Which of the following phase changes are associated with q > 0 (heat absorbed by the system)?

• A. Condensation (gas to liquid) and melting (solid to liquid)
• B. Melting (solid to liquid) and vaporization (liquid to gas)
• C. Freezing (liquid to solid) and condensation (gas to liquid)
• D. Deposition (gas to solid) and freezing (liquid to solid)

Answer: (B)

Verified step by step guidance

1. Understand that the sign of q (heat) depends on whether the system absorbs or releases heat during a phase change. If q > 0, the system absorbs heat (endothermic process).
2. Recall the common phase changes and their heat flow: melting (solid to liquid) and vaporization (liquid to gas) require energy input, so q > 0 for these processes.
3. Recognize that condensation (gas to liquid), freezing (liquid to solid), and deposition (gas to solid) release heat to the surroundings, so q < 0 for these processes.
4. Identify that the phase changes associated with q > 0 are those where the substance gains energy to overcome intermolecular forces and change to a higher energy phase.
5. Conclude that melting and vaporization are the phase changes where heat is absorbed by the system, corresponding to q > 0.