Thermodynamics WS 4
A Heating Curve
Use the graph on p. 7 of your Thermodynamics notes to help you answer these questions.
1. The graph shows what happens when ice at -40oC, is gradually heated to more than 120oC. What is happening in the
region between -40oC and 0oC?
The ice is warming up. 20 kcal of heat is being added to the ice. The ice remains a solid even though its
temperature rises 40oC.
2. At 0oC, the temperature does not change even though 80 kcal of heat is added. Why does the temperature not
change?
All the energy is going into changing the state of matter from solid to liquid. The ice is melting, and liquid and solid
water coexist. All the heat that is absorbed is converting the solid into a liquid, so the temperature does not change
as long as both states coexist.
3. What is happening in the region between 0oC and 100oC?
The liquid water is heating up. By now, all the solid water has been converted to liquid water, the 100 kcal of heat
that is being added increases the temperature of the water.
4. When the water reaches 100oC, the temperature does not change even though 540 kcal of heat is added. Why does
the temperature not change?
When the temperature reaches 100oC, the liquid water boils, and liquid and gaseous water coexist. All the heat that
is being absorbed is converting the liquid to the gas, so the temperature does not change as long as both states
coexist.
5. Compare the amount of heat needed to convert liquid water to water vapor with the amount needed to convert ice to
liquid water. Explain the difference.
More heat is needed to convert liquid water to water vapor than is needed to convert ice to liquid water. It takes
more energy to separate the particle in a liquid far enough to form a gas (especially due to the H-bonds) than it takes
for water molecules in a solid to slide past each other to form a liquid.
6. What is happening in the region above 100oC?
All the water is in the gaseous state, and the temperature of the gas rises as more heat is added.
7. If you continue to add heat, what will happen to the water vapor?
The temperature of the water vapor will continue to rise.
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Thermodynamics WS 4
A Heating Curve
The heating curve shown in the figure is a plot of temperature vs. time. It represents the heating of what is
initially ice at - 10°C at a constant rate of heat transfer.
Answer the following questions.
1.
a. What phase or phases are present during
1.
a._______solid ice_________________
segment A?
b. What is happening to the energy being
b.______warming the ice____________
absorbed from the heat source? (Answer in terms
of potential and/or kinetic energy.)
c._______none____________________
c. What phase change, if any, is taking place?
2.
a. What phase or phases are present during
2.
a.________ice and liquid water________
segment B?
b. What is happening to the energy being
b.________changing state____________
absorbed?
c._________solid  liquid___________
c. What phase change, if any, is taking place?
d. What is the significance of the temperature
d.______normal melting point, Hfus____
0°C?
3.
a. What phase or phases are present during
3.
a.__________liquid__________________
segment C?
b. What is happening to the energy being
b._________heating the water_________
absorbed?
c. What phase change, if any, is taking place?
c.__________none___________________
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Thermodynamics WS 4
4.
a. What phase or phases are present during
4.
a._______liquid and vapor__________
segment D?
b. What is happening to the energy being
b.______changing state____________
absorbed?
c._______liquid  gas_____________
c. What phase change, if any, is taking place?
d. What is the significance of the temperature
d._____normal boiling point, Hvap_____
100°C?
5. What would you expect to happen if the
5.
heating were continued?
After all the liquid is converted into gas, the gas increases
in temperature
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