If energy is limited in evaporation, why can the rate remain limited even with a large humidity deficit?

Correct answer: Because water molecules need latent heat to become vapour.

The correct answer is A. A humidity deficit means that the surrounding air is capable of receiving more water vapour, so it can promote evaporation. However, evaporation is not controlled by humidity deficit alone. Each water molecule that escapes from the liquid surface requires latent heat to change from liquid to vapour. If the available energy supply is small—for example, because of weak heating or limited energy transfer—only a limited number of molecules can make that phase change per unit time. The process is then energy-limited even though the air is dry. A is correct because it identifies the necessary energy requirement. B is wrong because dry air generally accepts additional vapour readily. C is wrong because humidity deficit does not eliminate atmospheric pressure. D is wrong because evaporation is the conversion of water into vapour and therefore requires a liquid water source. The deficit provides demand, while energy provides the ability to meet that demand. Memory cue: dry air may invite evaporation, but latent heat pays for it.