### States of Matter

Chapter 2 Energy and Matter
2.6
Changes of State
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
1
Melting and Freezing
A substance
• is melting while it changes from a solid to a liquid.
• is freezing while it changes from a liquid to a solid.
• such as water has a freezing (melting) point of 0°C.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
2
Calculations Using Heat of Fusion
The heat of fusion
• is the amount of heat released when 1 gram of
liquid freezes (at its freezing point).
• is the amount of heat needed to melt 1 gram of a
solid (at its melting point).
• for water (at 0°C) is
80. cal
1 g water
3
Calculation Using Heat of Fusion
The heat needed to freeze (or melt) a specific mass
of water (or ice) is calculated using the heat of
fusion.
Heat = g water x 80. cal
1 g water
Example: How much heat in cal is needed to melt
15.0 g of water?
15.0 g water x
80. cal
1 g water
= 1200 cal
4
Learning Check
A. How many calories are needed to melt 5.00 g of
ice at 0°C?
1) 80. cal
2) 400 cal
3) 0 cal
B. How many calories are released when 25.0 g of
water at 0°C freezes?
1) 80. cal
2) 0 cal
3) 2000 cal
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Solution
A. How many calories are needed to melt 5.00 g of ice
at 0°C?
2) 400 cal
5.00 g x 80. cal
1g
B. How many calories are released when 25.0 g of
water at 0°C freezes?
3) 2000 cal
25 g x 80. cal
1g
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Sublimation
Sublimation
• occurs when particles
change directly from solid
to a gas.
• is typical of dry ice, which
sublimes at -78C.
• takes place in frost-free
refrigerators.
• is used to prepare freezedried foods for long-term
storage.
7
Evaporation and Condensation
Water
• evaporates when
molecules on the
surface gain sufficient
energy to form a gas.
• condenses when gas
molecules lose energy
and form a liquid.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
8
Boiling
At boiling,
• all the water molecules
acquire enough energy
to form a gas.
• bubbles appear
throughout the liquid.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
9
Heat of Vaporization
The heat of vaporization is the amount of heat
• absorbed to vaporize 1 g of a liquid to gas at the
boiling point.
• released when 1 g of a gas condenses to liquid at
the boiling point.
Boiling Point of Water = 100°C
Heat of Vaporization (water)
=
540 cal
1 g water
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Learning Check
How many kilocalories (kcal) are released when 50.0 g
of steam from a volcano condenses at 100°C?
1) 27 kcal
2) 540 kcal
3) 2700 kcal
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Solution
How many kilocalories (kcal) are released when 50.0 g
of steam in a volcano condenses at 100°C?
1) 27 kcal
50.0 g steam x
540 cal
1 g steam
x 1 kcal = 27 kcal
1000 cal
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Summary of Changes of State
13
Heating Curve
A heating curve
• illustrates the changes
of state as a solid is
heated.
• uses sloped lines to
show an increase in
temperature.
• uses plateaus (flat
lines) to indicate a
change of state.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
14
Learning Check
A. A flat line on a heating curve represents
1) a temperature change.
2) a constant temperature.
3) a change of state.
B. A sloped line on a heating curve represents
1) a temperature change.
2) a constant temperature.
3) a change of state.
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Solution
A. A flat line on a heating curve represents
2) a constant temperature.
3) a change of state.
B. A sloped line on a heating curve represents
1) a temperature change.
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Cooling Curve
A cooling curve
• illustrates the changes
of state as a gas is
cooled.
• uses sloped lines to
indicate a decrease in
temperature.
• uses plateaus (flat
lines) to indicate a
change of state.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
17
Learning Check
Use the cooling curve for water to answer each.
A. Water condenses at a temperature of
1) 0°C.
2) 50°C.
3) 100°C.
B. At a temperature of 0°C, liquid water
1) freezes.
2) melts.
3) changes to a gas.
C. At 40°C, water is a
1) solid.
2) liquid.
3) gas.
D. When water freezes, heat is
1) removed.
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Solution
Use the cooling curve for water to answer each.
A. Water condenses at a temperature of
3) 100°C.
B. At a temperature of 0°C, liquid water
1) freezes.
C. At 40 °C, water is a
2) liquid.
D. When water freezes, heat is
1) removed.
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Heat Calculations
To reduce a fever, an infant is packed in 250 g of ice. If
the ice (at 0°C) melts and warms to body temperature
(37.0°C), how many calories are removed from the body?
A. 3.0 x 105 cal
B. 2.0 x 104 cal
C. 1.0 x 103 cal
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Heat Calculations
To reduce a fever, an infant is packed in 250 g of ice. If
the ice (at 0°C) melts and warms to body temperature
(37.0°C), how many calories are removed from the body?
Step 1: Diagram the changes
T = 37.0°C - 0°C = 37.0°C
37°C
temperature increase
0°C
solid
liquid
melting
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Heat Calculations
Step 2: How is the ice changing
Ice is melting from 0°C to 37.0°C, use the heat of
fusion.
Heat = g of ice x 80.0 cal
1 g ice
Step 3: Plug value into equation
Heat = 250 g of ice x 80.0 cal
1 g ice
= 2.0x104 cal (B) or 2.0x101 kcal
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