Change of Phase

1
Change of Phase

• Evaporation
• Condensation
• Fogs and Clouds
• Boiling
• Melting and Freezing
• Energy and Change of Phase

2
Evaporation

• Why do you feel chilly when getting out of a
  swimming pool?

3
Evaporation

• Why do you feel chilly when getting out of a
  swimming pool?
• Fast molecules escape the liquid, taking energy,
  and leaving the liquid cooler.

4
Evaporation

• Cool coffee by blowing on it
• Evaporation is a cooling process!
• Wrap a wet cloth around a container to cool it
  off at a picnic.

5
Condensation

• When in a shower and you turn off the water, why
  does is stay warm in the stall?

6
Condensation

• When in a shower and you turn off the water, why
  does is stay warm in the stall?
• The number of molecules in condensation is
  greater than the ones in evaporation.
• Condensation is a warming process
• Molecules that condense have a higher average
  kinetic energy

7
Condensation

• Why does a cold drink get wet on the outside?
• What causes Dew, fog, clouds?

8
Cloud formation

• Convection upward and expansion
• Cooling due to expansion
• Condensation due to cooling
• Cloud formation
• Condensation is enhanced by particles of dust

9
Boiling

• Boiling is a cooling process
• Temperature is 100 degrees C no matter how much
  heat we give the water.
• Water boils as fast as it is being warmed by
  heating.
• The steam carries away the heat.

10
Boiling Water

• At what temperature does water boil in the
  mountains?

11
Energy in Condensation/Vaporization

• The Heat of Vaporization is 540 calories per gram
  of condensed water at 100 degrees.

12
Melting and Freezing
13
Energy and Phase Changes
14
Energy
15
Class Problem

• What is the minimum amount of 100C steam
  required to melt 1 gram of 0C ice?
• 1) 0.125 g
• 2) 0.148 g
• 3) 6.75 g
• 4) 8.0 g
• 5) none of these

16
Class Problem

• What is the minimum amount of 100C steam
  required to melt 1 gram of 0C ice?
• 0.125 gram of 100C steam will provide the 80
  calories required to melt 1 gram of ice. The H2O
  in the form of steam will give up 540 calories
  per gram when it condenses to boiling water, and
  another 100 calories per gram when the water is
  cooled from 100C to 0C. So the steam will give
  up a total of 640 calories per gram to the ice.
  But the ice needs only 80 calories to melt. So
  only 80/640 gram (0.125 gram) of steam will do
  the job.

17
Class Problem

• Suppose 4 grams of boiling water are spread over
  a large surface so 1 gram rapidly evaporates. If
  evaporation takes 540 calories from the remaining
  3 grams of water, and no other heat transfer
  takes place, what will be the temperature of the
  remaining 3 grams?

18
Class Problem

• The remaining 3 grams will turn to 0C ice under
  conditions where all 540 calories are taken from
  the remaining water (like when the surroundings
  are below freezing and don't contribute energy).
  540 calories from 3 grams means each gram gives
  up 180 calories. 100 calories from a gram of
  boiling water reduces its temperature to 0C, and
  80 more calories taken away turns it to ice. This
  is why hot water so quickly turns to ice in a
  freezing-cold environment.

19
Class Problems
1. To say that evaporation is a cooling process
is to say that the A) the less energetic
particles escape. B) more energetic particles
escape. C) more energetic particles remain in
the water. 2. When a gas is changed to a
liquid state, the gas A) absorbs energy. B)
releases energy. C) neither releases nor absorbs
energy. D) both releases and absorbs energy.
3. A refrigerator A) causes internal energy
to disappear. B) produces cold. C) removes
internal energy from inside the refrigerator. D)
changes heat into cold. E) none of these
20
Class Problems
4. On a humid day, water condenses on the outside
of a glass of ice water. This phenomenon occurs
mainly because of A) capillary action. B)
adhesion of water molecules to glass. C)
evaporation. D) the saturation of cooled air.
E) the porosity of glass. 5. When a block of
ice at zero degrees Celsius melts, the ice A)
absorbs energy from its environment. B) releases
energy and gets warmer. C) absorbs energy and
gets warmer. D) releases energy to its
environment. E) absorbs energy but does not
change its temperature.