Evaporation is the change of a liquid into a gas without boiling. This process only occurs at the surface of the liquid.
During evaporation, some liquid molecules on the surface of the liquid obtain enough energy to escape into the gas state.
Enough energy must be absorbed by these molecules to overcome the intermolecular attractive forces.
Evaporation can only occur for molecules on the surface of the liquid because they are not surrounded by other molecules.
The overall intermolecular forces holding these particles in the liquid are therefore weaker.
Evaporation is very slow because only a small proportion of liquid surface molecules gain enough energy to evaporate.