Solids & Liquids Test Review
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Describe on a molecular level why solids have a definite shape and volume.
Describe on a molecular level why liquids have a definite volume but no shape.
Give three properties that solids and liquids have in common.
Choose three phase changes and explain what is happening at the molecular level.
Explain the change in temperature and energy as water approaches each phase change, passes through it,
and continues after the phase change for the three phases not used in question #3.
6. Compare and contrast evaporation and boiling (vaporization).
7. Explain what equilibrium is and give an example of equilibrium between two phases.
8. How is La Chalelier’s principle related to equilibrium? Give a specific example.
9. Draw a time/temperature diagram for water starting at –10 C and continuing through boiling. Label the
phases and phase changes. Explain what is happening during each phase or phase change at both the
molecular level and the change in energy.
10. Compare and contrast volatile and non-volatile.
11. Explain how freezing and melting can happen at the same temperature.
12. Describe what is happening at the triple point.
13. Can water exist as a liquid in a true vacuum (no pressure)? Why or why not?
14. If a sample of water has 2 atm of pressure over it, at what temperature will it boil?
15. If it takes 975 J of energy to melt 18 g of isopropyl alcohol (C3H7OH), what is its heat of fusion?
16. If the molar heat of fusion of water is 6.009 kJ/mol, how much energy is released when 97 g of water
freeze?
17. Why does ice float on water?
Solids & Liquids Test Review Key
1. Describe on a molecular level why solids have a definite shape and volume.
The molecules are attracted to all surrounding molecules and held in position, so the object has a
fixed shape. The molecules are also held in position so they can not expand or compress so the volume
does not change.
2. Describe on a molecular level why liquids have a definite volume but no shape.
The molecules are attracted to many surrounding molecules, but can move. This enables the
molecules to move and shape to change, but they can not expand or compress so the volume does not
change.
3. Give three properties that solids and liquids have in common.
Incompressible, high density, diffusion, specific melting/boiling temp
4. Choose three phase changes and explain what is happening at the molecular level.
Melting – attraction between molecules in solid decreases, allowing shape to change
Freezing – attraction between molecules in liquid increases, allowing shape to harden into place
Boiling – vapor pressure matches air pressure, attraction between molecules break allowing gas to
form
Condensation – gas molecules get close enough to be attracted to each other, forming liquid
Sublimation – attraction between molecules in solid break, allowing molecules to separate into gas
Solidification – gas molecules get close enough with either high pressure or low temperature and form
a solid as they are attracted to each other
5. Explain the change in temperature and energy as water approaches each phase change, passes through it,
and continues after the phase change for the three phases not used in question #3.
Melting – temp below 0 C, slowly heating up (increasing temp), then temp stabilizes at 0 while it melts
(absorbing more heat), then temp increases again after melting
Boiling – temp below 100 C, slowly heating up (increasing temp), then temp stabilizes at 100 while it
vaporizes (abosorbing more heat), then temp increases again after vaporizing
Freezing – temp above 0 C, slowly cooling (decreasing temp), then temp stabilizes at 0 while it freezes
(releasing heat), then temp decreases again after freezing
Condensing – slowly cooling (decreasing temp), then temp stabilizes while it condenses (releasing
more heat), then temp decreases again after condensation
Sublimation – temp below 0 C, slowly heating up (increasing temp), then temp stabilizes below 0
while it changes into a gas (absorbing more heat), then temp increases again
Deposition– temp below 0 C, gas temp stabilizes below 0 while it changes into a solid (releasing heat),
then temp decreases again
6. Compare and contrast evaporation and boiling (vaporization).
Evaporation and vaporization change from a liquid to a gas. Evaporation can happen at any
temperature, vaporization only happens when the vapor pressure of the liquid matches that of the
surrounding atmosphere
7. Explain what equilibrium is and give an example of equilibrium between two phases.
Equilibrium involves a balance between changes (phase changes here). At 100% humidity, the vapor
of water and water in a cup are in equilibrium – evaporating and condensing at the same rate. A cup
of ice water at 0 C is in equilibrium – melting and freezing at the same rate
8. How is La Chalelier’s principle related to equilibrium? Give a specific example.
A change in the system (pressure, temp, humidity, etc.) will cause the system to form a new
equilibrium. A dehumidifier decreases the humidity in the air, so more liquid water evaporates.
9. Draw a time/temperature diagram for water starting at –10 C and continuing through boiling. Label the
phases and phase changes. Explain what is happening during each phase or phase change at both the
molecular level and the change in energy.
10. Compare and contrast volatile and non-volatile.
Both dealing with the ability of a liquid to evaporate. Volatile liquids evaporate easily, non-volatile
don’t
11. Explain how freezing and melting can happen at the same temperature.
Both the solid and liquid of a material can exist at the same temperature. The two phase changes
simply give more energy to molecules or remove energy so the molecules either move around more
(liquid) or less (solid)
12. Describe what is happening at the triple point.
All three phases exist in equilibrium
13. Can water exist as a liquid in a true vacuum (no pressure)? Why or why not?
No, water can not be liquid below .006 atm.
14. If a sample of water has 2 atm of pressure over it, at what temperature will it boil?
It looks to be about 110 C
15. If it takes 975 J of energy to melt 18 g of isopropyl alcohol (C3H7OH), what is its heat of fusion?
Heat of Fusion is kJ/mol
975 J = .975 kJ
18 g C3H7OH (1 mol C3H7OH/60.11 g C3H7OH) = .30 mol C3H7OH
.975 kJ/.30 mol C3H7OH = 3.25 kJ/mol C3H7OH
16. If the molar heat of fusion of water is 6.009 kJ/mol, how much energy is released when 97 g of water
freeze?
97 g H2O (1 mol H2O/18.02 g H2O) = 5.39 mol H2O
6.009 kJ/mol H2O * 5.39 mol H2O = 32.39 kJ
17. Why does ice float on water?
Ice is less dense than liquid water