Change of Phase
Phases of Matter
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Bose-Einstein Condensate
Solid
Liquid
Gas
Plasma
Low energy
High energy
The phase of matter depends on both temperature and
pressure. And when a substance undergoes a phase change,
there is usually some sort of transfer of energy involved.
Phase Changes
Su
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on
ati
m
Phase Changes
p
Va
o
Liquid
n
sio
Fu
Solid
C
l
tal
rys
iz
on
ati
p
De
a
riz
n
Co
itio
os
Gas
n
tio
de
Gas
on
ati
ns
Liquid
n
Solid
Temperature of matter
Heating/Cooling Curve
Temperature of matter
Heating/Cooling Curve
Gas
Liquid
Solid
Solid &
liquid
Q = mcΔT
Q = mcΔT
Q = mL
Heat added
€
Q = mcΔT
Q = mL
€
€
Liquid
& gas
Heat added
€
€
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Energy & Phase Change
Adding heat to a solid will eventually do five things:
1.  raise the temperature of the solid until it reaches its
melting point;
2.  convert the material from solid to liquid;
3.  raise the temperature of the liquid until it reaches its
boiling point;
4.  convert the material from liquid to gas;
5.  raise the temperature of the gas.
During this process, energy must be continually added to the
system, which absorbs this energy.
The exact opposite thing happens when energy is released by
the system (or taken away from the system).
Heat required to change T
• Specific Heat Capacity = c = amount of heat
required to raise the temperature of a gram of
matter by 1°C. !
• c=1 cal /g/°C for water!
• c=0.50 cal /g/°C for ice!
• c= 0.50 cal /g/°C for steam!
Q = mcΔT
Amount of heat!
mass of matter!
change in
temperature!
specific heat
capacity of
matter!
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Heat req’d to change phase
Heat of fusion = L = amount of heat required to
change a gram of matter from solid to liquid.
(L=80cal /g for water).!
Example 1
How much energy is required to raise the temperature of 20g
of water from 80°C to steam at 100°C?
Heat of vaporization = L = amount of heat required
to change a gram of matter from liquid to gas.
(L=540cal /g for water).!
heat of fusion
or heat of
vaporization!
Q = mL
Amount of heat!
mass of matter!
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Example 2
Example 3
How much energy is required to melt a 50 gram ice cube?
A stove can supply 1000 Joules/second of heat. How long will
it take to completely boil away 350 milliliters of water that
starts out at room temperature (20 °C)?
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Evaporation
Evaporative Cooling
Temperature is a related to the average KE of the particles in a
substance. The higher the KE of those particles, the greater
the temperature of the substance.
What happens when a particle gets bumped so hard that it
leaves the surface of a liquid?
What does this imply for the average KE of the remaining
particles in the liquid?
People
Dogs
Soup
Cloth-covered canteens
Evaporation is a cooling process (for the liquid left behind).
Condensation
Gas Laws
Condensation is the opposite process: converting a high-energy
gas molecule into a lower-energy liquid molecule by slowing it
down, and taking away some of its KE.
When fast moving water vapor hits a cold mirror in the
bathroom or a cold can of soda, the vapor condenses into a
liquid form.
Warm air can hold more water vapor than cold air. When air
gets saturated (100% humidity), water droplets start to form.
http://www.accuweather.com/en/us/pasadena-ca/91106/
current-weather/38136_pc
What’s the story with clouds and fog?
Boiling
Boiling & Vapor Pressure
Liquid-to-gas phase change that happens below the surface.
To boil something, you add heat to it, which ___________
the average KE of the molecules.
When higher energy particles leave the liquid, what happens
to the average KE of the remaining molecules in the liquid.
For a liquid to boil, vapor pressure (inside the bubble) has to be
large enough to overcome atmospheric pressure and water
pressure from outside the bubble.
If we place a cap over the water and increase the external
pressure on the water, does that make it easier or harder for
the water to boil?
“Boiling is a cooling process.”
1. 
2. 
3. 
4. 
5. 
PiVi Pf V f
=
Ti
Tf
Earth heats air.
Warm air rises.
Pressure decreases at altitude.
Temperature decreases.
Water vapor in air condenses, forming clouds.
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High pressure increases the boiling point. (Boiling temp is higher.)
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Boiling & Vapor Pressure
Pressure Cooker
For a liquid to boil, vapor pressure (inside the bubble) has to be
large enough to overcome atmospheric pressure and water
pressure from outside the bubble.
If we decrease the external pressure on the water, does that
make it easier or harder for the water to boil?
Mountain climbers and other people living at high altitudes
often use pressure cookers—a closed pot with a lid that locks
down tight—when preparing their meals. Explain why.
Low pressure decreases the boiling point. (Boiling temp is lower.)
Freezing
Regelation
During freezing, high-energy particles in a liquid slow down
and move closer to each other, and eventually settle into fixed
positions in the matter, which has now become a solid. The
particles continue to vibrate, but they are not free to flow as
they did when they were in the liquid state.
Ice normally melts at 0°C, but if pressure is applied, the ice
crystals are literally crushed out of their solid bonds, and
converted to liquid H2O. When the pressure is removed,
refreezing occurs.
This process of “melting under pressure, then refreezing” is
called regelation. http://www.youtube.com/watch?v=gM3zP72-rJE
If you add sugar or salt to a
liquid, the foreign molecules
inhibit freezing—they make
it more difficult for the
material to freeze, so it
doesn’t turn to a solid until
it reaches a lower
temperature.
Side note
Do scientists ever make it sound as if we have it all figured
out?
We don’t have it all figured out!
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