What happens to the rate of evaporation when humidity is high?

• A. Evaporation slows down because the air already contains a lot of water vapor, reducing the driving gradient.
• B. Evaporation speeds up with humidity.
• C. Humidity has no effect on evaporation.
• D. Evaporation only depends on surface area.

Answer: (A)

Evaporation is driven by a vapor pressure gradient between the liquid surface and the surrounding air. When humidity is high, the air already contains a lot of water vapor, so the gradient is smaller and fewer water molecules can escape into the air without already being saturated. This makes the evaporation rate slower. In dry air, the gradient is larger, so water can leave the surface more readily and evaporate faster. Humidity is one factor among others like temperature and surface area, but high humidity specifically reduces the rate by shrinking the driving force for evaporation.