Suppose the pressure is 1 atm and we put water (at 25 °C) in an open container. After some time the water will disappear as the water evaporates. We know that water at 25 °C and 1 atm has a vapour pressure. And this vapour pressure is obviously less than the atmospheric pressure or external pressure as no boiling is occurring at this pressure, temperature. If they were equal, boiling would occur which is clearly not happening.
So, if the vapour pressure is less than the atmospheric pressure, then how are are liquid molecules converting into gaseous molecules? I mean the atmospheric pressure above the liquid is exerting a greater force on the liquid and it's vapour pressure is less than the atmospheric pressure.
Then how are the liquid molecules overcoming the atmospheric pressure and changing into gaseous molecules if their vapour pressure is less than the atmospheric pressure? They can only change into gaseous molecules when they can overcome the atmospheric pressure which can occur when their vapour pressure equals atmospheric pressure.