of phase change of phase - youngbull science...

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How Do Clouds Form?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.

Analyze and Conclude1. Observing What did you observe when you

pulled the glove out of the jar?

2. Predicting What do you suppose would hap-pen if you were to pull the glove out of the jar more slowly?

3. Making Generalizations What factors are nec-essary for cloud formation?

discover!

CHANGE OF PHASE

The 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 tempera-ture and the pressure that is exerted upon it. Changes of phase involve a transfer of energy.

3Changes of phase involve a transfer of energy.

THE BIG

IDEA ......

..

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CHANGE OF PHASE

Objectives• Describe how evaporation

affects a liquid’s temperature. (23.1)

• Describe how condensation affects temperature. (23.2)

• 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)

The material in this chapter is not a prerequisite for the chapters that follow.

discover!

MATERIALS gallon jar, water, matches, rubber glove

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

See Expected Outcome.

It would take longer for water drops to form.

Water vapor and smoke or dust particles

1.

2.

3.

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CHAPTER 23 CHANGE OF PHASE 451

23.1 EvaporationWater 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, mol ecules 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 evapora-tion when the cloth covering on the sides is kept wet. As the faster-moving water molecules leave the cloth, the temperature of the cloth decreases. The cool cloth in turn cools the metal canteen by conduc-tion, which in turn cools the water inside.

When the human body overheats, sweat glands produce perspi-ration. 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.

CONCEPTCHECK ...... How does evaporation affect a liquid’s temperature?

� FIGURE 23.2Pigs lack sweat glands. They wallow in mud to cool themselves.

FIGURE 23.1 �The cloth covering on the sides of the canteen pro-motes cooling when it is wet.

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23.1 Evaporation

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.

� 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.

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.

Evaporation is a process that cools the

liquid left behind.

T e a c h i n g R e s o u r c e s

• Conceptual Physics Alive! DVDs Heat: Change of State

CONCEPTCHECK ...

...CONCEPTCHECK ...

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23.2 CondensationThe 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 sub-stance 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 mea-sure 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 low-temperature 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 mov-ing slowly as shown in Figure 23.4. At higher speeds, they can bounce apart and remain in the gaseous phase. The faster the water mol-ecules move, the less able they are to condense to form droplets.

FIGURE 23.3 �Heat is given up by steam when it condenses inside the radiator.

A camel’s best source of water is its over-sized nose, with an inside structure that recaptures most of the moisture in water-saturated air coming from its lungs. So it withdraws water from its own exhaled breath.


452

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!

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.


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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 mol-ecules to cause condensation—provided there is enough water vapor present. What ever the temperature, it is the slower molecules that are more likely to stick.

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.

CONCEPTCHECK ...... How does condensation affect temperature?

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

think!

� FIGURE 23.4a. At high speeds, molecules of water vapor bounce apart and remain a gas. b. At lower speeds, mol-ecules of water vapor are more likely to stick together and form a liquid.

Cloud formation can be stimulated by “seeding“ the air with appropriate particles or ions.

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� 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.

Condensation warms the area where the

liquid forms.


• Transparency 44

CONCEPTCHECK ...

...CONCEPTCHECK ...

...

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.

DemonstrationDemonstration


454

23.3 Evaporation and Condensation Rates

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 envi-ronment, 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 tempera-ture. 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 appar-ent evaporation takes place, you might conclude that nothing is hap-pening. 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.

CONCEPTCHECK ...

... How can evaporation and condensation takeplace at the same time?

23.4 BoilingEvaporation 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 liq-uid, 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 pres-sure. 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.

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.

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

23.3 Evaporation 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.


• Reading and Study Workbook

• Laboratory Manual 66

• PresentationEXPRESS

• Interactive Textbook

• Next-Time Question 23-1

23.4 Boiling

Key Term boiling

Students are often confused by the idea that boiling is a cooling process. Proceed slowly!

Common Misconception Boiling is a warming process.

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.

� Teaching Tip Explain how a geyser is like a pressure cooker. It erupts when a certain pressure is reached.

CONCEPTCHECK ...

...CONCEPTCHECK ...

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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, pres-sure 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 boil-ing can occur. The increased temperature of the water cooks the food faster.

Low Pressure It is important to note that it is the high tempera-ture 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.

CONCEPTCHECK ...

... What is the effect of pressure on the boiling temperature of a liquid?

Can You See Steam?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?

discover!

FIGURE 23.7 �Heating and boiling are two distinct processes. Heating warms the water, and boil-ing cools it.

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

think!

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� 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!

Ask I n 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.


• Laboratory Manual 61

• Probeware Lab Manual 11

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.)


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.)


CONCEPTCHECK ...

...CONCEPTCHECK ...

...

discover!

MATERIALS teakettle, water, heat source, candle, matches

EXPECTED OUTCOME The cloud of condensed steam disappears.

THINK The heat from the flame causes the condensed water droplets to evaporate.


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23.5 FreezingWhen 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 exam-ple of this process. When energy is extracted from water at a temper-ature 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 six-sided structure of an ice crystal.

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 mol ecules or ions among liquid water mol-ecules that remain increases. Con nections become more difficult.

In general, dissolving anything in a liquid lowers the liquid’s freezing temperature. Antifreeze in an automobile engine is a practi-cal application of this process.

CONCEPTCHECK ...

... What effect does dissolving anything in a liquid have on the liquid’s freezing temperature?

23.6 Boiling and Freezing at the Same Time

Suppose that a dish of water at room temperature is placed in a vac-uum 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 tempera-ture. As the pressure is further reduced, more and more of the faster remaining slow-moving molecules boil away.

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.


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23.5 Freezing

Key Term freezing

� 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.

23.6 Boiling and Freezing at the Same Time

CONCEPTCHECK ...

...CONCEPTCHECK ...

...

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.



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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.

CONCEPTCHECK ...

... What can cause boiling and freezing to take place at the same time?

23.7 RegelationThe open-structured crystals of ice can be crushed by the applica-tion 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 melt-ing 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 phenom-enon 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.

CONCEPTCHECK ...

... Why do so few substances undergo regelation?

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 insu-lated from the base by a polystyrene cup.

FIGURE 23.10 �Regelation allows the wire to cut through the ice, but leaves the ice in a single solid piece.

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Lowering the pressure can cause

boiling and freezing to take place at the same time.





23.7 Regelation

Key Term regelation

Common Misconception Ice melts only when heat is added.

FACT Ice can also melt under increased pressure.

Regelation can occur only in substances

that expand when they freeze.





CONCEPTCHECK ...

...CONCEPTCHECK ...

...

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.


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.


CONCEPTCHECK ...

...CONCEPTCHECK ...

...


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FIGURE 23.11 �The change in the internal energy of a substance causes the change of phase.

23.8 Energy and Changes of PhaseIf 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.

Examples of Phase Changes The general behavior of many substances can be illustrated with a description of the changes of phase of H

2O. 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.

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.

PHASE

PHASE

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.

Physics in the Kitchen


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23.8 Energy and Changes of Phase

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.

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.


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.



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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 addi-tional 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 tempera-ture 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 poten-tial 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.

VAPORIZING

HEAT (CALORIES)

MELTING

PHASE CHANGES OF WATER

54080 100

ICE

STEA

M

100

0

-50

TEM

PERA

TURE

(% C

)

WATER

FIGURE 23.12 �The graph shows the energy involved in the heating and the change of phase of 1 gram of H2O.

How much energy is released when a gram of steam at 100°C con-denses to water at 100°C?Answer: 23.8.1

think!

Water’s heat of vapor-ization 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.

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� 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).

� 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 P oint 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?


460

FIGURE 23.14 �The refrigeration cycle in a common refrigerator keeps the inside cool.

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 cool-ing 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 open-ing 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, appropri-ately 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 vapor-ization and condensation are reversed, the air conditioner becomes a heater. A device that moves heat is called a heat pump.

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:Visit:Web Code: –

Links on Phases of Matter www.SciLinks.org csn 2308


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� 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.

� 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.


11/13/07 11:02:39 AM


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 fin-ger—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 fire-wood. (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, rela-tively little heat is conducted to the feet, just as little heat is con-ducted 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.

CONCEPTCHECK ...

... How is energy related to phase changes?

Fire Fighting 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.

Physics on the Job

When H2O in the vapor phase condenses, is the surrounding air warmed or cooled?Answer: 23.8.2

think!

A refrigerator is a “heat pump.” It transfers heat out of a cold environ-ment and into a warm environment. When the process is reversed, the heat pump is an air con-ditioner. In both cases, external energy oper-ates the device.

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� 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.

The change in the internal energy of a

substance causes the change of phase.


• 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

CONCEPTCHECK ...

...CONCEPTCHECK ...

...


For:Visit:Web Code: –

Self-Assessment PHSchool.com csa 2300

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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 condensa-tion.

• 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 ••••••

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 satura-tion 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 cool-ing 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.

think! Answers

3 REVIEW

phases (p. 450)

evaporation (p. 451)

condensation(p. 452)

saturated (p. 452)

relative humidity(p. 452)

boiling (p. 454)

freezing (p. 456)

regelation (p. 457)

heat pump (p. 460)


462

REVIEW


• TeacherEXPRESS

• Virtual Physics Lab 23

• Conceptual Physics Alive! DVDs Heat: Change of State


11/13/07 11:02:42 AM

ASSESS


Check Concepts ••••••

Section 23.1 1. Do all the molecules or atoms in a liquid

have about the same speed, or much differ-ent speeds?

2. What is evaporation, and why is it also a cooling process?

3. Why does a hot dog pant?

Section 23.2 4. What is condensation, and why is it also a

warming process?

5. Why is being burned by steam more damag-ing than being burned by boiling water of the same temperature?

6. Which usually contains more water vapor—warm air or cool air?

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?

9. Why do you feel less chilly if you dry your-self inside the shower stall after taking a shower?

Section 23.4 10. What is the difference between evaporation

and boiling?

11. Why does the temperature at which a liquid boils depend on atmospheric pressure?

12. Why is a pressure cooker even more useful when cooking food in the mountains than when cooking at sea level?

Section 23.5 13. Why does antifreeze or any soluble sub-

stance put in water lower its freezing tem-perature?

Section 23.6 14. How can water be made to both boil and

freeze at the same time?

3

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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.

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.

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


ASSESSASSESS (continued)

464

Section 23.7 15. What is regelation, and what does it have to

do with the open-structured crystals in ice?

Section 23.8 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?

17. When a vapor turns to a liquid, does it give off energy or does it absorb energy?

18. What is the effect of rapid evaporation on the temperature of water?

19. In a refrigerator, does the food cool when a vapor turns to a liquid, or vice versa?

20. Why is it important that a finger be wet before it is touched to a hot clothes iron?

Plug and Chug ••••••

Use the following information to help you answer Questions 21–26.

Quantity of heat energy required for change of phase � (mass) � (heat of fusion or heat of vaporization), or in equation form, Q mL.

Quantity of heat energy responsible for a temperature change � (mass) � (specific heat) � (change in temperature), or in equation form,Q mc T.

For water, heat of fusion � 80 cal/g; heat of vaporization � 540 cal/g.

21. Calculate the energy absorbed by 20 grams of water that warms from 30°C to 90°C.

22. Calculate the energy needed to melt 50 grams of 0°C ice.

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.

3


464

15. Melting under pressure; the pressure crushes open ice crystals.

16. a. 1 b. 80 c. 540

17. Gives off energy

18. It causes a reduction of temperature.

19. When liquid turns to vapor

20. The energy that could cause a burn will be reduced by the energy that causes a phase change of the water.

Plug and Chug 21. Q 5 mcDT 5 (20 g) 3

(1 cal/g°C)(90°C 2 30°C) 5 1200 cal

22. Q 5 mL 5 (50 g)(80 cal/g) 5 4000 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

24. Q 5 mL 5 (20 g)(540 cal/g) 5 10,800 cal

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.


11/13/07 11:02:44 AM

CHAPTER 23 CHANGE OF PHASE 465CHAPTER 23 CHANGE OF PHASE 465

Think and Explain •••••

27. a. Evaporation is a cooling process. What cools and what warms during evapora-tion?

b. Condensation is a warming process. What warms and what cools during condensa-tion?

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 mol-ecules in a particular liquid had the same speed, and some were able to evaporate, the remaining liquid would not undergo cool-ing. Do you agree or disagree, and what is your explanation?

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?

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 sometimesit 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 be-neath a park bench?

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 can-not occur on very cold days. Explain.

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28. Wind increases evaporation, which cools you.

29. Agree; remaining liquid would have the same energy per molecule before and after evaporation.

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.


ASSESSASSESS (continued)

466

3 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 ca-pacity than the wood. Which will melt more ice before cooling to 0°C?

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?

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 boil-ing 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?)

46. How can water be brought to a boil without heating it?

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?

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, surround-ing 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.

49. Room-temperature water will boil spon-taneously in a vacuum—on the moon, for example. Could you cook an egg in this boiling water? Defend your answer.

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 consump-tion 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.

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?


466

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.

49. No; low-temperature boiling won’t cook! Cooking is caused by the high temperature, not by the bubbling water.

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.


11/13/07 11:02:50 AM

CHAPTER 23 CHANGE OF PHASE 467CHAPTER 23 CHANGE OF PHASE 467

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?

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 mi-croscopic bubbles can collect long enough to become big bubbles. Nucleation sites are also important for phase changes of con-densation and solidification. Snowflakes and raindrops typically form around dust particles, for example.

More Problem-Solving PracticeAppendix F

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 con-densed 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?

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?

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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

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

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.


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• Chapter and Unit Tests


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