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AP Chemistry Practice Quiz: Energy of Phase Changes

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 9 questions to check your progress.

Question 1 of 9

During the process of condensation, a substance transitions from a gas to a liquid. Which of the following describes the energy change of the system?

All Questions (9)

During the process of condensation, a substance transitions from a gas to a liquid. Which of the following describes the energy change of the system?

A) Energy is absorbed, and the system's energy increases.

B) Energy is released, and the system's energy decreases.

C) Energy is absorbed, and the system's energy decreases.

D) No energy change occurs, but the temperature drops.

Correct Answer: B

According to the provided content, energy is released during condensation, which causes the system's energy to decrease. The temperature remains constant during the phase change.

A sample of a pure substance is heated at its boiling point. As the substance changes from a liquid to a gas, what happens to its temperature?

A) The temperature increases steadily.

B) The temperature decreases as energy is used for the phase change.

C) The temperature remains constant.

D) The temperature fluctuates until all the liquid is gone.

Correct Answer: C

The provided text states that 'The temperature of a pure substance remains constant during a phase change.' Boiling is a phase change.

The molar enthalpy of vaporization (boiling) for a certain substance is +40.7 kJ/mol. What is the molar enthalpy of condensation for the same substance?

A) +40.7 kJ/mol

B) +20.35 kJ/mol

C) -40.7 kJ/mol

D) -20.35 kJ/mol

Correct Answer: C

The energy absorbed during vaporization is equal in magnitude to the energy released during the complementary phase change, condensation. Therefore, the sign is reversed. ΔH_condensation = -ΔH_vaporization.

Which of the following phase transitions requires an input of energy, thereby increasing the system's energy?

A) Freezing

B) Condensation

C) Melting

D) Deposition (gas to solid)

Correct Answer: C

The content states that 'Energy must be transferred to a system to cause melting or boiling, increasing the system's energy.' Freezing and condensation are processes that release energy.

If it takes an amount of heat 'q' to melt 1 mole of a solid substance, how much heat is involved when 1 mole of the same substance freezes from a liquid to a solid?

A) An amount of heat 'q' is absorbed.

B) An amount of heat 'q' is released.

C) An amount of heat '2q' is released.

D) No heat is involved in the freezing process.

Correct Answer: B

The energy absorbed during a phase change (melting) is equal in magnitude to the energy released during the complementary phase change (freezing). If 'q' is absorbed for melting, then 'q' must be released for freezing.

The heat 'q' absorbed or released during a phase transition is calculated based on the amount of substance in moles 'n' and the molar enthalpy of the phase transition 'ΔH'. How would the heat absorbed during the melting of 2 moles of a substance compare to the heat absorbed by 1 mole of the same substance?

A) It would be half as much.

B) It would be the same.

C) It would be twice as much.

D) It would be four times as much.

Correct Answer: C

The heat absorbed or released is directly proportional to the amount of the substance in moles. Therefore, melting twice the number of moles requires twice the amount of heat.

A system containing 3 moles of a liquid is completely vaporized at its boiling point. If the molar enthalpy of vaporization is ΔH_vap, which expression correctly represents the heat, q, absorbed by the system?

A) q = 3 / ΔH_vap

B) q = ΔH_vap / 3

C) q = -3 * ΔH_vap

D) q = 3 * ΔH_vap

Correct Answer: D

The heat absorbed (q) is calculated by multiplying the amount of substance in moles (n) by the molar enthalpy of the phase transition (ΔH). Here, n = 3 moles and the transition is vaporization, so q = n * ΔH_vap = 3 * ΔH_vap. The sign is positive because boiling is an endothermic process.

When a substance freezes, its energy decreases. What happens to the energy that is released by the system?

A) It is destroyed.

B) It is converted into mass.

C) It is transferred to the surroundings.

D) It remains within the system, lowering the temperature.

Correct Answer: C

The content implies the conservation of energy. If energy is released by the system (decreasing the system's energy), it must be transferred somewhere, which is the surroundings. The temperature of the system itself remains constant during the phase change.

Which statement correctly contrasts the energy changes during melting and freezing of a pure substance?

A) Melting absorbs energy while freezing releases energy of an equal magnitude; temperature changes during both processes.

B) Melting and freezing both release energy, but the temperature only remains constant during freezing.

C) Melting absorbs energy while freezing releases energy of an equal magnitude; temperature remains constant during both processes.

D) Melting releases energy while freezing absorbs energy of an equal magnitude; temperature remains constant during both processes.

Correct Answer: C

This question combines all three content points. Melting is endothermic (absorbs energy), while freezing is exothermic (releases energy). The magnitudes are equal. For a pure substance, temperature remains constant during any phase change.