In a scenario where heat transfers from the water surface to the air directly at the interface, which mechanism is responsible?

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

Answer: (C)

Energy crossing from water into air at a contact interface happens mainly by conduction through the boundary layer. The hotter water transfers energy to the adjacent air molecules via direct molecular interactions across the surface, and that energy then diffuses into the air (aided by convection moving the air away). Radiation can contribute energy exchange between surfaces, but when the two media are in direct contact, conduction across the interface is the primary mechanism for crossing the boundary. Evaporation can remove some energy as latent heat, but that involves phase change and mass transfer, not the direct crossing of energy through the boundary itself.