Change of Phase

Report
Matter exists in four common phases:
• Solid phase (ice)
• Liquid phase (ice melts to water)
• Gaseous phase (water turns to vapor; addition of
more energy vaporizes water to vapor)
• Plasma phase (vapor disintegrates to ions and
electrons)
 The
phase depends upon: temperature
and pressure.
from Solid  Liquid  Gas 
Plasma requires energy to be added to
the material.
 Change
 Energy
causes the molecules to move
more rapidly.
Evaporation
 Change
of phase from liquid to gas
 EVAPORATION
 Molecules
IS A COOLING PROCESS!
are randomly moving
 Some
gain KE, others lose KE
 Some
can break free of the liquid—become gas
 Average
KE drops, the liquid becomes cooler
 Bodies
overheat, produce perspiration
(sweat)
 The water evaporates and cools us
 Keeps body temperature stable
Sublimation
 Form
of phase change directly from solid
to gas
Example: dry ice (solid carbon dioxide
molecules), frozen water




are less energetic.
have decreased average speeds.
result in lowered temperature.
All of these.




are less energetic.
have decreased average speeds.
result in lowered temperature.
All of these.




evaporation.
condensation.
sublimation.
regelation.




evaporation.
condensation.
sublimation.
regelation.
Condensation process





Opposite of evaporation
Warming process from a gas to a liquid
Gas molecules near a liquid surface are attracted to the
liquid
They strike the surface with increased KE, becoming
part of the liquid
Overall KE of liquid increases, so it gets warmer!




Steam releases much energy when it condenses to a
liquid and moistens the skin—hence, it produces a
more damaging burn than from same-temperature
100C boiling water.
A cold soda can is wet in warm air because slowmoving molecules make contact with the cold surface
and condense.
The Shower Dilemma: You’re cold when you get out of
the shower because the water is evaporating, but if you
stay in the shower stall, it feels warmer. Why?
The water vapor in the stall is condensing with the
water molecules causing it to be warmer!
 Humidity-
amount of water vapor in the
air
 Relative Humidity- ratio of the amount of
water vapor in the air at a given temp. to
the largest amount of water vapor the air
can contain at that temp. (Weather
reports)
 Temperature
drops:
water vapor
condenses
 Slow moving
molecules combine:
create droplets of
water
 Warm
air rises, it expands.
 As it expands, it cools.
 As it cools, water vapor
molecules begin sticking
together
 They combine with other
particles in the air to form
clouds!
 Fog is a cloud closer to the
ground based on temp.
difference between water
and the ground




evaporation.
condensation.
sublimation.
regelation.




evaporation.
condensation.
sublimation.
regelation.
Condensation
CHECK YOUR NEIGHBOR
When you step out after a hot shower you feel cold,
but you can feel warm again if you step back into the
shower area. Which process is responsible for this?
A.
B.
C.
D.
Evaporation
Condensation
Both of these.
None of the above.
Condensation
CHECK YOUR ANSWER
When you step out after a hot shower you feel cold, but you
can feel warm again if you step back into the shower area.
Which process is responsible for this?
A.
B.
C.
D.
Evaporation
Condensation
Both of these.
None of the above.
Explanation:
When you step back into the
shower area, the steam that is
present condenses on your
body, causing it to warm up.
 Evaporation
and Condensation
 Evaporation
beneath the surface causes
bubbles (turns to vapor in the liquid!)
 Bubbles rise to the surface
 Only occurs when the pressure of the
vapor can resist the pressure of the liquid
and atmosphere above it.
 For
water, that point is 100 degrees C at
regular atmospheric pressure
 Water at 100 degrees C is in thermal
equilibrium, it is being cooled as much
as it is being warmed
 This makes boiling a cooling process
 Add more heat to boiling water…what
happens to temperature?
 Depends
on Pressure!
 Higher altitude means less air pressure
 The lower the pressure, the lower the
boiling point of water
 Higher the pressure, boiling point
increases
 Denver: boiling point is 95 degrees C
Boiling
CHECK YOUR NEIGHBOR
A. cools the water being boiled.
B.
depends on atmospheric pressure.
C. is a change of phase below the water surface.
D. All of the above.
Boiling
CHECK YOUR ANSWER
A.
B.
C.
D.
cools the water being boiled.
depends on atmospheric pressure.
is a change of phase below the water surface.
All of the above.
 Change
from solid to liquid
 Heat a solid: molecules move faster and
more violently
 Forces can’t hold the molecules together,
solid melts!
 Opposite
of melting
 Freeze at the same temp. as they melt
 Liquid gives off energy, motion slows
down, molecules link together
 Become a solid!
 A pure substance has a definite melting
or freezing point at a certain pressure.




warm the water.
cool the water.
both warm and cool the water at the
same time.
have no effect on water’s temperature.




warm the water.
cool the water.
both warm and cool the water at the
same time.
have no effect on water’s temperature.




decreased temperatures.
decreased atmospheric pressure.
increased temperatures.
increased atmospheric pressure.




decreased temperatures.
decreased atmospheric pressure.
increased temperatures.
increased atmospheric pressure.
Comment: This is shown in the chapter-opener photo of
Ron Hipschman with the Water Freezer exhibit at the
Exploratorium in San Francisco.




higher than the freezing point.
lower than the freezing point.
the same as the freezing point.
unrelated to the freezing point.




higher than the freezing point.
lower than the freezing point.
the same as the freezing point.
unrelated to the freezing point.
Comment: Water freezes at 0°C and ice melts at 0°C.
Likewise for the melting and freezing points of other
substances.
 Add
energy to a piece of -50°C ice
 Temperature rises
 After it hits 0°C, the temperature stays
here even as we add heat!
 This heat is being used to Melt the ice
 This is a change of phase!
 When all the ice melts, the temperature
of the water can begin to increase
 When
it hits 100°C, again the temperature
stays the same as heat is applied.
 This heat is vaporizing the water to steam
 After all the water is steam, the
temperature will rise
 It
takes 80 calories to melt one gram of
ice to water
 This process is reversible, it takes 80
calories to freeze one gram of water
 This energy (80 cal per gram) is called
the Latent Heat Of Fusion for water.
 If you have a 25 gram sample of ice, how
much energy is needed to melt it?
 Q = mass x Latent Heat = 25 x 80 = 2000
cal
 It
takes 540 calories to vaporize a gram of
water
 Again, reversible 540 calories to condense
steam to water
 This energy (540 calories per gram) is
called the Latent Heat of Vaporization for
water
 If you have a 25 gram sample of 100°C
water, how much heat is needed to vaporize
it?
 Q = mass x Latent Heat = 25 x 540 = 13500
cal
Phase Change:
Q(gained or lost) = m x Latent Heat (fusion or vaporiztion)
Temperature Change:
Q (gained or lost) = m c ΔT
Problem
How much energy is required to heat 25 g of liquid water
from 25C to 100C and change it to steam?
Step 1: Calculate the energy needed to heat the water from 25C to
100C
Q = m  c  T
Q = 25g  1.0 cal g-1 C –1  75 C = 1875 cal
Step 2: Vaporization: Use the Latent Heat to calculate the energy
required to vaporize 25g of water at 100C
Q = 25.0 g  540 cal/g = 13500 cal
.25g  1mol H2O / 18g mol-1 H2O = 1.4 mol H2O
vap H (H2O) = 1.4 mol H2O  40.6kJ/mol = 57 kJ
Total energy change is:
Heat from temperature change + Heat of vaporization:
1875 cal + 13500 cal = 15375 cal
 Latent
heat of fusion: 80 cal per gram
 Latent heat of vaporization: 540 cal per
gram
 One
more step:
 How much energy is needed to vaporize
a 50 gram sample of ice completely to
steam?
 Hint: You must go through three different
heats exchanges!
 Heat
of fusion:
 Q = m x 80 = 50 x 80 = 4000 cal
 Heat for temperature change 0 – 100°C:
 Q = m c ΔT = 50 x 1 x (100-0) = 5000 cal
 Heat of vaporization:
 Q = m x 540 = 50 x 540 = 27000 cal
 Total Heat needed: 4000 + 5000 + 27000 =
36000 cal!
Heating and cooling curve for water heated at a constant rates.
A-B = Solid ice, temperature is increasing.
Particles gain kinetic energy, vibration of particles increases.
Ice
B-C = Solid starts to change state from solid to liquid. Temperature remains constant
as energy is used to break inter-molecular bonds.
H2O (s)  H2O () energy required  80 cal/g
0ºC
C-D = temperature starts to rise once all the solid has melted. Particles gain kinetic
energy.
Liquid
water
D-E = Liquid starts to vaporize, turning from liquid to gas. The temperature remains
constant as energy is used to break inter-molecular forces.
H2O ()  H2O (g) energy required  540 calg
100ºC
E-F = temperature starts to rise once all liquid is vaporized. Gas particles gain kinetic
energy.
steam
Heat & Changes of State
sublimation
sublimation
boiling
melting
vaporization
condensation
freezing
deposition

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absorbed by the steam.
released by the steam.
conserved as the phase change occurs.
changed to a different form.
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absorbed by the steam.
released by the steam.
conserved as the phase change occurs.
changed to a different form.

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absorb energy.
release energy.
retain their energy.
lose the quality of wetness.
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absorb energy.
release energy.
retain their energy.
lose the quality of wetness.

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absorbed.
given off.
unchanged.
in effect multiplied.




absorbed.
given off.
unchanged.
in effect multiplied.




absorbed.
given off.
unchanged.
in effect multiplied.




absorbed.
given off.
unchanged.
in effect multiplied.
Comment: This occurs when snowflakes form in the air.

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