Which of the following phase changes is exothermic?

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

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, evaporation, and sublimation 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 form liquid, releasing heat to the surroundings, making it exothermic.
4. Analyze each option: melting (solid to liquid), evaporation (liquid to gas), and sublimation (solid to gas) all require energy input, so they are endothermic.
5. Conclude that condensation (gas to liquid) releases energy, so it is the exothermic phase change among the options.