Which one of the following phase changes is exothermic?

• A. Evaporation of liquid ethanol to ethanol vapor
• B. Sublimation of dry ice (solid CO_2) to CO_2 gas
• C. Condensation of water vapor to liquid water
• D. Melting of ice to liquid water

Answer: (C)

Verified step by step guidance

1. Understand that an exothermic process releases heat to the surroundings, meaning the system loses energy during the phase change.
2. Recall that phase changes from a higher energy state to a lower energy state are exothermic. For example, condensation (gas to liquid) and freezing (liquid to solid) release energy.
3. Analyze each option: Evaporation (liquid to gas) and sublimation (solid to gas) require energy input, so they are endothermic, not exothermic.
4. Condensation (gas to liquid) involves gas molecules losing energy to form a liquid, which releases heat, making it exothermic.
5. Melting (solid to liquid) requires energy absorption to break the solid structure, so it is endothermic, not exothermic.