Question

Why do we say evaporation is a surface phenomenon ?

Answer 1

During Evaporation, the upper surface absorbs heat energy and the molecules of the upper surface absorbs that energy and when their energy is exceed from the binding energy , they leaves the upper surface.
So, it is called a surface phenomenon.

Answer 2

Evaporation is a surface phenomenon because molecules present in the water surface are bonded so weak in comparison with the inner molecules. If the temperature increases the bond of hydrogen and oxygen breaks, the surface temperature is high than the inner due to the bond breakage the water molecules will evaporate from the surface.

For example: If you keep water under sunlight as the temperature increases the water molecules gets energy to breakdown the attractive forces and get converted into vapour.