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Chapter 2 Energy and Matter

2.6 Changes of State

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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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

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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

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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 = 1200 cal

1 g water

Calculation Using Heat of Fusion

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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

Learning Check

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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 1 g

B. How many calories are released when 25.0 g of water at 0°C freezes?

3) 2000 cal 25 g x 80. cal 1 g

Solution

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Sublimation

Sublimation• occurs when particles

change directly from solid to a gas.

• is typical of dry ice, which sublimes at -78C.

• takes place in frost-free refrigerators.

• is used to prepare freeze-dried foods for long-term storage.

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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.

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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

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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 gof steam in a volcano condenses at 100°C?

1) 27 kcal

50.0 g steam x 540 cal x 1 kcal = 27 kcal 1 g steam 1000 cal

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Summary of Changes of State

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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.

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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.

Learning Check

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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.

Solution

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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

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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.2) added.

Learning Check

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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.

Solution

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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

Heat Calculations

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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

37°C T = 37.0°C - 0°C = 37.0°C

temperature increase

0°C solid liquid

melting

Heat Calculations

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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 = 2.0x104 cal (B) or 2.0x101 kcal 1 g ice

Heat Calculations