Which statement best describes evaporation in heat transfer?

• A. transfer of heat that occurs as a liquid absorbs energy and changes form into a vapor
• B. direct transfer of heat from a radiation energy source of higher temp to one of cooler temp
• C. heating that occurs when nonthermal energy is absorbed into tissue
• D. decreased nerve conduction velocity in treated tissue

Answer: (A)

Evaporation is the heat transfer process where a liquid absorbs energy and changes into a vapor. It’s an endothermic phase change: energy goes into breaking intermolecular forces so molecules at the surface escape into the gas phase. Because this energy is used for the phase change, the liquid’s temperature can stay relatively constant during evaporation, and the liquid can cool as higher-energy molecules leave. The rate depends on surface area, temperature difference with the surroundings, airflow, and humidity. This description fits the idea of a liquid absorbing energy to become vapor, unlike heat transfer by radiation, nonthermal tissue heating, or nerve conduction changes, which describe other phenomena.