1. business and industrial
2. energy
3. oil

CHANGE
OF PHASE
3 CHANGE OF PHASE
Objectives
• Describe how evaporation
affects a liquid’s temperature.
(23.1)
........
• Describe how condensation
affects temperature. (23.2)
THE BIG
IDEA
• Explain how evaporation and
condensation can take place at
the same time. (23.3)
• Describe how pressure affects
boiling point. (23.4)
• Describe the effect of dissolving
anything in a liquid on the
liquid’s freezing temperature.
(23.5)
• Describe how something can
boil and freeze at the same
time. (23.6)
• Explain why so few substances
undergo regelation. (23.7)
• Explain the relationship
between energy and phase
change. (23.8)
Changes of phase involve a
transfer of energy.
T
he four possible forms of matter—solid,
liquid, gas, and plasma—are called phases.
Matter can change from one phase (or state,
as it is also sometimes called) to another. Ice, for
example, is the solid phase of H2O. Add energy,
and the rigid molecular structure breaks down to
the liquid phase, water. Add more energy, and the
liquid changes to the gaseous phase as the water
boils to become steam.
The phase of matter depends on its temperature and the pressure that is exerted upon it.
Changes of phase involve a transfer of energy.
The material in this chapter
is not a prerequisite for the
chapters that follow.
discover!
gallon jar, water,
matches, rubber glove
MATERIALS
EXPECTED OUTCOME The
water vapor will condense
around the smoke. The smoke
particles in the jar serve as
nucleation sites, about which
cloud droplets coalesce.
ANALYZE AND CONCLUDE
1. See Expected Outcome.
2. It would take longer for
water drops to form.
discover!
How Do Clouds Form?
Analyze and Conclude
1. Cover the bottom of a gallon jar with a thin
layer of water.
2. Drop a lit match into the jar.
3. Quickly place the fingers of a rubber glove
inside the jar and stretch the open end of the
glove over the jar’s mouth.
4. Put your fingers in the glove and quickly pull
the glove out of the jar.
1. Observing What did you observe when you
pulled the glove out of the jar?
2. Predicting What do you suppose would happen if you were to pull the glove out of the
jar more slowly?
3. Making Generalizations What factors are necessary for cloud formation?
3. Water vapor and smoke or
dust particles
450
450
23.1 Evaporation
23.1 Evaporation
Water in an open container will eventually evaporate, or dry
up. The liquid that disappears becomes water vapor in the air.
Evaporation is a change of phase from liquid to gas that takes
place at the surface of a liquid.
The temperature of anything is related to the average kinetic
energy of its molecules. Molecules in the liquid phase continuously
move about in all directions and bump into one another at different
speeds. Some of the molecules gain kinetic energy while others lose
kinetic energy. Those molecules at the surface of the liquid that gain
kinetic energy by being bumped from below may have enough energy
to break free of the liquid. They can leave the surface and fly into the
space above the liquid. They now comprise a vapor, molecules in the
gaseous phase.
The increased kinetic energy of molecules bumped free of the
liquid comes from molecules remaining in the liquid. This is
“billiard-ball physics”: When balls bump into one another and some
gain kinetic energy, the other balls lose this same amount of kinetic
energy. So the average kinetic energy of the molecules remaining
behind in the liquid is lowered. Evaporation is a process that
cools the liquid left behind.
A canteen, such as the one in Figure 23.1, keeps cool by evaporation when the cloth covering on the sides is kept wet. As the fastermoving water molecules leave the cloth, the temperature of the cloth
decreases. The cool cloth in turn cools the metal canteen by conduction, which in turn cools the water inside.
Key Terms
phases, evaporation
Common Misconception
All the molecules of a substance at a
certain temperature have the same
energy.
FACT Temperature is a measure
of the average KE of the
molecules. Some molecules will
have more or less KE than this
average.
FIGURE 23.1 The cloth covering on the
sides of the canteen promotes cooling when it is
wet.
Ask Why does a canvas
bag of water cool when the
bag is slung over the bumper
of a car driven in hot weather?
Water seeps through the canvas.
The faster-moving molecules
vaporize, leaving less energy per
molecule behind. Name at least
two ways to cool a hot cup of
coffee. Increase evaporation by
(1) blowing on it, or (2) pouring
it into the saucer to increase
the evaporating area. Cool it by
conduction by (3) pouring it into
a cooler saucer, or (4) putting
silverware in it to absorb heat
and to provide a radiating
antenna.
FIGURE 23.2
Pigs lack sweat glands.
They wallow in mud to
cool themselves.
......
When the human body overheats, sweat glands produce perspiration. As the sweat evaporates, it cools us and helps us maintain a
stable body temperature. Animals that lack sweat glands, such as the
pig in Figure 23.2, must cool themselves in other ways. For example,
dogs cool themselves by panting.
CONCEPT
CHECK
......
Evaporation is a
process that cools the
liquid left behind.
CONCEPT
CHECK
Teaching Resources
• Conceptual Physics Alive!
DVDs Heat: Change of
State
How does evaporation affect a liquid’s temperature?
CHAPTER 23
Teaching Tip Begin by
mentioning the familiar
experience of leaving the
shower and feeling chilly in the
air. Explain the cooling of a
liquid from an atomic point of
view, and reinforce the idea of
temperature being a measure of
the average molecular KE, which
means there are molecules that
move faster and slower than the
average.
CHANGE OF PHASE
451
451
23.2 Condensation
23.2 Condensation
Key Terms
condensation, saturated,
relative humidity
Teaching Tip Explain that
condensation is the process
opposite to evaporation. It is a
warming process.
Teaching Tip Make the point
that a change of phase from
liquid to gas or the opposite is
not entirely one or the other.
Condensation and evaporation
occur together. The net effect
is usually what is spoken about.
Make clear what is cooling when
evaporation occurs, and what
is warming when condensation
occurs. To say that one thing
cools is to say that another
warms. When a cup of hot
coffee cools by evaporation,
the surrounding air is warmed.
Conservation of energy reigns!
FIGURE 23.3 Heat is given up by steam
when it condenses inside
the radiator.
Ask 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 warm on a muggy
day? You are warmed by the
condensation of vapor on you.
Ask Does humidity make
us feel warmer or colder—or
both? If you’re already cold,
more humidity makes you feel
colder. If you’re already hot,
more humidity makes you feel
hotter. At pleasant temperatures,
a little humidity makes us more
comfortable.
452
A camel’s best source
of water is its oversized nose, with an
inside structure that
recaptures most of
the moisture in watersaturated air coming
from its lungs. So
it withdraws water
from its own exhaled
breath.
452
The process opposite to evaporation is condensation. Condensation
is the changing of a gas to a liquid. The formation of droplets of
water on the outside of a cold soda can is an example. Water vapor
molecules collide with the slower-moving molecules of the cold can
surface. The vapor molecules give up so much kinetic energy that
they can’t stay in the gaseous phase. They condense.
Condensation also occurs when gas molecules are captured by
liquids. In their random motion, gas molecules may hit a liquid and
lose kinetic energy. The attractive forces exerted on them by the
liquid may hold them. Gas molecules become liquid molecules.23.2.1
Condensation warms the area where the liquid forms.
Kinetic energy lost by condensing gas molecules warms the surface
they strike. A steam burn, for example, is more damaging than a burn
from boiling water of the same temperature. Steam gives up energy
when it condenses to the liquid that wets the skin. The radiator in
Figure 23.3 also works by condensation of steam.
The effects of condensation can be seen in the atmosphere. The
air always contains some water vapor. This water vapor can make the
air feel humid, or it can lead to the formation of fog and clouds.
Relative Humidity At any given temperature and pressure, there
is a limit to the amount of water vapor in the air. When any substance contains the maximum amount of another substance, the first
substance is saturated. The ratio of how much water vapor is in
the air to the maximum amount that could be in the air at the same
temperature is the relative humidity. Relative humidity is not a measure of how much water vapor is in the air. On a hot day with a low
relative humidity, there may be more water vapor in the air than on a
cold day with high relative humidity.
At a relative humidity of 100%, the air is saturated. More water
vapor is required to saturate high-temperature air than lowtemperature air. The warm air of tropical regions is capable of
containing much more moisture than cold Arctic air.
For saturation, there must be water vapor molecules in the air
undergoing condensation. When slow-moving molecules collide,
some stick together—they condense. To understand this, think of
a fly making grazing contact with flypaper. At low speed it would
surely get stuck, whereas at high speed it is more able to rebound into
the air. Similarly, when water vapor molecules collide, they are more
likely to stick together and become part of a liquid if they are moving slowly as shown in Figure 23.4. At higher speeds, they can bounce
apart and remain in the gaseous phase. The faster the water molecules move, the less able they are to condense to form droplets.
Demonstration
FIGURE 23.4
a. At high speeds, molecules
of water vapor bounce apart
and remain a gas.
b. At lower speeds, molecules of water vapor are
more likely to stick together
and form a liquid.
......
Fog and Clouds Warm air rises. As it rises, it expands. As it
expands, it cools. As it cools, water vapor molecules begin sticking
together after colliding, rather than bouncing off one another. If
there are larger and slower-moving particles or ions present, water
vapor condenses upon these particles, and we have a cloud.
Fog is basically a cloud that forms near the ground. Flying
through a cloud is much like driving through fog. Fog occurs in areas
where moist air near the ground cools. For example, moist air that
has blown in from over an ocean or lake may pass over cooler land.
Some of the water vapor condenses out of the air as it cools, and we
have fog.23.2.2 A key feature of fog and cloud formation is a slowing
down of water vapor molecules in air.
CONCEPT
CHECK
think!
Is it correct to say that
relative humidity is a
measure of the amount of
water vapor in the air at
a particular temperature?
Explain.
Answer: 23.2
Teaching Tip Ask why a glass
containing an iced drink becomes
wet on the outside. State that
the reason is . . . and then write a
big “23.4” on the board. Ask why
the walls of the classroom would
become wet if the temperature
of the room were suddenly
reduced. State that the answer
is . . . and then underline your
“23.4.” Ask why dew forms on
the morning grass, and state that
the answer is . . . another
underline for “23.4.”Ask why fog
forms, and how the clouds form,
and each time point back to
your “23.4.” By now your class is
wondering about the significance
of “23.4.” Announce you’re
discussing Figure 23.4, and go on
to discuss the formation of fog
and clouds.
Teaching Tip Help students
to remember the process of cloud
formation by pointing out that
it is a 4-C process: convection
(causes expansion), cooling (due
to expansion), condensation (due
to cooling), and cloud formation.
Cloud formation can be
stimulated by “seeding“
the air with appropriate
particles or ions.
Condensation warms
the area where the
liquid forms.
......
Although condensation in the air occurs more readily at low
temperatures, it can occur at high temperatures also. Recall that
temperature is a measure of average kinetic energy. There are always
some molecules moving faster than average, and some moving
slower. Even at high temperature, there will be enough slow molecules to cause condensation—provided there is enough water vapor
present. Whatever the temperature, it is the slower molecules that are
more likely to stick.
Repeat the collapsing can
demo (Section 20.2) but
this time invert the can into
boiling water. No crunch! Lead
your class to understand that
the net effect is no change, as
condensation of steam is met
with just as much vaporization
from the boiling water. If the
water is not boiling, then the
can will collapse.
CONCEPT
CHECK
Teaching Resources
How does condensation affect temperature?
• Transparency 44
CHAPTER 23
CHANGE OF PHASE
453
453
23.3 Evaporation
23.3 Evaporation and
Condensation Rates
and Condensation
Rates
Teaching Tip The canceling
effects of evaporation and
condensation may also be
expressed as neutralizing effects.
......
The molecules and
energy leaving a
liquid’s surface by evaporation
can be counteracted by as many
molecules and as much energy
returning by condensation.
CONCEPT
CHECK
Teaching Resources
• Reading and Study
Workbook
• Laboratory Manual 66
• PresentationEXPRESS
FIGURE 23.5 If you feel chilly outside
the shower stall, step back
inside and be warmed by
the condensation of the
excess water vapor there.
• Interactive Textbook
• Next-Time Question 23-1
......
CONCEPT How can evaporation and condensation take
23.4 Boiling
CHECK
Key Term
boiling
Common Misconception
Boiling is a warming process.
Teaching Tip Explain how a
geyser is like a pressure cooker. It
erupts when a certain pressure is
reached.
454
place at the same time?
23.4 Boiling
Students are often confused by
the idea that boiling is a cooling
process. Proceed slowly!
FACT When a substance boils,
the molecules having the highest
KE escape. This lowers the
average KE of the molecules of
the substance. The substance
then has a lower temperature
than it would have had if those
molecules had not escaped.
When you emerge from a shower into a dry room, you often feel
chilly because evaporation is taking place quickly. If you stay in the
shower stall, you will not feel as chilly. When you are in a moist environment, moisture from the air condenses on your skin and warms
you, counteracting the cooling of evaporation. If as much moisture
condenses as evaporates, you will feel no change in body temperature. That’s why you are more comfortable if you stay in the stall.
If you leave a covered dish of water for several days and no apparent evaporation takes place, you might conclude that nothing is happening. You’d be mistaken, for much activity is taking place at the
molecular level. Evaporation and condensation occur continuously at
equal rates. The molecules and energy leaving a liquid’s surface
by evaporation can be counteracted by as many molecules and as
much energy returning by condensation. The water level doesn’t
change because evaporation and condensation have canceling effects.
Evaporation and condensation normally take place at the same
time. If evaporation exceeds condensation, the liquid is cooled. If
condensation exceeds evaporation, the liquid is warmed.
FIGURE 23.6 The motion of molecules in
the bubble of steam (much
enlarged) creates a gas
pressure that counteracts
the water pressure against
the bubble.
454
Evaporation takes place at the surface of a liquid. A change of phase
from liquid to gas can also take place beneath the surface of a liquid, causing bubbles. The bubbles are buoyed upward to the surface,
where they escape into the surrounding air. The change of phase
from liquid to gas beneath a liquid’s surface is called boiling.
The pressure of the vapor within the bubbles in a boiling liquid
must be great enough to resist the pressure of the surrounding water.
Unless the vapor pressure is great enough, the surrounding pressures
will collapse any bubbles that may form. At temperatures below the
boiling point, the vapor pressure is not great enough. Bubbles do not
form until the boiling point is reached.
As the atmospheric pressure is increased, the molecules in the
vapor are required to move faster to exert increased pressure within
the bubble in order to counteract the additional atmospheric pressure. Increasing the pressure on the surface of a liquid raises the
boiling point of the liquid. Conversely, lowered pressure (as at high
altitudes) decreases the boiling point. Thus, boiling depends not only
on temperature but on pressure also.
discover!
discover!
teakettle, water,
heat source, candle, matches
MATERIALS
Can You See Steam?
EXPECTED OUTCOME The
cloud of condensed steam
disappears.
1. Bring a teakettle full of water to a
boil and watch the spout. Where
do you see the cloud form?
2. Hold a lighted candle in the
cloud of condensed steam. What
do you see?
3. Think What does the heat
from the flame do to the
condensed steam?
The heat from the flame
causes the condensed water
droplets to evaporate.
THINK
Low Pressure It is important to note that it is the high temperature of the water that cooks the food, not the boiling process itself.
At high altitudes, water boils at a lower temperature. In Denver,
Colorado, the “mile-high city,” for example, water boils at 95°C,
instead of the 100°C boiling temperature characteristic of sea level.
If you try to cook food in boiling water of a lower temperature, you
must wait a longer time for proper cooking. A “three-minute” boiled
egg in Denver is runny. If the temperature of the boiling water were
very low, food would not cook at all.
Boiling, like evaporation, is a process that cools the liquid left
behind. Heating water is one thing; boiling is another. When 100°C
water at atmospheric pressure is boiling, heat is taken away as fast as
it is added. Figure 23.7 shows the water is being cooled by boiling as
fast as it is being heated by energy from the heat source. If cooling
did not take place, continued application of heat to a pot of boiling
water would result in a continued increase in temperature.
think!
Since boiling is a cooling
process, would it be a
good idea to cool your
hot and sticky hands by
dipping them into boiling
water? Explain.
Answer: 23.4
FIGURE 23.7 Heating and boiling are two
distinct processes. Heating
warms the water, and boiling cools it.
Demonstration
Evacuate air from a flask
of water that is at room
temperature, enough that
the water in the flask will boil
from the heat of the students’
hands as it is passed around
the classroom. (Do this only
for a thick-walled flask that
won’t implode.)
Ask In bringing water to a
boil on a high mountain, is the
time required to bring the water
to a boil longer or shorter than
at sea level? Shorter Is the time
required for cooking longer or
shorter? Longer
Increasing the
pressure on the
surface of a liquid raises the
boiling point of the liquid.
......
High Pressure A pressure cooker is based on this fact. A pressure
cooker has a tight-fitting lid that does not allow vapor to escape until
it reaches a certain pressure greater than normal air pressure. As the
evaporating vapor builds up inside the sealed pressure cooker, pressure on the surface of the liquid is increased, which prevents boiling.
A pressure cooker reaches a higher temperature because the increased
pressure forces the water to reach a higher temperature before boiling can occur. The increased temperature of the water cooks the food
faster.
Teaching Tip Explain that
the temperature of boiling water
remains at 100ºC because the
water is cooled by boiling as
fast as it is warmed by heating.
Explain that the hot steam above
gets its energy from the boiling
water; so energy is leaving the
water—that’s what is meant by
cooling!
CONCEPT
CHECK
......
CONCEPT What is the effect of pressure on the boiling
CHECK
temperature of a liquid?
Teaching Resources
• Laboratory Manual 61
• Probeware Lab Manual 11
CHAPTER 23
CHANGE OF PHASE
455
455
23.5 Freezing
23.5 Freezing
Key Term
freezing
When energy is continually withdrawn from a liquid, molecular
motion slows until the forces of attraction between the molecules
cause them to get closer to one another. The molecules then vibrate
about fixed positions and form a solid. Water provides a good example of this process. When energy is extracted from water at a temperature of 0°C and at atmospheric pressure, ice is formed. The liquid
water gives way to the solid ice phase. The change in phase from
liquid to solid is called freezing. Figure 23.8 shows the open sixsided structure of an ice crystal.
Teaching Tip Recall the open
structure of ice crystals discussed
in Section 21.9. This model
illustrates why foreign molecules
that do not fit into the structure
lower the freezing point. (It also
explains why pressure causes
regelation.)
Ask Why is rock salt spread
on icy roads in winter? A short
answer is that salt makes ice
melt. Why involves the fact
that salt in water separates
into sodium and chlorine ions.
When these ions join water
molecules, heat is given off,
which melts microscopic parts of
an icy surface. The pressure of
automobiles rolling along the
salt-covered icy surface forces the
salt into the ice, enhancing the
melting process.
......
In general, dissolving
anything in a liquid
lowers the liquid’s freezing
temperature.
CONCEPT
CHECK
FIGURE 23.8 Pure ice crystals have
an open, hexagonal
structure.
Although streams can
freeze over in cold
weather, most often
they don’t. Why?
Because streams are
usually fed with warmer
groundwater.
23.6 Boiling and
Freezing at the Same
Time
......
CONCEPT What effect does dissolving anything in a liquid
CHECK
have on the liquid’s freezing temperature?
23.6 Boiling and Freezing
at the Same Time
Demonstration
Place a drop or two of water
in a dish that is insulated from
the base of a vacuum jar by a
polystyrene cup. As you slowly
reduce the pressure using a
vacuum pump, the water will
start to boil. As you reduce the
pressure further, the boiling
causes the temperature of the
water to drop until it reaches
its freezing point and ice
forms over the surface of the
bubbling water.
456
Interestingly enough, if sugar or salt is dissolved in the water,
the freezing temperature will be lowered. These “foreign” molecules
or ions get in the way of water molecules that ordinarily would join
together. As ice crystals do form, the hindrance is intensified, for the
proportion of foreign molecules or ions among liquid water molecules that remain increases. Connections become more difficult.
In general, dissolving anything in a liquid lowers the liquid’s
freezing temperature. Antifreeze in an automobile engine is a practical application of this process.
Suppose that a dish of water at room temperature is placed in a vacuum jar, as shown in Figure 23.9. If the pressure in the jar is slowly
reduced by a vacuum pump, the vapor pressure of the molecules
within the water will be high enough to form bubbles, and the water
will start to boil. The boiling process takes higher-energy molecules
away from the water left in the dish, which cools to a lower temperature. As the pressure is further reduced, more and more of the faster
remaining slow-moving molecules boil away.
456
......
Lowering the
pressure can cause
boiling and freezing to take
place at the same time.
CONCEPT
Continued boiling results in a lowering of temperature until the
freezing point of approximately 0°C is reached. Continued cooling by
boiling causes ice to form over the surface of the bubbling water.
Lowering the pressure can cause boiling and freezing to take
place at the same time! This must be witnessed to be appreciated.
Frozen bubbles of boiling water are a remarkable sight.
If some drops of coffee are sprayed into a vacuum chamber,
they too will boil until they freeze. Even after they are frozen, the
water molecules will continue to evaporate until little crystals of
coffee solids are left. This is how freeze-dried coffee is made. The
low temperature of this process tends to keep the chemical structure
of coffee solids from changing. When hot water is added, much of
the original flavor of the coffee is preserved.
CHECK
place at the same time?
23.7 Regelation
......
CHECK
• Reading and Study
Workbook
• PresentationEXPRESS
• Interactive Textbook
FIGURE 23.9 The apparatus shown can be
used to demonstrate that
water will freeze and boil at
the same time in a vacuum.
A gram or two of water is
placed in a dish that is insulated from the base by a
polystyrene cup.
23.7 Regelation
Key Term
regelation
Common Misconception
Ice melts only when heat is added.
FACT Ice can also melt under
increased pressure.
The open-structured crystals of ice can be crushed by the application of pressure. Whereas ice normally melts at 0°C, the application
of pressure lowers the melting point. The crystals are simply crushed
to the liquid phase. At twice standard atmospheric pressure, the melting point is lowered to –0.007°C. Quite a bit more pressure must be
applied for an observable effect.
When the pressure is removed, refreezing occurs. The phenomenon of melting under pressure and freezing again when the pressure
is reduced is called regelation. It is one of the properties of water
that make it different from other substances. Regelation can occur
only in substances that expand when they freeze.
You can see regelation if you suspend a fine wire that supports
heavy weights over an ice block, as shown in Figure 23.10. The wire
will slowly cut its way through the ice, but its track will refill with ice.
You will see the wire and weights fall to the floor, leaving the ice in a
single solid piece!
To make a snowball, you use regelation. When you compress the
snow, you cause a slight melting, which helps to bind the snow into
a ball. Making snowballs is difficult in very cold weather, because the
pressure you can apply may not be enough to melt the snow.
Once, it was thought that an ice skate’s pressure lowered the
freezing point of ice. Now, we know that this is not sufficient to
explain ice-skating. Ice has a thin layer of liquid on its surface even at
very low temperatures.
CONCEPT
Teaching Resources
Demonstration
Attach a heavy weight to each
end of a piece of copper wire.
Place the wire over a block of
ice with the weights hanging
freely on each side. The
pressure of the wire on the ice
causes the ice to melt. As the
wire makes its way through
the ice, the ice refreezes above
the wire and melts under it.
FIGURE 23.10 Regelation allows the wire
to cut through the ice, but
leaves the ice in a single
solid piece.
Regelation can occur
only in substances
that expand when they freeze.
......
......
CONCEPT What can cause boiling and freezing to take
CHECK
CONCEPT
CHECK
Teaching Resources
• Reading and Study
Workbook
• PresentationEXPRESS
Why do so few substances undergo regelation?
CHAPTER 23
• Interactive Textbook
CHANGE OF PHASE
457
457
23.8 Energy and
Changes of Phase
Physics in the Kitchen
Note that the unit calorie is
used to express the heat of
fusion and vaporization of
water. SI units have their
merits, and they have their
drawbacks too. I have a strong
bias toward saying 1 calorie will
raise the temperature of 1 g
of water by 1°C, rather
than 4184 J will raise the
temperature of 1 kg of water
by 1°C, and that 80 calories will
melt 1 g of ice and 540 calories
will vaporize 1 g of boiling
water, rather than the SI
figures 334.88 kJ/kg and
2260 kJ/kg. I find the
SI values somewhat more
conceptually difficult. Note
23.8.1 gives the SI units, so
you can choose to lecture with
SI units and point out the few
places where the unit calorie
occurs.
The Egg Test
Physics can help with even the simplest of all
cooked creations—the boiled egg. Test the egg for
freshness by placing it in water. If it sinks and lies
on its side, it’s fresh. If it floats, it’s rotten. An egg
loses density as it ages because it loses moisture
through pores in its shell, eventually becoming
less dense than water. To test that the egg is raw,
spin it on a tabletop. If it wobbles, it’s uncooked.
The wobbling indicates that the yolk is moving
within the egg, thus changing the egg’s center of
gravity. Eggs sometimes crack while boiling due
to an air pocket inside. With heat, the air pressure
in the pocket increases enough to crack the shell.
If you carefully pierce the egg’s big end with a
small, clean pin before boiling, it won’t crack.
Finally be sure you actually boil the water. You can
heat an egg indefinitely at lower temperatures,
but it doesn’t cook. Cooking requires exceeding a
threshold temperature so that the long-stranded
molecules of the egg become cross-linked. That’s
why an egg won’t cook by boiling at very high
altitudes—the boiling water is not hot enough to
cook the egg.
23.8 Energy and Changes of Phase
If you heat a solid sufficiently, it will melt and become a liquid. If you
heat the liquid, it will vaporize and become a gas. Energy must be
put into a substance to change its phase from solid to liquid to gas.
Conversely, energy must be extracted from a substance to change
its phase from gas to liquid to solid. Figure 23.11 shows the flow of
energy.
PHASE
FIGURE 23.11 Demonstration
Demonstrate heat of fusion
with a beaker containing
about 400 mL of crushed ice.
Place a thermometer into
the beaker and note the 0ºC
temperature. Then place
a Bunsen burner beneath
the beaker and continue to
note the temperature as the
ice melts. Students see no
temperature change until
after all the ice melts.
458
The change in the
internal energy of a
substance causes the
change of phase.
Heat of fusion is either
the energy needed to
separate molecules
from the solid phase,
or the energy released
when bonds form in a
liquid and change it to
the solid phase.
458
PHASE
Examples of Phase Changes The general behavior of many
substances can be illustrated with a description of the changes of
phase of H2O. To make the numbers simple, suppose we have a
1-gram piece of ice at a temperature of –50°C in a closed container,
and it is put on a stove to heat. A thermometer in the container
reveals a slow increase in temperature up to 0°C. (It takes about half
of a calorie to raise the temperature of the gram of ice by 1°C.) Once
it reaches 0°C, the temperature of the ice remains at 0°C even though
heat input continues. Rather than getting warmer, the ice melts.
Teaching Tip Emphasize the
SI equivalents for heat of fusion
of water (3.34 3 105 J/kg),
heat of vaporization of water
(2.26 3 106 J/kg), and specific
heat capacities of
ice (2060 J/kg?K),
liquid water (4180 J/kg?K),
and steam (2020 J/kg?K).
0
MELTING
STE
AM
VAPORIZING
100
ER
AT
W
IC
E
TEMPERATURE (%C)
PHASE CHANGES OF WATER
-50
80
100
540
HEAT (CALORIES)
FIGURE 23.12 The graph shows the energy involved in the heating
and the change of phase of 1 gram of H2O.
In order for the whole gram of ice to melt, 80 calories
(335 joules) of heat energy must be absorbed by the ice. Not until
all the ice melts does the temperature again begin to rise. Each additional calorie absorbed by the gram of water increases its temperature
by 1°C until it reaches its boiling temperature, 100°C. Again, as heat
is added, the temperature remains constant while more of the gram
of water is boiled away and becomes steam. The water must absorb
540 calories (2255 joules) of heat to vaporize the whole gram.23.8.1
Finally, when all the water has become steam at 100˚C, the temperature begins to rise once more. It continues to rise as long as heat is
added (again taking about a half calorie per gram for each 1°C rise in
temperature). This process is shown graphed in Figure 23.12.
Reversibility of Phase Changes The phase change sequence is
reversible. When the molecules in a gram of steam condense to form
boiling water, they liberate 540 calories (2255 joules) of heat to the
environment. When the water is cooled from 100°C to 0°C,
100 additional calories are liberated to the environment. When ice
water fuses to become solid ice, 80 more calories (335 joules) of
energy are released by the water.
The 540 calories (2255 joules) required to vaporize a gram of
water is a relatively large amount of energy—much more than is
required to change a gram of ice at absolute zero to boiling water
at 100°C. Although the molecules in steam and boiling water at
100°C have the same average kinetic energy, steam has more potential energy, because the molecules are free of each other and are not
bound together in the liquid. Steam contains a vast amount of energy
that can be released during condensation.
CHAPTER 23
think!
How much energy is
released when a gram
of steam at 100°C condenses to water at 100°C?
Answer: 23.8.1
Teaching Tip Ask if it
is possible to add heat to a
substance without raising its
temperature, and why a steam
burn is more damaging than a
burn from boiling water at the
same temperature. In answering
these, discuss the change of
phase graph of Figure 23.12, and
relate this to Figure 23.11. After
giving examples of changes of
phase where energy is absorbed,
give examples where energy is
released—such as raining and
snowing.
Teaching Tip Point out that
the reciprocals of the slopes of
the graph in the solid, liquid, and
gas portions are proportional
to the specific heat capacities of
the respective phases at standard
pressure.
The Dutch philosopher Baruch
Spinoza said, “Nature abhors
a vacuum.” Since nature is
indifferent, wouldn’t it be
more correct to say that it is
we investigators who abhor a
vacuum?
Water’s heat of vaporization is huge. The
energy needed to
vaporize a quantity of
boiling water is nearly
seven times the energy
needed to melt the
same amount of ice.
CHANGE OF PHASE
459
459
Teaching Tip Describe the
interesting example of energy
absorbed during a change of
phase in the heat shields that
protect spacecraft on re-entry
into the atmosphere. The KE
of a spacecraft in orbit is many
times greater than the amount
of energy needed to vaporize
the craft. The shields are made of
synthetic resin or plastic ablative
material that dissipates heat
by melting and vaporizing. At
altitudes from 25 km to about
40 km, almost all of the KE is
dissipated within about a period
of 1 minute, heating the shield to
several thousand degrees Celsius.
Because of the shield’s very
low conductivity, only a small
portion of the heat generated
on re-entry is absorbed by the
craft. A centimeter or two of the
ablative material is consumed by
ablation, radiating about 80% of
the heat to the surrounding air.
If the re-entry trajectory is too
steep, heating will be too severe
to deal with by ablative cooling.
If the trajectory is too flat, the
spacecraft might “bounce off”
Earth’s atmosphere and overshoot
into space.
FIGURE 23.13 When a car is washed on
a cold day, hot water will
freeze more readily than
warm water because of
the energy that the rapidly
evaporating water takes
with it.
For: Links on Phases of Matter
Visit: www.SciLinks.org
Web Code: csn – 2308
Teaching Tip Opening a
refrigerator door lets warm air
in, which takes energy to cool.
The more empty your fridge, the
more cold air is swapped with
warm air. So keep your fridge
full for lower operating costs—
especially if you’re an excessive
open-and-close-the-door type.
Teaching Tip Ask about
cooling a room by leaving the
refrigerator door open, and
compare it to putting an air
conditioner in the middle of a
room instead of mounting it in
a window. Ask what the result
would be of mounting an air
conditioner backwards in a
window.
460
FIGURE 23.14 The refrigeration cycle
in a common refrigerator
keeps the inside cool.
460
The large value of 540 calories per gram explains why under some
conditions hot water will freeze faster than warm water.23.8.2 This
occurs for water hotter than 80°C. It is evident when the surface area
that cools by rapid evaporation is large compared with the amount of
water involved. Examples are a car washed with hot water on a cold
winter day, and a skating rink flooded with hot water to melt and
smooth out the rough spots and refreeze quickly. The rate of cooling by rapid evaporation is very high because each gram of water
that evaporates draws at least 540 calories from the water left behind.
This is an enormous quantity of energy compared with the 1 calorie
per Celsius degree that is drawn for each gram of water that cools by
thermal conduction. Evaporation truly is a cooling process.
Applications of Phase Changes A refrigerator’s cooling cycle
is a good example of the energy interchanges that occur with the
changes of phase of the refrigeration fluid (not water!). The liquid is
pumped into the cooling unit, where it is forced through a tiny opening to evaporate and draw heat from the things stored in the food
compartment. The gas is then directed outside the cooling unit to
coils located in the back. As the gas condenses in the coils, appropriately called condensation coils, heat is given off to the surrounding
air. The liquid returns to the cooling unit, and the cycle continues. A
motor pumps the fluid through the system, where it enters the cyclic
processes of vaporization and condensation. The next time you’re
near a refrigerator, place your hand near the condensation coils in the
back (or bottom), and you will feel the heat that has been extracted
from the inside.
An air conditioner employs the same principles. It simply pumps
heat from one part of the unit to another. When the roles of vaporization and condensation are reversed, the air conditioner becomes a
heater. A device that moves heat is called a heat pump.
......
A way that some people judge the hotness of a clothes iron is
to touch it briefly with a finger. This is also a way to burn the finger—unless it is first moistened. Energy that ordinarily would go
into burning the finger goes, instead, into changing the phase of the
moisture on it. The energy converts the moisture to a vapor, which
additionally provides an insulating layer between the finger and the
hot surface.
Similarly, you may have seen news photos or heard stories about
people walking barefoot without harm over red-hot coals from firewood. (CAUTION! Never try this on your own; even experienced
“firewalkers” have received bad burns when the conditions were
not just right.) The primary factor here is the low conductivity of
wood—even red-hot wood. Although its temperature is high, relatively little heat is conducted to the feet, just as little heat is conducted by air when you put your hand briefly into a hot pizza oven
(because air is a poor conductor). But if you touch metal in the hot
oven, OUCH! Similarly, a firewalker who steps on a hot piece of
metal or another good conductor will be burned. A secondary factor
is skin moisture. Perspiration on the soles of the feet decreases heat
transfer to the feet. Much of the heat that would go to the feet instead
goes to vaporizing the moisture—again, like touching a hot clothes
iron with a wetted finger. Temperature is one thing; heat transfer is
another.
In brief, a solid absorbs energy when it melts; a liquid absorbs
energy when it vaporizes. Conversely, a gas emits energy when it
liquefies; a liquid releases energy when it solidifies.
CONCEPT
CHECK
A refrigerator is a “heat
pump.” It transfers heat
out of a cold environment and into a warm
environment. When the
process is reversed, the
heat pump is an air conditioner. In both cases,
external energy operates the device.
think!
When H2O in the vapor
phase condenses, is the
surrounding air warmed
or cooled?
Answer: 23.8.2
How is energy related to phase changes?
Teaching Tip Ask students
to consider a pair of molecules
before and after collision. Draw
them on the board as shown,
each with the same KE.
If they bounce from each other
with the same speed, then they
each have the same KE after the
collision as before. If they collide
such that one gains speed, the
other one has a smaller KE.
Again, the total KE before and
after is the same. Then ask what
happens if the molecule that
loses KE is a water molecule. If it
is hit by a fast-moving molecule
of any kind, it will be brought
up to high speed and high KE
again, but if it instead encounters
another slow-moving water
molecule, one that has similarly
given its energy to another
molecule in collision, the two
will probably stick together.
Suppose this happens for many
water molecules in a sample of
gas. Then the KE per molecule of
remaining gas should increase as
water condenses.
Physics on the Job
......
The change in the
internal energy of a
substance causes the change of
phase.
CONCEPT
Fire Fighting
CHECK
Firefighters regularly enter burning buildings to save lives and property.
In order to perform their job effectively and safely, firefighters must be
knowledgeable about the physics of heat. The most common fire control is
dousing a flame with water. In some cases, a fine mist is more effective in
quenching a fire. Why? Because the fine mist readily turns to steam, and in
so doing quickly absorbs energy and cools the burning material. Properly
dealing with flames saves lives, including their own. To firefighters, the
physics of heat is much more than a classroom assignment. It’s a matter of
staying alive. Job opportunities exist for firefighters with city or county fire
departments and the National Forest Service.
Teaching Resources
• Concept-Development
Practice Book 23-1, 23-2
• Laboratory Manual 62, 63,
64, 65
• Probeware Lab Manual 12
• Transparencies 45, 46
• Next-Time Question 23-2
CHAPTER 23
CHANGE OF PHASE
461
461
REVIEW
3 REVIEW
Teaching Resources
• TeacherEXPRESS
• Virtual Physics Lab 23
For: Self-Assessment
Visit: PHSchool.com
Web Code: csa – 2300
• Conceptual Physics Alive!
DVDs Heat: Change of State
Concept Summary
•
•
•
•
•
•
•
•
Evaporation cools the liquid left behind.
Condensation warms the area where the
liquid forms.
The molecules and energy leaving a
liquid’s surface by evaporation can be
counteracted by as many molecules and
as much energy returning by condensation.
Increasing the pressure on the surface
of a liquid raises the boiling point of the
liquid.
In general, dissolving anything in a liquid
lowers the liquid’s freezing temperature.
Lowering the pressure can cause boiling
and freezing to take place at the same
time.
Regelation can occur only in substances
that expand when they freeze.
Energy must be put into a substance to
change its phase from solid to liquid to
gas. Conversely, energy must be extracted
from a substance to change its phase
from gas to liquid to solid.
Key Terms
••••••
phases (p. 450)
evaporation (p. 451)
condensation
(p. 452)
saturated (p. 452)
relative humidity
(p. 452)
462
••••••
462
boiling (p. 454)
freezing (p. 456)
regelation (p. 457)
heat pump (p. 460)
think! Answers
23.2
No. Humidity is a measure of the amount
of water vapor per volume of air, whatever
the temperature. Relative humidity, on the
other hand, is the amount of vapor in the
air compared with the amount for saturation at a particular temperature. Relative
humidity is a ratio, expressed as a percent.
Air with 60% of the vapor contained by
saturated air at the same temperature has a
relative humidity of 60%.
23.4
No, no, no! When we say boiling is a cooling process, we mean that the water (not
your hands!) is being cooled. A dip in
100°C water would be most uncomfortable
for your hands!
23.8.1 One gram of steam at 100°C releases 540
calories of energy when it condenses to
become water at the same temperature.
23.8.2 The surrounding air is warmed because the
change of phase is from vapor to liquid,
which releases energy.
ASSESS
3 ASSESS
Check Concepts
1. A wide distribution of various
speeds
2. Change of phase from liquid
to gas; the remaining liquid
loses KE and cools.
Check Concepts
••••••
Section 23.1
9. Why do you feel less chilly if you dry yourself inside the shower stall after taking a
shower?
1. Do all the molecules or atoms in a liquid
have about the same speed, or much different speeds?
3. To cool by evaporation from
the mouth and throat
4. Change of phase from gas
to liquid; the existing liquid
gains KE and warms.
5. Steam has more internal
energy than boiling water.
6. Warm air
7. It expands and cools, and
the slower-moving water
molecules stick together.
8. The water level in an open
container stays the same.
2. What is evaporation, and why is it also a
cooling process?
3. Why does a hot dog pant?
Section 23.2
Section 23.4
10. What is the difference between evaporation
and boiling?
4. What is condensation, and why is it also a
warming process?
11. Why does the temperature at which a liquid
boils depend on atmospheric pressure?
5. Why is being burned by steam more damaging than being burned by boiling water of
the same temperature?
12. Why is a pressure cooker even more useful
when cooking food in the mountains than
when cooking at sea level?
6. Which usually contains more water vapor—
warm air or cool air?
Section 23.5
7. Why does warm moist air form clouds when
it rises?
Section 23.3
8. How can you tell if the rate of evaporation
equals the rate of condensation?
13. Why does antifreeze or any soluble substance put in water lower its freezing temperature?
9. The greater condensation
inside the shower area
reduces the cooling effect
of evaporation.
10. Evaporation occurs only at
the surface, whereas boiling
occurs throughout a liquid.
11. Atmospheric pressure tends to
squash vapor bubbles.
12. It provides pressure in a lower
pressure region, thereby
raising the temperature.
13. It inhibits the formation of
the hexagonal ice structure.
14. By reducing the pressure
drastically
Section 23.6
14. How can water be made to both boil and
freeze at the same time?
CHAPTER 23
CHANGE OF PHASE
463
463
15. Melting under pressure; the
pressure crushes open ice
crystals.
3 ASSESS
16. a. 1
b. 80
c. 540
17. Gives off energy
(continued)
18. It causes a reduction of
temperature.
19. When liquid turns to vapor
Section 23.7
20. The energy that could cause
a burn will be reduced by the
energy that causes a phase
change of the water.
15. What is regelation, and what does it have to
do with the open-structured crystals in ice?
Plug and Chug
25. Q 5 mL 5 (20 g)(540 cal/g) 5
10,800 cal
26. Q 5 mL 1 mcDT 1 mL 5
(20 g)(540 cal/g) 1
(20 g)(1 cal/g°C)(100°C) 1
(20 g)(80 cal/g) 5 14,400 cal
Think and Explain
27. a. The liquid cools and the
environment warms.
b. The liquid warms and the
environment cools.
••••••
Use the following information to help you answer
Questions 21–26.
22. Q 5 mL 5 (50 g)(80 cal/g) 5
4000 cal
24. Q 5 mL 5 (20 g)(540 cal/g) 5
10,800 cal
20. Why is it important that a finger be wet
before it is touched to a hot clothes iron?
Plug and Chug
21. Q 5 mcDT 5 (20 g) 3
(1 cal/g°C)(90°C 2 30°C) 5
1200 cal
23. Q 5 mL 1 mcDT 5 (100 g) 3
(80 cal/g) 1 (100 g)(1 cal/g°C) 3
(30°C) 5 11,000 cal
19. In a refrigerator, does the food cool when a
vapor turns to a liquid, or vice versa?
Section 23.8
Quantity of heat energy required for change of
phase (mass) (heat of fusion or heat of
vaporization), or in equation form, Q mL.
16. a. How many calories are needed to raise the
temperature of 1 gram of water by 1°C?
b. How many calories are needed to melt 1
gram of ice at 0°C?
c. How many calories are needed to vaporize
1 gram of boiling water at 100°C?
Quantity of heat energy responsible for a
temperature change (mass) (specific heat)
(change in temperature), or in equation
form,Q mcRT.
17. When a vapor turns to a liquid, does it give
off energy or does it absorb energy?
21. Calculate the energy absorbed by 20 grams
of water that warms from 30°C to 90°C.
18. What is the effect of rapid evaporation on
the temperature of water?
22. Calculate the energy needed to melt
50 grams of 0°C ice.
For water, heat of fusion 80 cal/g; heat of
vaporization 540 cal/g.
23. Calculate the energy needed to melt 100
grams of 0°C ice and then heat it to 30°C.
24. Calculate the energy absorbed by 20 grams
of 100°C water that is turned into 100°C
steam.
25. Calculate the energy released by 20 grams of
100°C steam that condenses to 100°C water.
26. Calculate the total energy released when
20 grams of steam condenses to water, cools,
and then turns to ice at 0°C.
464
464
28. Wind increases evaporation,
which cools you.
29. Agree; remaining liquid
would have the same energy
per molecule before and after
evaporation.
Think and Explain
•••••
27. a. Evaporation is a cooling process. What
cools and what warms during evaporation?
b. Condensation is a warming process. What
warms and what cools during condensation?
28. You’re not chilly when swimming in warm
water. But when emerging from warm water
on a warm summer day, you feel chilly if the
wind is blowing. Explain.
29. Classmate Matthew says that if all the molecules in a particular liquid had the same
speed, and some were able to evaporate, the
remaining liquid would not undergo cooling. Do you agree or disagree, and what is
your explanation?
33. Why does dew form on the surface of a cold
soft-drink can?
34. Air-conditioning units contain no water
whatever, yet it is common to see water
dripping from them when they’re running
on a hot day. Explain.
35. Why do clouds often form above mountain
peaks? (Hint: Consider the updrafts.)
36. Sometimes moisture forms on the inside
of your windows at home. And sometimes
it forms on the outside. What is your
explanation?
37. If a large tub of water is kept in a small
unheated room, even on a very cold day the
temperature of the room will not go below
0°C. Why not?
38. On a clear night, why does more dew form
in an open field than under a tree or beneath a park bench?
30. You can determine wind direction if you
wet your finger and hold it up into the air.
Explain.
31. Give two reasons why pouring a hot cup of
coffee into a saucer results in faster cooling.
32. At a picnic, why would wrapping a bottle
in a wet cloth be a better method of cooling
than placing the bottle in a bucket of cold
water?
39. Machines used for making snow at ski areas
blow a mixture of compressed air and water
through a nozzle. The temperature of the
mixture may initially be well above the
freezing temperature of water, yet crystals
of snow are formed as the mixture is ejected
from the nozzle. Explain how this happens.
40. People who live where snowfall is common
will attest to the fact that air temperatures
are generally higher on snowy days than on
clear days. Some people get cause and effect
mixed up when they say that snowfall cannot occur on very cold days. Explain.
CHAPTER 23
CHANGE OF PHASE
465
30. Moisture from the side of
the finger in the wind more
readily evaporates, making
that side of the finger cooler.
31. More surface area results
in greater evaporation and
will produce more cooling.
The saucer warms up by
conduction and the coffee
cools.
32. The evaporation of liquid
from the cloth will decrease
the temperature further.
33. Water vapor in air is chilled
upon contact with the can
(Figure 23.4).
34. Water vapor in air condenses
on their cold surfaces
(Figure 23.4).
35. As up-drafted air rises it
cools; water molecules in it
condense (Figure 23.4).
36. It forms on the cooler side
of the window (or side with
greatest relative humidity) via
Figure 23.4.
37. The continual release of
energy by the freezing
water (80 cal/g) keeps the
temperature of the unheated
room from going below 0°C.
38. Trees and benches, etc., lower
the net radiation of Earth,
so those regions are warmer
than open regions. Dew forms
in the cooler regions.
39. The mixture expands when
ejected from the nozzle, and
cools to freezing temperature.
40. In snowfall, water goes from
vapor to solid and causes the
surrounding air to become
warmer.
465
41. Wood; its greater specific
heat means it releases more
energy in cooling.
42. Every gram of water freezing
releases 80 cal to cellar.
Continual energy release
keeps cellar temperature
from going below 0°C. Sugar
and salts in canned goods
prevents them from freezing
at 0°C. Cellar can’t go below
0°C until all the water has
frozen.
43. Similar to answer to Question
42. Every gram that freezes
releases 80 cal to fruit.
Additionally, the ice coating is
an insulator.
44. Madison is correct: Food
cooks by temperature, which
is the same in both cases.
45. Heat input with no change
in temperature indicates that
the heat energy is going into
the change of phase from
liquid to water vapor, which
leaves the liquid as steam.
46. Reduce the pressure
47. Agree with Elizabeth;
decreased pressure lessens
the squeezing of molecules,
favoring their tendency to
separate and form vapor.
48. Jar reaching 100°C is in
thermal equilibrium with
surrounding water. So no
further heat enters to cause
boiling.
3 ASSESS
(continued)
41. A piece of metal and an equal mass of wood
are both removed from a hot oven at equal
temperatures and dropped onto blocks of
ice. The metal has a lower specific heat capacity than the wood. Which will melt more
ice before cooling to 0°C?
47. Elizabeth says that the boiling temperature
of water decreases when the water is under
reduced pressure. Austin says the opposite
is true—that reduced pressure increases the
boiling point. Whom do you agree with and
why?
42. Why is it that, in cold winters, a tub of water
placed in a farmer’s canning cellar helps
prevent canned food from freezing?
48. Nick suspends a small jar of water in a
saucepan, careful that the bottom of the jar
doesn’t rest on the bottom of the saucepan.
Nick then puts water in the pan, surrounding the jar. He puts the saucepan on a hot
stove and is puzzled to see that although the
water in the pan comes to a boil, the water
in the jar doesn’t. He looks to you for an
explanation. Explain.
43. Why will spraying fruit trees with water
before a frost help to protect the fruit from
freezing?
44. Andrew says that potatoes will cook faster in
vigorously boiling water than in gently boiling water. Madison disagrees. Whom do you
agree with, and why?
45. Why is the constant temperature of boiling
water on a hot stove evidence that boiling is
a cooling process? (What would happen to
its temperature if boiling were not a cooling
process?)
49. No; low-temperature boiling
won’t cook! Cooking is caused
by the high temperature, not
by the bubbling water.
50. Your inventor friend proposes a design for
cookware that will allow boiling to take
place at a temperature of less than 100°C so
that food can be cooked with the consumption of less energy. Comment on this idea.
51. Hydrothermal vents are openings in the
ocean floor that discharge very hot water.
Water emerging at nearly 280°C from one
such vent off the Oregon coast, some
2400 m beneath the surface, is not boiling.
Provide an explanation.
50. As in previous answer, high
temperature rather than
boiling cooks food.
51. As in a pressure cooker, high
pressure prevents boiling.
52. As in a pressure cooker, high
pressure prevents boiling.
46. How can water be brought to a boil without
heating it?
466
49. Room-temperature water will boil spontaneously in a vacuum—on the moon, for
example. Could you cook an egg in this
boiling water? Defend your answer.
466
52. In the power plant of a nuclear submarine,
the temperature of the water in the reactor
is above 100°C. How is this possible?
Think and Solve
53. a. From 2273°C “ice” to 0°C
ice requires (273)(0.48) 5
130 cal; 0°C ice to 0°C water,
80 cal; 0°C water to 100°C
water, 100 cal; total 310 cal.
b. Q 5 mL 5 (1 g)(540 cal/g) 5
540 cal
Think and Solve
••••••
53. The specific heat capacity of ice is
0.48 cal/g°C. Make the assumption that it
remains at that value all the way to absolute
zero (at very low temperatures it’s lower,
which we’ll ignore here).
a. Show that the heat required to change a
1-gram ice cube at absolute zero
(273°C) to 1 gram of boiling water is
about 310 calories.
b. Show that more energy is needed to turn
100°C water to 100°C steam.
54. How much steam at 100°C must be condensed in order to melt 1 gram of 0°C ice
and have the resulting ice water remain at
0°C? (The answer is not 0.148 grams!)
55. How many calories are given off by
1 gram of 100°C steam that changes
phase to 1 gram of ice at 0°C?
56. If 20 grams of hot water at 80°C is poured
into a cavity in a very large block of ice at
0°C, what will be the final temperature of
the water in the cavity? How much ice must
melt in order to cool the hot water down to
this temperature?
59. If that same amount of energy (answer to
question 58) were used to warm 4 kg of
water (8 times as much!) initially at 0° C,
what would be the final temperature of the
water?
60. The heat of vaporization of ethyl alcohol is
8.5 105 J/kg. If 2 kg of it were allowed to
vaporize in a refrigerator, how much energy
would be drawn from the air molecules?
61. How much energy is needed to change 1 kg
of ice at –10°C to steam at 120°C?
54. (540 cal/g)m 1 (m)(1 cal/g°C) 3
(100°C) 5 80 cal; m 5
(80 cal) 4 (640 cal/g) 5
0.125 g
55. (540 1 100 1 80) 5 720 cal
56. The water will cool to the
temperature of the ice, 0°C.
Quantity of heat of cooling
water 5 quantity of heat to
melt ice. So (20 g)(1 cal/g°C) 3
(80°C) 5 m(80 cal/g) and m 5
20 g.
57. 1.67 3 106 J
58. 1.13 3 106 J
59. 67.5°C
60. 1.7 3 106 J
Activity
••••••
62. Boil some water in a pan and note that
bubbles form at particular regions of the
pan. These are nucleation sites—scratched
or flawed regions of the pan, or simply bits
of dirt. When water reaches the boiling
point these sites provide havens where microscopic bubbles can collect long enough
to become big bubbles. Nucleation sites are
also important for phase changes of condensation and solidification. Snowflakes
and raindrops typically form around dust
particles, for example.
61. [(4800 1 80,000 1 100,000 1
540,000 1 10,000) cal] 3
(4.184 J/cal) 5 3.074 3 106 J
Activity
62. Students will see bubbles
emanating from these
nucleation sites.
Answer Questions 57–61 in terms of joules rather
than calories.
57. How much energy is needed to melt 5 kg of
ice at 0°C?
58. How much energy is given to your body
when 0.5 kg of steam condenses on your
skin?
Teaching Resources
More Problem-Solving Practice
Appendix F
CHAPTER 23
CHANGE OF PHASE
467
• Computer Test Bank
• Chapter and Unit Tests
467