Which of the following phase changes is exothermic?

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

Answer: (D)

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 the common phase changes and their energy characteristics: melting, sublimation, and evaporation are endothermic because they require energy input to overcome intermolecular forces.
3. Recognize that condensation is the reverse of evaporation, where gas molecules lose energy and transition to the liquid phase, releasing heat in the process.
4. Analyze each option: melting ice, sublimation of dry ice, and evaporation all require energy absorption (endothermic), while condensation releases energy (exothermic).
5. Conclude that the phase change which is exothermic is the condensation of water vapor to liquid water because it involves the release of heat as gas molecules form intermolecular bonds in the liquid phase.