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.
• C. Humidity has no effect on evaporation.
• D. Evaporation stops completely.

Answer: (A)

The main idea is that evaporation depends on the vapor pressure difference between the liquid surface and the surrounding air. When humidity is high, the air already contains a lot of water vapor, so the partial pressure of water vapor in the air is large. This makes the gradient driving molecules to leave the liquid smaller, so fewer molecules escape per unit time. In other words, the air’s capacity to take more water vapor is reduced, which slows the net evaporation rate. It doesn’t stop completely, and it doesn’t speed up—humid air simply lowers the rate because the surrounding atmosphere is closer to saturation. In dry air, evaporation would proceed faster because the gradient is larger.