Where does evaporation occur?

• A. At the surface of a liquid
• B. Inside the liquid as bubbles form
• C. Throughout the liquid equally
• D. Only when the liquid boils

Answer: (A)

Evaporation is a surface process: it happens when molecules at the boundary between the liquid and air gain enough energy to break free and escape into the gas phase. Molecules inside the liquid are surrounded by other molecules, so they don’t easily escape into the air. This is why evaporation occurs at the surface even when the liquid isn’t boiling. Boiling, by contrast, involves vapor forming throughout the liquid and producing bubbles. So the correct description is that evaporation occurs at the surface of the liquid.