Questions about Heats of Fusion and Solidification
Multiple-Choice Questions: Heats of Fusion and Solidification
1. What does the heat of fusion represent?
A) The energy required to vaporize a liquid
B) The energy released during freezing
C) The energy required to melt a solid
D) The energy absorbed during condensation
E) The temperature at which melting occurs
2. Which of the following is true about the heat of solidification?
A) It is positive because energy is absorbed
B) It occurs during boiling
C) It is equal in magnitude but opposite in sign to the heat of fusion
D) It only applies to gases
E) It increases with volume
3. What is the unit for heat of fusion in SI units?
A) J/mol
B) L/mol
C) J/g
D) kPa
E) °C
4. Which process involves the heat of fusion?
A) Boiling water
B) Melting ice
C) Condensing steam
D) Freezing ethanol
E) Subliming dry ice
5. The heat of solidification refers to:
A) Heat required to change a solid into a gas
B) Heat needed to melt a substance
C) Heat released when a liquid becomes solid
D) Heat required to boil water
E) Heat absorbed during sublimation
6. When a substance melts, its heat of fusion is:
A) Absorbed by the system
B) Released to the surroundings
C) Converted to sound
D) Remains constant
E) Negative in sign
7. Which of the following equations correctly expresses the heat absorbed or released during fusion or solidification?
A) Q = mcΔT
B) Q = PΔV
C) Q = mLf
D) Q = nRT
E) Q = kAΔT
8. What is the heat of fusion of water approximately?
A) 100 J/g
B) 334 J/g
C) 4.18 J/g°C
D) 2257 J/g
E) 1000 J/g
9. If 10 g of ice melts at 0°C, how much energy is absorbed? (Use 334 J/g for water)
A) 33.4 J
B) 3.34 kJ
C) 3340 J
D) 0.334 kJ
E) 668 J
10. During solidification, the temperature of the substance:
A) Increases
B) Decreases sharply
C) Remains constant
D) Depends on pressure
E) Varies randomly
11. Why does temperature remain constant during fusion?
A) Heat is used to raise temperature
B) Heat increases kinetic energy
C) Heat breaks intermolecular bonds
D) Heat is lost to surroundings
E) No heat is involved
12. Which phase change releases heat to the surroundings?
A) Melting
B) Boiling
C) Evaporation
D) Freezing
E) Sublimation
13. The amount of heat needed to melt a substance depends on:
A) Temperature only
B) Pressure only
C) Mass and heat of fusion
D) Volume only
E) Specific heat capacity
14. In the equation Q = mLf, "Lf" represents:
A) Latent fusion
B) Linear force
C) Latent heat of fusion
D) Loss of frequency
E) Lateral flux
15. If 500 J are released during solidification of a substance with heat of fusion 250 J/g, how many grams solidified?
A) 1 g
B) 2 g
C) 0.5 g
D) 5 g
E) 250 g
16. Which of the following is a cooling process?
A) Fusion
B) Condensation
C) Boiling
D) Melting
E) Heating
17. What occurs at the molecular level during melting?
A) Molecules slow down and get closer
B) Bonds break and molecules move more freely
C) Molecules are compressed
D) Molecules ionize
E) Energy is lost
18. A student melts 20 g of a substance with a heat of fusion of 200 J/g. What is the energy absorbed?
A) 4000 J
B) 20 J
C) 200 J
D) 10,000 J
E) 1000 J
19. Which graph correctly represents the temperature vs. time curve of ice melting at constant pressure?
A) Linear increase
B) Sudden spike, then flat
C) Plateau during phase change
D) Decrease and then rise
E) Random fluctuations
20. Which statement is true about heat of fusion?
A) It’s used only for gases
B) It’s irrelevant in phase changes
C) It is measured only in °C
D) It varies with the substance
E) It only applies to water
Answers and Explanations
1. C – Heat of fusion is the energy required to melt a solid into a liquid at its melting point.
2. C – Heat of solidification is equal in magnitude and opposite in sign to the heat of fusion.
3. A – SI unit for heat of fusion is joules per mole (J/mol), though J/g is also used.
4. B – Melting involves absorption of heat, described by heat of fusion.
5. C – Solidification releases heat as a liquid becomes a solid.
6. A – During melting, energy is absorbed to overcome intermolecular forces.
7. C – Q = mLf is used for phase changes involving fusion or solidification.
8. B – Water’s heat of fusion is 334 J/g.
9. B – Q = 10 g × 334 J/g = 3340 J = 3.34 kJ
10. C – During phase changes, temperature stays constant as heat is used to break bonds.
11. C – Heat energy goes into breaking molecular attractions, not raising temperature.
12. D – Freezing releases energy (exothermic).
13. C – Q = mLf, depends on mass and heat of fusion.
14. C – Lf stands for latent heat of fusion.
15. B – m = Q/Lf = 500 J ÷ 250 J/g = 2 g
16. B – Condensation and freezing release heat; they're cooling processes.
17. B – Melting involves breaking of intermolecular forces; molecules move more freely.
18. A – Q = 20 g × 200 J/g = 4000 J
19. C – Temperature plateaus during phase changes.
20. D – Heat of fusion differs among substances due to molecular structure.
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