Pure substances

PURE SUBSTANCES
Prepared by – Manmeet Singh
Assistant Professor
Prepared By Manmeet Singh, Assistant
Professor

PV diagram for pure substances
• Heating of water from -10 degree Celsius to
100 degree Celsius boiling and above
• 1-2 – heating at constant pressure from -10 to
0 degree Celsius. Volume ice increases.
Temperature also increases.
• 2-3 – melting of ice. Volume decreases from 2
to 3 which is peculiarity of water. Temperature
and pressure remains constant.
• 3-4- Water is heated from 0 to 100 degree
Celsius. Volume of water increases. Pressure
constant at 1 atm.
• 4-5- Boiling of water at 1 atm pressure and
constant temperature of 100 degree Celsius.
Volume increases from 4 to 5 . Saturated liquid
converts to saturated vapor.
• 5-6- Superheating at constant pressure.
Volume increases from 5 to 6. Temperature
increases from 100 to some other temperature
say 250 degree Celsius.
Professor

•Water existed in
•Solid phase between 1 and 2.
•In liquid phase between 3 and 4
•In vapor phase between 5 and 6
or above
•Between 2 and 3 – Latent heat of
fusion (Solid to liquid)
•Between 4 and 5- Latent heat of
vaporization (liquid to vapor)
•Both phase change at constant
temperature and constant
pressure.
•State 2,3,4 and 5 are known as
saturation states-
•Saturation state is a state from
which change of phase may
occur without change of
temperature and pressure.
State 2 is saturated solid state.
State 3 and 4 are saturated liquid. State 3
liquid is saturated wrt to solidification and
state 4 liquid is saturated wrt to
vaporization
State 5 – saturated vapor
Professor

• If heating of ice from -10 degree
celsius to 250 degree celsius or
above is done at different constant
pressure, similar saturation states
are obtained with different
temperature ,volume and other
different properties.
• And all these can be plotted on P-V
coordinates, when heated at
different constant pressure.
• All saturation solid states (i.e 2) are
joined by a line- Saturation solid
line
• All saturated liquid states (i.e. 3 and
4) are joined – Saturated liquid
lines
• The line passing through saturated
vapor states – Saturated vapor line
Professor

Professor

• Triple point – all
three phases exists in
equilibrium.
• Below triple point
pressure , the
substance cannot
exist in the liquid
phase, and substance
when heated
transform into solid
to vapor – called
Sublimation
Professor

Triple point
• Sublimation
• Dry ice, sometimes referred to as
"cardiceis the solid form of carbon
dioxide.
• It is used primarily as a cooling
agent. Its advantages include lower
temperature than that of water ice
and not leaving any residue (other
than incidental frost from moisture
in the atmosphere). It is useful for
preserving frozen foods
where mechanical cooling is
unavailable.
• Dry ice sublimates at −78.5 °C
(−109.3 °F) at Earth atmospheric
pressures. This extreme cold makes
the solid dangerous to handle
without protection due to burns
caused by freezing (frostbite).
Professor

• As working fluid in power
cycles is generally liquid –
vapor ,so solid region is
omitted.
• And P-V diagram become
as shown.
Professor

• State A –vapor is compressed
slowly and isothermally.
Pressure rise until the vapor
become saturated.
• If compression continued temp.
and pressure remaining
constant, condensation takes
place between B and C. Btw B
and C liuid and vapor are in
equilibrium.
• After C since a large pressure is
required to compress liquid, CD
is almost vertical with very less
change in volume.
• ABCD is typical isotherm of a
pure substance on PV diagram.
Professor

• As temperature
increases, transition
zone decreases (BC)
and becomes zero at
critical point.
• Above critical point
liquid suddenly
flashes into vapor or
vapor on cooling
suddenly condenses
into liquid.
Professor

Vaporization
• Evaporation
• In evaporation, molecules on the surface of a liquid which
have enough kinetic energy to overcome intermolecular
forces between water molecules escape the surface of the
liquid to become gaseous. Some of these molecules may
eventually rejoin water molecules on the liquid surface and
become a part of the liquid again (condensation).
Eventually, the condensation rate would be equal to the
evaporation rate, and a constant vapor pressure is
established. In this situation, the air above the liquid surface
is saturated with molecules that have been evaporated.
• Evaporation is responsible for drying clothes and
transpiration in plants. Water never comes to a “boil” in
these cases but rather evaporate away.
• Boiling
• This is vaporization that happens when the liquid is at
its boiling temperature. Boiling occurs when the vapor
pressure is equal to the external pressure. In this condition,
liquid molecules below the surface which have enough
kinetic energy to overcome intermolecular forces can join
up to form bubbles, which can then rise through the liquid.
Professor

Phase change
• Phase change occur at constant temperature and pressure
called saturation temperature and saturation pressure.
Professor

P-T diagram for pure substance
• Upon slow heating at different
constant pressure as shown on P-
V plane. If these were plotted on
P-T coordinates , the diagram as
shown here is obtained.
• At 1 atm heating of ice from -10
to 250 degree Celsius
• 1-2 –solid ice heating from -10 to
0 degree Celsius
• 2-3- melting of ice at 0 degree
Celsius.
• 3-4- liquid heating (0 to 100)
• 4-5- vaporization of water at 100
degree Celsius.
• 5-6 – heating in vapor phase.Prepared By Manmeet Singh, Assistant
Professor

• Curve passing through 2,3 points is
called – Fusion Curve
• Curve passing through 4,5 points is
called – Vaporization Curve
• If the vapor pressure of a solid is
measured at different
temperatures and these are
plotted, the Sublimation curve will
be obtained.
• All three above curve meet at
triple point
• For water 4.58 mm Hg and 273.16
K Prepared By Manmeet Singh, Assistant
Professor

• For substances like water which expand on freezing
,slope of fusion curve is negative.
Professor

P-V-T Surface
Professor

Temperature –
Entropy plot
Professor

h-s or Mollier diagram
Consider heating of ice from -5 degree
Celsius to 250 degree Celsius
Similarly, isobars at different
pressure are drawn on h-s
diagram
Constant
temperature heating
them slope is
constant
Professor

H-s or Mollier diagram
Professor

Quality or dryness fraction
Professor

Constant property lines T-s diagram
Professor

Constant property lines h-s diagram
Professor

Professor

Steam tables
Professor

Practice questions steam table
Professor

Questions 2
Professor

Question no 3
Professor

Question no. 4
Professor

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