for the phase change processes.
Triple Phase:
• When all three phases of a pure substance co-exist in equilibrium. It is called 
triple phase.
• Triple phase states form a line called the triple line.
• The triple line appears as a point on the p-T diagram and therefore is often 
called the triple point.
p V diagram of pure substance
• No substance can exist in the liquid phase in stable equilibrium at pressure 
below the triple point pressure.
• The same can be said for temperature for substance that contract on 
freezing.
• Substances at high pressure can exist in the liquid phase at temperatures 
below the triple point temperature.
The p-T Diagram:
• Solid - Liquid = Fusion
• Liquid - Vapour = Vaporisation
• Solid - Vapour = Sublimation
Substances that Substances that 
expand on contract on
Enthalpy
• Enthalpy is a measure of the total energy of a thermodynamic system.
• It includes energy required to create a system and the amount of energy 
required to make room for it by displacing its environment and establishing its 
volume and pressure.
• For certain type of processes particularly in power generation and 
refrigeration.
o Enthalpy (H )= U + pV(kJ) Or
per unit mass h = u + pV(kJ/kg)
Saturated Liquid and Saturated Vapour States:
• Subscript f-» properties of saturated liquid
• Subscript g -? properties of saturated vapour
• Vf _ specific volume of saturated liquid
• Vg _ specific volume of saturated vapour
• Vfg _ difference between Vg and Vfnthai
• Vfg =Vg-Vf
• hfg -? enthalpy of vaporisation or latent heat of vaporisation (amount of 
energy need to vaporise a unit mass of saturated liquid at a given 
temperature of pressure).
• The magnitude of latent heat depends on temperature and pressure at which 
phase change occurs.
• For example at 1 atm pressure, the latent heat of fusion of water is 333.7 
kj/kg and latent heat of vaporization is 2257.1 kj/kg.
• At below triple point pressure, substance begins to change directly gas.
• Enthalpy of vaporization decreases as the temperature or pressure increases 
and become zero at the critical point.
Saturated Liquid Vapour Mixture:
• During vaporisation process, a substance exists as part liquid and part 
vapour.
• A new property quality x is the ratio of mass of vapour to the total mass of the 
mixture.
Quality or dryness fraction: 
x = — - —
m total = m liquid + m vapour = m f + m g
where, mf = mass of the saturated liquid, and mg = mass of the saturated vapour •
• Quality has significance for saturated mixtures only.
• It has no meaning in the compressed liquid or superheated region.
• Its value lie between 0 (saturated liquid) and 1 (saturated vapour).