Which thermal mode involves a phase change to transfer heat without direct contact?

Which thermal mode involves a phase change to transfer heat without direct contact?

• A. Conduction
• B. Convection
• C. Evaporation
• D. Radiation

Answer: (C)

Heat transfer through evaporation relies on a phase change from liquid to vapor, which requires latent heat of vaporization. The liquid absorbs this energy at the surface, and as molecules escape into the air, they carry that energy away, effectively transferring heat without the need for direct contact with a solid heat source. Radiation also transfers heat without contact, but it does not involve a phase change, so evaporation is the correct mechanism when a phase change is specified. This surface-based phase-change cooling is why sweating or a liquid evaporating on a surface cools things down.

Heat transfer through evaporation relies on a phase change from liquid to vapor, which requires latent heat of vaporization. The liquid absorbs this energy at the surface, and as molecules escape into the air, they carry that energy away, effectively transferring heat without the need for direct contact with a solid heat source. Radiation also transfers heat without contact, but it does not involve a phase change, so evaporation is the correct mechanism when a phase change is specified. This surface-based phase-change cooling is why sweating or a liquid evaporating on a surface cools things down.