1. Phase Changes Contributions by:
John L. Falconer & Will Medlin Department of Chemical and Biological Engineering University of Colorado Boulder, CO
Supported by the National Science Foundation

2. B A C Which lines could be constant volume processes? Pressure A B C
Solid
Liquid
Gas
Temperature
Pressure C B A A B C
All of these.
None of these.
ANSWER: A

3. B A C Which lines could be constant volume processes? Pressure A B C
Solid
Liquid
Gas
Temperature
Pressure C B A A B C
All of these.
None of these.
ANSWER: D. All of these.

4. A B C Which lines could be constant volume processes? Pressure A B C
Solid
Liquid
Gas
Temperature
Pressure C B A A B C
All of these.
None of these.
ANSWER: B.

5. Which line corresponds to an isotherm?
Volume B
Pressure E A C D
ANSWER: C. For ideal gas, as pressure increases, volume decreases at constant temperature so that P*V is constant, and this is represened by curve C

6. A solid phase change takes place for iron at 75°C
A solid phase change takes place for iron at 75°C. The phase transformation from the a phase to the b phase is endothermic. The entropy of the iron _______.
increases
decreases
remains constant
depends on the history of treatment of the iron
ANSWER: A. increases. The enthalpy change divided by T is the entropy change.

7. Liquid ethanol becomes a solid when the pressure increases isothermally. The entropy of the ethanol ________________.
increases
decreases
remains the same
ANSWER: B. decreases. Because the temperature of the system is constant, delta S = Q/T. Heat is being removed from the system as the pressure increases to remain isothermal, so delta S is a negative value. This means the final entropy is less than the initial entropy. Liquid has higher entropy than a solid.

8. Solid water becomes a liquid when the pressure increases isothermally
Solid water becomes a liquid when the pressure increases isothermally. The entropy of the water _______.
increases
decreases
remains the same
ANSWER: A. increases. Because the temperature of the system is constant, delta S = Q/T. Heat is being removed from the system as the pressure increases to remain isothermal, so delta S is a negative value. This means the final entropy is greater than the initial entropy. Liquid has higher entropy than a solid.

9. What is the condition of water at 300°C and 20 MPa pressure?
Log10 [ V(cm3/mol)] 1
1.5 2
2.5 3
3.5 4
4.5 5 10 20 30 40 50
Pressure (MPa)
Temperature (°C)
100
Critical point
supercritical
200
300
400
Sat’d vapor
500
Sat’d liquid
ANSWER: B. Liquid.
Vapor
Vapor-liquid
Liquid
Supercritical

10. What is the approximate temperature in a 100 cm3
vessel containing 1 mole of water at 9 MPa?
Log10 [ V(cm3/mol)] 1
1.5 2
2.5 3
3.5 4
4.5 5 10 20 30 40 50
Pressure (MPa)
Temperature (°C)
100
Critical point
supercritical
200
300
400
Sat’d vapor
500
Sat’d liquid
ANSWER: B. 200°C
100°C
200°C
300°C
400°C
500°C

11. What is the approximate steam quality for a 100 cm3 vessel containing 1 mole of water at 9 MPa?
Log10 [ V(cm3/mol)] 1
1.5 2
2.5 3
3.5 4
4.5 5 10 20 30 40 50
Pressure (MPa)
Temperature (°C)
100
Critical point
supercritical
200
300
400
Sat’d vapor
500
Sat’d liquid
ANSWER: C. 50%
0 %
20 %
50 %
80 %
100%

12. Solid-Vapor Region (Ice-Steam)
Steam enters a reversible turbine. It is expanded from the initial state 1 to a final temperature of T1. What is the state of the fluid?
Entropy T1
Temperature
Liquid-Vapor
Region
(Water-Steam)
Critical
Point
Liquid Region
(Water)
Solid-Vapor Region (Ice-Steam)
Vapor
(Steam)
Final 1
Pure liquid
Pure vapor
Vapor‐liquid mixture
ANSWER: B. Pure vapor.

13. What is the phase of methane at P = 2 MPa and
T = 150 K? The critical temperature of methane is 190 K.
ρ (mol/cm3) 1
P (MPa) 2 3 4 5 6
Tr = 1.1
1.3
1.0
0.9
0.8
0.01
0.02
Vapor
Liquid
Mixture
ANSWER: B. Liquid.

14. You have a vessel with a piston for compression and a pressure gauge
You have a vessel with a piston for compression and a pressure gauge. The vessel contains a fluid, but you don’t know whether it’s a liquid or vapor. If you push down on the piston, and the __________.
volume decreases significantly as the pressure increases, its a vapor.
walls of the vessel get warm as you increase pressure, it’s a liquid
Both of the above.
ANSWER: A. volume decreases significantly as the pressure increases, its a vapor.

15. A rigid, constant volume container containing a mass that could be solid, liquid and/or gas is brought into contact with a much hotter object. The temperature of the contents ____________.
always increases  
always decreases  
always increases or remains the same  
could increase, decrease or remain the same  
ANSWER: C. always increases or remains the same. Heat goes into sensible heat or phase change, which takes place at constant temperature

16. A well-insulated container has 0. 5 kg ice and 0
A well-insulated container has 0.5 kg ice and 0.5 kg liquid water at 0°C in equilibrium. Liquid ethanol (at 0°C, 0.5 kg), which has a lower freezing point than water, is added to the system. What happens?
Some ice melts, Tf = 0°C.
All ice melts, Tf = 0°C.
Some ice melts, Tf < 0°C.
Some ice melts, Tf > 0°C.
All ice melts, Tf < 0°C.
ANSWER: C. Some ice melts, Tf < 0°C. The ice will start to melt because of the change in fugacity of the liquid; this will absorb energy, causing the temperature to drop. Only part of the ice will melt because the T drop will lower the fugacity of the solid to meet the liquid.

17. A well-insulated container has 0. 5 kg ice and 0
A well-insulated container has 0.5 kg ice and 0.5 kg liquid water at 0°C in equilibrium. Liquid ethanol (at 0°C, 0.5 kg), which has a lower freezing point than water, is added to the system. What happens?
Ice-alcohol solid, Tf < 0°C.
All ice melts, Tf = 0°C.
Some ice melts, Tf < 0°C.
Some ice melts, Tf > 0°C.
Ethanol freezes
ANSWER: C. Some ice melts, Tf < 0°C. The ice will start to melt because of the change in fugacity of the liquid; this will absorb energy, causing the temperature to drop. Only part of the ice will melt because the T drop will lower the fugacity of the solid to meet the liquid.

18. Ethanol at 0°C is added to a ice-liquid water mixture at 0°C in an adiabatic container. The temperature of the mixture _______________.
decreases
increases
remains the same
ANSWER: A. All the ice melts as it tries to reach equilibrium. Since adiabatic, energy goes into melting the ice, so it must come from lowering the temperature of the mixture. The ice melts because the fugacity of the water in the liquid mixture is decreased by adding ethanol but does not affect the fugacity of the solid, therefore driving the melting.

19. A water/ethanol mixture is at -5°C
A water/ethanol mixture is at -5°C. You drop an ice cube that was also at -5°C into the liquid. What happens?
Nothing happens, the system stays at -5°C
Some ice melts
All ice melts
Some water freezes
ANSWER: C. By adding the ice, the system tries to reach equilibrium. Since the fugacity of the liquid water will be less than the ice cube (since it was diluted with ethanol), all the ice will melt.