Change of Phase

1
Chapter 23

• Change of Phase

2
Change of Phase

• Three common phases
• Solids
• Liquids
• Gases
• Phase of matter depends on temperature and
  pressure
• Changes in phase usually involve energy transfer

3
Evaporation

• Evaporationthe change in phase from a liquid to
  a gas that takes place at the surface of a liquid
• Molecules in a liquid bump into each other an
  transfer KE
• Molecules at the surface are bumped from below
  and may gain enough energy to break free from the
  surface
• These molecules are now a vapor (in the gaseous
  phase)

4
Evaporation

• The molecules that are bumped into the gaseous
  phase have an increased KE that comes from the
  molecules of the remaining liquid (having a
  decreased KE)
• Remember that just because the molecules have
  certain (same) temperature, they dont have the
  same energy
• Temperature is the measure of the average KE of
  the molecules
• Some molecules will have more or less KE
• The molecules remaining in the liquid phase have
  a decrease KE, therefore evaporation is a cooling
  process
• Sweatthe evaporation of sweat cools us and helps
  the body maintain a certain temperature
• If we didnt have sweat glands, then we would
  have to find another way to cool ourselves on a
  hot day
• Dogs pant to cool themselves
• Pigs use mud to cool themselves

5
Evaporation

• How can you cool a cup of hot coffee?
• Blow on it (to increase evaporation)
• Pour it into the saucer (to increase the
  evaporating area)
• Pour it into a saucer (to cool it by conduction)
• Put silverware in it (to absorb heat by
  conduction and radiate the heat)

6
Condensation

• Condensationthe changing of a gas to a liquid
• The opposite of evaporation
• Evaporation ? cooling process
• Loss of KE at the surface of liquid
• Condensation ? warming process
• Gain of KE at the surface of liquid
• Water droplets on the outside of a cup of cold
  liquid
• Water vapor molecules collide with the molecules
  of the cool surface
• The molecules give up so much KE they cannot
  remain is gaseous phase and condense into a liquid

7
Condensation

• Occurs when gas molecules are captured by
  liquids
• Gas molecules strike the surface of a liquid
  losing KE
• Attractive forces exerted by the liquid hold the
  gas molecules in the liquid
• Gas molecules become liquid molecules
• Some solids go directly to the gas
  phasesublimation (dry ice)

8
Condensation in the Atmosphere

• Air contains some water vapor
• Relative humidity is a measure of the water vapor
  in the air compared to the limit for the
  temperature
• Air is saturated when the limit of water vapor in
  the air for a given temperature is reached
• For saturation, water vapor molecules must be
  condensing
• The slower the molecules are moving, the more
  likely they are to stick together when they
  collide and condense
• At high temperatures molecules move faster, but
  there are still enough slow molecules to cause
  condensation

9
Fog and Clouds

• Warm air rises ? as it rises it expands ? as it
  expands it cools ? as it cools water vapor
  molecules stick together and condense ? A CLOUD
  IS FORMED
• Fog is a cloud that forms near the ground
• Moist air near the ground cools and fog forms
• Cloud formation is a 4-C process
• Convection (causes expansion)
• Cooling (due to expansion)
• Condensation (due to cooling)
• Cloud formation

10
Evaporation and Condensation

• When rubbing alcohol is applied to your skin, why
  do you feel a chilly sensation?
• You are chilled by the rapid evaporation of the
  alcohol.
• Why do you feel extra muggy on a warm muggy day?
• You are warmed by the condensation of water vapor
  on you.

11
Evaporation and Condensation Rates

• Evaporation and condensation take place at the
  same time
• If evaporation exceeds condensation, a liquid is
  cooled
• If condensation exceeds evaporation, a liquid is
  warmed
• If evaporation and condensation occur equally,
  the liquid is in equilibrium
• Evaporation and condensation have canceling
  effects
• Leave a dish of water on a table for several days
  and no apparent evaporation takes place. Does
  this mean that nothing is happening in the water?
• No, it just means that the liquid is in
  equilibrium

12
Boiling

• Evaporation takes place at the surface of a
  liquid
• Change of phase can also occur beneath the
  surface
• Boilinggas forms beneath the surface of a
  liquid, the bubbles rise to the surface and the
  gas escapes to the surrounding air

13
Boiling

• Pressure in the bubbles must be great enough to
  resist the pressure of the surrounding liquid
• If the temperature is below the boiling point
  vapor pressure is not great enough and bubbles do
  not form
• Boiling depends on both pressure and temperature
• At higher altitudes (lower atmospheric pressure)
  liquids have a lower boiling point
• In Denver (the mile-high city), the boiling point
  of water is 95ºC instead of 100ºC at sea level
• It takes longer to cook food in boiling water in
  Denver because of the smaller temperature

14
Boiling

• Boiling is a cooling process
• As the liquid begins to boil, the vapor escapes
  (taking KE and temperature with it)
• Liquid is being cooled by boiling as fast as it
  is being heated
• The temperature of boiling water remains at 100ºC
  regardless of the amount of heat applied
• If this cooling did not take place, boiling would
  result in a constant increase in temperature
• A pressure cooker can reach high temperatures
  because it prevents boiling, thus preventing
  cooling

15
Freezing

• Freezingthe change in phase from liquid to a
  solid
• Freezing occurs when
• energy is continually withdrawn from liquid
  slowing molecular motion
• forces of attraction between molecules cause them
  to get closer to each other
• the molecules vibrate to form a solid
• If a foreign particle is dissolved in the liquid
  the freezing temperature will be lowered
• These particles get in the way of the molecules
  that would ordinarily join together (connections
  become more difficult)
• Putting salt on the sidewalk before/during/after
  snow storm

16
Regelation

• Regelationthe phenomenon of melting under
  pressure and freezing again when the pressure is
  removed
• Making a snowball
• Compression by your hands melts the snow slightly
  and helps you make the snow into a ball
• Ice skating
• Ice has a thin layer of liquid on its surface
  even at low temperatures because of the pressure
  from the skates

17
Energy and Changes of Phase

• Energy must be put into a substance to change its
  phase from a solid to a liquid to a gas
• Heat an ice cube ? it forms water (liquid)
• Heat water ? it forms water vapor (gas)
• Energy must be extracted (taken out of) a
  substance to change its phase from a gas to a
  liquid to a solid

18
Energy and Changes of Phase

• Heat 1 g of ice at -50C
• The specific heat of ice is .5 cal/gC so it
  takes about 25 calories for the ice to reach 0C
• Add 80 calories for the ice to melt
• Note the temperature of the substance does not
  change until ALL of the ice melts
• 80 calories/gram to melt ice ? heat of fusion
• In SI units 1 cal 4.184 J heat of fusion of
  water (80)(4.184)(1000) 3.35 x 105 J/kg
• Add 100 calories (the specific heat of water is 1
  cal/gC) until the water boils (boiling point for
  water is 100C)
• Add 540 calories for the water to vaporize and
  become steam
• As the water is vaporizing the temperature is
  constant (100C) until the whole gram of water
  becomes steam
• 540 calories/gram to vaporize water ? heat of
  vaporization
• In SI units 1 cal 4.8184 J heat of
  vaporization of water (540)(4.184)(1000) 2.26
  x 106 J/kg
• As heat is added, the temperature will continue
  to rise ( .5 cal/gram for each C)

19
Energy and Changes of Phase

• The process works in reverse too!
• When 1 gram of steam condenses to form boiling
  water, it gives off 540 calories of heat to the
  environment
• When 1 gram water is cooled from 100C to 0C,
  100 additional calories are released to the
  environment as heat
• When 1 gram of water is becomes solid ice, 80
  more calories of energy are released to
  environment

20
Energy and Changes of Phase

• Hot water will freeze more rapidly than warm
  water (not cold water) because more of it
  evaporates leaving less of it to freeze.
• Each gram of water that evaporates takes 540
  calories from the remaining water with it.
• This is a large amount of energy, since 1
  calorie/C is taken out of water that is cooling
  by thermal conduction.
• This is why a skating rink is flooded with hot
  water to melt and smooth out the rough spots and
  refreeze quickly

21
Energy and Phase Changes
540 calories taken from the remaining three
grams. Each gram loses 540/3 180 calories of
energy. Each gram was at a temperature of 100C,
and the temperature of water changes 1C for each
calorie that is removed. 100 calories will be
given off to take each gram of water to 0C The
remaining 80 calories for each gram will be
release when each gram of water turns into ice.
22
Energy and Phase Changes

• Air conditioners and refrigerators use phase
  change to keep things cool
• Liquid is pumped through a cooling unit and
  forced through a small opening to evaporate and
  draw heat from the food compartment.
• The warmed gas is transferred outside the cooling
  unit to coils in the back.
• T he gas condenses and the heat is released to
  the surrounding air by the coils.
• The liquid returns to the cooling unit and the
  process continues.

23
Energy and Phase Changes

• When H2O in water vapor condenses, is the
  surrounding air warmed or cooled?
• Warmedenergy is released when something is
  cooled
• When H20 in water vaporizes, is the surrounding
  air warmed or cooled?
• Cooledenergy is consumed when something is heated

24
Energy and Phase Changes

• Heat of fusionthe quantity of heat needed per
  kilogram to melt a solid (or solidify a liquid)
  at a constant temperature and atmospheric
  pressure
• ?Q mhf
• Heat of vaporizationthe quantity of heat needed
  per kilogram to vaporize a liquid (or a liquify a
  gas) at a constant temperature and atmospheric
  pressure
• ?Q mhv

25
Energy and Phase Changes

• An igloo is made of 224 blocks ice at 0C, each
  with a mass of 12.0 kg. How much heat must be
  gained by the ice to melt the entire igloo? (heat
  of fusion of ice is 3.35 x 105 J/kg)
• Gus is cooking soup in his hot pot and finds that
  he has added too much water. If Gus needs to
  boil off 0.200 kg of water in order for his soup
  to have the correct consistency, how much
  additional heat must Gus add once the soup is
  boiling? (heat of vaporization of water is 2.26 x
  106 J/kg)
• To cool her 0.200 kg cup of 75.0C hot chocolate
  (mostly water), Heidi drops a 0.0300 kg ice cube
  at 0C into her insulated foam cup. What is the
  temperature of the hot chocolate after all the
  ice is melted?