When water vapor is cooled at constant pressure, which phase change is most likely to occur?

• A. It condenses to form liquid water.
• B. It remains as water vapor.
• C. It sublimates to form solid ice.
• D. It decomposes into hydrogen and oxygen gases.

Answer: (A)

Verified step by step guidance

1. Understand the scenario: Water vapor is being cooled at constant pressure. Cooling means lowering the temperature of the water vapor.
2. Recall the phase changes of water: Water vapor (gas) can change to liquid water by condensation, or directly to solid ice by deposition (the reverse of sublimation).
3. Consider the typical behavior of water vapor when cooled at constant pressure: As temperature decreases, water vapor loses energy and molecules come closer together, favoring a transition to the liquid phase.
4. Eliminate unlikely options: Water vapor does not remain as vapor when cooled significantly; it does not spontaneously decompose into hydrogen and oxygen gases under normal conditions; sublimation is the transition from solid to gas, so the reverse (deposition) would be from gas to solid, but this usually requires lower temperatures or different conditions.
5. Conclude that the most likely phase change when water vapor is cooled at constant pressure is condensation, where water vapor turns into liquid water.