# How change of state take place ?

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Energy Simply stated energy is the ability to do work or cause change. The energy of motion is called kinetic energy. Particles within matter are in constant motion. The amount of motion of these particles depends on the kinetic energy they possess.
1. Standards—8.3.9: Demonstrate, using drawings and models, the movement of atoms in a solid, liquid, and
gaseous state. Explain that atoms and molecules are perpetually in motion. 8.3.10: Explain that increased tem-
perature means that atoms have a greater average energy of motion and that most gases expand when
heated.
Also covers: 8.2.8 (Detailed standards begin on page IN8.)
Changes of State
Thermal Energy and Heat
Shards of ice fly from the sculptor’s chisel. As the crowd
looks on, a swan slowly emerges from a massive block of ice. As
the day wears on, however, drops of water begin to fall from the ■ Define and compare thermal
sculpture. Drip by drip, the sculpture is transformed into a pud- energy and temperature.
dle of liquid water. What makes matter change from one state to ■ Relate changes in thermal
another? To answer this question, you need to think about the energy to changes of state.
■ Explore energy and temperature
particles that make up matter.
changes on a graph.
Energy Simply stated, energy is the ability to do work or cause
change. The energy of motion is called kinetic energy. Particles Matter changes state as it heats up
within matter are in constant motion. The amount of motion of or cools down.
these particles depends on the kinetic energy they possess. Particles
with more kinetic energy move faster and farther apart. Particles Review Vocabulary
with less energy move more slowly and stay closer together. energy: the ability to do work or
The total kinetic energy of all the particles in a sample of cause change
matter is called thermal energy. Thermal energy, an extensive New Vocabulary
property, depends on the number of particles in a substance as
well as the amount of energy each particle has. If either the
•• thermal energy
temperature
number of particles or the amount of energy in each particle •• heat
melting
changes, the thermal energy of the sample changes. With identi-
cally sized samples, the warmer substance has the greater ther- •• freezing
vaporization
mal energy. In Figure 7, the particles of hot water from the hot
spring have more thermal energy than the particles of snow on
• condensation
the surrounding ground.
Figure 7 These girls are enjoy-
ing the water from the hot spring.
Infer why the girls appear to be
comfortable in the hot spring while
there is snow on the ground.
SECTION 2 Changes of State 99
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2. Figure 8 The particles in hot tea move faster than
those in iced tea. The temperature of hot tea is higher
than the temperature of iced tea.
Identify which tea has the higher kinetic energy.
M642-06C-MS
Standard Check
Temperature Not all of the particles in a sample of matter have
8.3.10: Explain that increased the same amount of energy. Some have more energy than others.
temperature means that atoms The average kinetic energy of the individual particles is the
have a greater average energy of temperature, an intensive property, of the substance. You can find
motion . . . an average by adding up a group of numbers and dividing the
Which tea has a lower average total by the number of items in the group. For example, the aver-
kinetic energy—iced tea or hot tea? age of the numbers 2, 4, 8, and 10 is (2 ⫹ 4 ⫹ 8 ⫹ 10) ⫼ 4 ⫽ 6.
Temperature is different from thermal energy because thermal
energy is a total and temperature is an average.
You know that the iced tea is colder than the hot tea, as
shown in Figure 8. Stated differently, the temperature of iced tea
is lower than the temperature of hot tea. You also could say that
the average kinetic energy of the particles in the iced tea is less
Types of Energy Thermal than the average kinetic energy of the particles in the hot tea.
energy is one of several dif-
ferent forms of energy. Heat When a warm object is brought near a cooler object, ther-
Other forms include the
chemical energy in chemical mal energy will be transferred from the warmer object to the
compounds, the electrical cooler one. The movement of thermal energy from a substance
energy used in appliances, at a higher temperature to one at a lower temperature is called
the electromagnetic energy heat. When a substance is heated, it gains thermal energy.
of light, and the nuclear Therefore, its particles move faster and its temperature rises.
energy stored in the When a substance is cooled, it loses thermal energy, which causes
nucleus of an atom. Make a
list of examples of energy
its particles to move more slowly and its temperature to drop.
that you are familiar with.
How is heat related to temperature?
100 CHAPTER 4 States of Matter
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3. Specific Heat
As you study more science, you will
discover that water has many unique
properties. One of those is the amount
of heat required to increase the temper-
ature of water as compared to most
other substances. The specific heat of a
substance is the amount of heat required
to raise the temperature of 1 g of a sub-
stance 1°C.
Substances that have a low specific
heat, such as most metals and the sand in
Figure 9, heat up and cool down quickly
because they require only small amounts
Figure 9 The specific heat of
of heat to cause their temperatures to rise. A substance with a high
water is greater than that of sand.
specific heat, such as the water in Figure 9, heats up and cools
The energy provided by the Sun
down slowly because a much larger quantity of heat is required to
raises the temperature of the sand
cause its temperature to rise or fall by the same amount.
much faster than the water.
Changes Between the
Solid and Liquid States Figure 10 Rather than melting
Matter can change from one state to another when thermal into a liquid, glass gradually soft-
energy is absorbed or released. This change is known as change of ens. Glass blowers use this charac-
state. The graph in Figure 11 shows the changes in temperature as teristic to shape glass into
thermal energy is gradually added to a container of ice. beautiful vases while it is hot.
Melting As the ice in Figure 11 is
heated, it absorbs thermal energy
and its temperature rises. At some
point, the temperature stops rising
and the ice begins to change into
liquid water. The change from the
solid state to the liquid state is called
melting. The temperature at which a
substance changes from a solid to a
liquid is called the melting point.
The melting point of water is 0°C.
Amorphous solids, such as rub-
ber and glass, don’t melt in the same
way as crystalline solids. Because
they don’t have crystal structures to
break down, these solids get softer
and softer as they are heated, as you
can see in Figure 10.
SECTION 2 Changes of State 101
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4. VISUALIZING STATES OF MATTER
Figure 11
L
ike most substances, water can exist in three
distinct states—solid, liquid, or gas. At certain
temperatures, water changes from one state
VAPORIZATION When water reaches its
to another. This diagram shows what changes occur
boiling point of 100ºC, water molecules are
as water is heated or cooled.
moving so fast that they break free of the
attractions that hold them together in the
MELTING When ice melts, its temperature liquid state. The result is vaporization—
remains constant until all the ice turns to the liquid be-comes a gas. The tempera-
water. Continued heating of liquid water ture of boiling water remains constant
causes the molecules to vibrate even until all of the liquid turns to steam.
FREEZING When liquid water freezes, it
releases thermal energy and turns into Gas
the solid state, ice.
Vaporization
100°C
Condensation
Temperature
Liquid
CONDENSATION When steam is cooled,
Melting
0°C it releases thermal energy and turns
Freezing into its liquid state. This process is
called condensation.
Solid Thermal energy
Solid state: ice Liquid state: water Gaseous state: steam
102 CHAPTER 4 States of Matter
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5. Freezing The process of melting a crystalline solid can be
reversed if the liquid is cooled. The change from the liquid state
to the solid state is called freezing. As the liquid cools, it loses
thermal energy. As a result, its particles slow down and come
Topic: Freezing Point
closer together. Attractive forces begin to trap particles, and the Study
crystals of a solid begin to form. As you can see in Figure 11, Visit in8.msscience.com for Web
The temperature at which a substance changes from the liq- Activity Make a list of several
uid state to the solid state is called the freezing point. The freez- substances and the temperatures
ing point of the liquid state of a substance is the same at which they freeze. Find out how
temperature as the melting point of the solid state. For example, the freezing point affects how the
solid water melts at 0°C and liquid water freezes at 0°C. substance is used.
During freezing, the temperature of a substance remains
constant while the particles in the liquid form a crystalline solid.
Because particles in a liquid have more energy than particles in
a solid, energy is released during freezing. This energy is released
into the surroundings. After all of the liquid has become a solid,
the temperature begins to decrease again.
How can ice save oranges?
uring the spring, Florida citrus
D farmers carefully watch the fruit
when temperatures drop close to freez-
ing. When the temperatures fall below
0°C, the liquid in the cells of oranges
can freeze and expand. This causes the
cells to break, making the oranges
mushy and the crop useless for sale. To
prevent this, farmers spray the oranges
with water just before the temperature
reaches 0°C. How does spraying
oranges with water protect them?
Identifying the Problem
Using the diagram in Figure 11, con-
sider what is happening to the water at
0°C. Two things occur. What are they?
Solving the Problem
1. What change of state and what energy
changes occur when water freezes?
2. How does the formation of ice on the
orange help the orange?
SECTION 2 Changes of State 103
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6. Changes Between the
Liquid and Gas States
After an early morning rain, you and your friends enjoy
Observing stomping through the puddles left behind. But later that after-
Vaporization noon when you head out to run through the puddles once more,
Procedure the puddles are gone. The liquid water in the puddles changed
1. Use a dropper to place one into a gas. Matter changes between the liquid and gas states
drop of rubbing alcohol through vaporization and condensation.
on the back of your hand.
2. Describe how your hand Vaporization As liquid water is heated, its temperature rises
feels during the next until it reaches 100°C. At this point, liquid water changes into
2 min.
3. Wash your hands. water vapor. The change from a liquid to a gas is known as
vaporization (vay puh ruh ZAY shun). You can see in Figure 11
that the temperature of the substance does not change during
1. What changes in the
appearance of the rubbing vaporization. However, the substance absorbs thermal energy.
alcohol did you notice? The additional energy causes the particles to move faster until
2. What sensation did you they have enough energy to escape the liquid as gas particles.
feel during the 2 min? How Two forms of vaporization exist. Vaporization that takes
can you explain this place below the surface of a liquid is called boiling. When a liq-
sensation? uid boils, bubbles form within the liquid and rise to the surface,
3. Infer how sweating cools
the body. as shown in Figure 12. The temperature at which a liquid boils
is called the boiling point. The boiling point of water is 100°C.
Vaporization that takes place at the surface of a liquid is
called evaporation. Evaporation, which occurs at temperatures
below the boiling point, explains how puddles dry up. Imagine
that you could watch individual water molecules in a puddle.
You would notice that the molecules move at different speeds.
Although the temperature of the water is constant, remember
that temperature is a measure of the average kinetic energy of
the molecules. Some of the fastest-moving molecules overcome
the attractive forces of other molecules and escape from the sur-
face of the water.
Figure 12 During boiling, liquid changes
to gas, forming bubbles in the liquid that
rise to the surface.
Define the word that describes a liquid
changing to the gas.
104 CHAPTER 4 States of Matter
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7. Figure 13 The drops of water
on these glasses and pitcher of
water vapor in the air lost enough
This process is called condensation.
Location of Molecules It takes more than speed for water
molecules to escape the liquid state. During evaporation, these
faster molecules also must be near the surface, heading in the
right direction, and they must avoid hitting other water mole-
cules as they leave. With the faster particles evaporating from the
surface of a liquid, the particles that remain are the slower,
cooler ones. Evaporation cools the liquid and anything near the
liquid. You experience this cooling effect when perspiration
Condensation Pour a nice, cold glass of lemonade and place Topic: Condensation
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it on the table for a half hour on a warm day. When you come
back to take a drink, the outside of the glass will be covered by condensation is involved in
drops of water, as shown in Figure 13. What happened? As a gas weather.
cools, its particles slow down. When particles move slowly
Activity Find out how conden-
enough for their attractions to bring them together, droplets of sation is affected by the tempera-
liquid form. This process, which is the opposite of vaporization, ture as well as the amount of
is called condensation. As a gas condenses to a liquid, it releases water in the air.
the thermal energy it absorbed to become a gas. During this
process, the temperature of the substance does not change. The
decrease in energy changes the arrangement of particles. After
the change of state is complete, the temperature continues to
drop, as you saw in Figure 11.
What energy change occurs during
condensation?
Condensation formed the droplets of water on the outside of
your glass of lemonade. In the same way, water vapor in the
atmosphere condenses to form the liquid water droplets in
clouds. When the droplets become large enough, they can fall to
the ground as rain.
SECTION 2 Changes of State 105
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8. Changes Between the
Solid and Gas States
Some substances can change from the solid state
to the gas state without ever becoming a liquid.
During this process, known as sublimation, the sur-
face particles of the solid gain enough energy to
become a gas. One example of a substance that
undergoes sublimation is dry ice. Dry ice is the solid
form of carbon dioxide. It often is used to keep
materials cold and dry. At room temperature and
pressure, carbon dioxide does not exist as a liquid.
Figure 14 The solid carbon Therefore, as dry ice absorbs thermal energy from the objects
dioxide (dry ice) at the bottom of around it, it changes directly into a gas. When dry ice becomes
this beaker of water is changing a gas, it absorbs thermal energy from water vapor in the air. As
directly into gaseous carbon dioxide. a result, the water vapor cools and condenses into liquid water
This process is called sublimation. droplets, forming the fog you see in Figure 14.
Summary Self Check
Thermal Energy and Heat 1. Describe how thermal energy and temperature are sim-
• Thermal energy depends on the amount of
the substance and the kinetic energy of parti-
ilar. How are they different?
2. Explain how a change in thermal energy causes matter
cles in the substance. to change from one state to another. Give two examples.
• Heat is the movement of thermal energy from
a warmer substance to a cooler one.
3. List the three changes of state during which energy is
absorbed.
4. Describe the two types of vaporization.
Specific Heat
5. Think Critically How can the temperature of a sub-
• Specific heat is a measure of the amount of
energy required to raise 1 g of a substance 1°C.
stance remain the same even if the substance is absorb-
ing thermal energy?
Changes Between Solid and Liquid States 6. Write a paragraph in your Science Journal that explains
• During all changes of state, the temperature
of a substance stays the same.
why you can step out of the shower into a warm bath-
room and begin to shiver.
Changes Between Liquid and Gas States
• Vaporization is the change from the liquid
state to a gaseous state.
7. Make and Use Graphs Use the data you collected in
the Launch Lab to plot a temperature-time graph.
• Condensation is the change from the gaseous
state to the liquid state.
Describe your graph. At what temperature does the
graph level off? What was the liquid doing during this
time period?
Changes Between Solid and Gas States
8. Use Numbers If sample A requires 10 calories to raise
• Sublimation is the process of a substance
going from the solid state to the gas state
the temperature of a 1-g sample 1°C, how many calo-
ries does it take to raise a 5-g sample 10°C?
without ever being in the liquid state.
106 CHAPTER 4 States of Matter in8.msscience.com/self_check_quiz
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