Why does a glass of water left out in air vaporize (eventually) even though it's not at boiling temperature?

Even at room temperature a few molecules of liquid have energy o break free of the surface into the air. If the water is in a closed container then eventually the water molecules recondense at the surgace of the water or on the sides of the container. A balanced system arises, the number of water molecules in the air (in gas state) is constant, some always leaving the surface of the water, some recondensing back into water. In this blanced system, the pressure arising in the gas state due to the water molecules is called the vapour pressure of the water.

If the system is open, though (no lid on the container) then some of the water molecules in the air will simply move frther and further away from the surface of the water without hitting anything else ... no recondensation so the system never reaches balance ... water molecules still leave the surface of the water but never make it back again ,,, so the water eventually all evaporates.

(As an aside ... if you heat the water up ... more molecules leave the surface of the water ... more molecules of water in the air just above the water ... so a higher pressure of water molecules (higher vapour pressure) in the air above the water. When you each 100 degrees C, this vapour pressure of water has risen to 1 atmosphere, the air pressure ... The presure of the water molecules in the air is enough to push the air away ... and the water boils)

like in the heat exchanging where the hotter passes to the cooler until they reach an equilibrium in both middles , so is the water vapor(humidity) with a drier air . Water is pure100%humidity but air is comparatively drier than water.So an exchanging process starts between water and air where the water loses some of its molecules to the dry air which consequently becomes more humid .if this humid air is not dissipated there will come a time where it becomes(the air )saturated and the evaporation process will stop > but air is never still, so,the air above the glass is always renewed and the humidity exchanging continues until all the water evaporates. Notice that evaporation does not need boiling temperature to start .Nature does not boil the sea to make clouds.

in response to the guy above, evaporation does not mean that water molecules are breaking apart. water vapor is still H2O, it is just not bound by intermolecular forces.

your book is just saying that molecules in the air are moving around and they sometimes strike the surface of the water. when that happens, enough energy can sometimes be imparted to the water molecules that they can break free of the other water molecules that are pulling on them.

actually the biggest determination of the rate of evaporation is the amount on sunlight reaching it. at any level of heat there will be some molecules at the high end of the bell curve that escape the bonds of their neighbours and evaporate into the air. with sunlight, there are high energy photons striking particular molecules and giving them extra energy for escape.

uh...to translate the above answer into common english...all he means is that water (H20) is 'breaking apart'. Water is made up of the molecules hydrogen and oxygen (which are both gases) and each molecule on the surface of the water escape into the air. Heat speeds up this process.