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MCAT Chemistry Question — Phase Change

Which phase change has the same magnitude of energy to the heat of vaporization for water?

A. sublimation

B. freezing

C. condensation

D. deposition

Click for Explanation

Phase changes require the same energy input or give off the same energy depending on the direction of phase change. Freezing and melting are paired, and the enthalpy change is called the heat of fusion. Evaporation and condensation are paired, and the enthalpy change is the heat of vaporization. Deposition and sublimation are paired, representing solid to gas change and there is no specific term for the enthalpy change. “Heat of sublimation” describes the enthalpy change.

A. sublimation, incorrect, sublimation refers to a phase change from solid to gas while vaporization refers to a phase change from liquid to gas.

B. freezing, incorrect, freezing refers to a phase change from liquid to solid.

C. condensation, correct, heat of vaporization and condensation both refer to a phase change from gas to liquid and are equal in magnitude and opposite in sense.

D. deposition, incorrect, deposition refers to a phase change from gas to solid. A deposition is also a term for legal testimony.

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