Why does spontanious evaporation take place?

Answer 1

At any temperature, above absolute zero, the atoms and molecules of a liquid have thermal energy, which causes the liquid molecules to move around and collide with each other. Thus a few of them will always have a kinetic energy more than that needed to break free from the liquid surface and go out. That in short is evaporation.

If the surface is open to the atmosphere, the molecules may not return to the liquid and thus fresh molecules continue to escape. If the surface is covered, then an equilibrium is reached and the evaporation stops. At the microscopic levels, molecules are going in and out of the liquid into the space above. It is a dynamic equilibrium.

Answer 2

All substances have the abilty to Evaporate at any temperature and, as Absolute Zero Temperature, at which all molecular motion is theoreticallly supposed to cease, and as this temperature cannot be attained then the molecules of all substances are always in motion ...i.e. they have Kinetic Energy and therefore contain Heat Energy.
E.g....In a closed container, at the surface of a liquid, the motion of the molecules will cause some to 'Jump Out' of the liquid into the 'Vapour Space' above it and, for any GIVEN, stable temperature, the number of molecules leaving the liquid equals the number returning to it and, the liquid and its vapour are said to be in 'EQUILIBRIUM' and the molecules are exerting a vapour pressure relative to the liquid temperature.
In the open, when there is a disturbance of air around the surface of the liquid, some of the molecules are moved away and can't return to the liquid.
This loss of molecules is the cause of the Evaporation of the liquid and can take place at any temperature.
An increase in temperature will increase evaporation rate as will an increase in air disturbance over the liquid. (And Vice-Versa).

Answer 3

The temperature of a system is only an average; there are some molecules with higher, some with lower. If some of these higher energy molecules have the energy required to evaporate, then they just might.