Which of the following exchanges of energy has a positive sign for heat (relative to the system)?

• A. snow melts on a mountaintop (snow is the system)
• B. water vapor condenses on a cold surface (water vapor is the system)
• C. a rock tumbles from a mountain (the rock is the system)
• D. both the first and second choices
• E. both the first and third choices
• F. both the second and third choices

Answer: (A)

Verified step by step guidance

1. Identify the system in each scenario: In the first scenario, the system is the snow; in the second, it is the water vapor; and in the third, it is the rock.
2. Understand the concept of heat exchange: A positive sign for heat indicates that the system is absorbing heat from the surroundings.
3. Analyze the first scenario: When snow melts, it absorbs heat from the surroundings to change from solid to liquid, indicating a positive heat exchange.
4. Analyze the second scenario: When water vapor condenses, it releases heat to the surroundings to change from gas to liquid, indicating a negative heat exchange.
5. Analyze the third scenario: When a rock tumbles, it does not involve a phase change or heat exchange in the context of this problem, so it is not relevant to the heat exchange question.