This equilibrium can be better understood if we consider the example of a transparent box carrying a U - tube with mercury ( manometer). Drying agent like anhydrous calcium chloride (or phosphorus penta - oxide) is placed for a few hours in the box.
After removing the drying agent by tilting the box on one side, a watch glass (or Petri dish) containing water is quickly placed inside the box. It will be observed that the mercury level in the right limb of the manometer slowly increases and finally attains a constant value, that is, the pressure inside the box increase and reaches a constant value.
Also the volume of water in the watch glass decreases. Initially there was no water vapour (or very less) inside the box. As water evaporated the pressure in the box increase due to addition of water molecules into the gaseous phase inside the box. The rate of evaporation is constant. However, the rate of increase in pressure decreases with time due to condensation of vapour into water.
Finally it leads to an equilibrium conditions when there is no net evaporation. This implies that the number of water molecules from the gaseous state into the liquid state also increases till the equilibrium is attained i.e.,
rate of evaporation = rate of condensation
H2O (1) ⇌H2O (vap)
At equilibrium the pressure exerted by the water molecules at a given temperature remains constant is called the equilibrium vapour pressure of water increase with temperature. It the above experimen is temperature, and the liquid which has a higher vapour pressure is called more volatile and has a lower boiling point.
If we expose three water glasses containing separately 1 mL each of acetone, ethyl alcohol, and water to atmosphere and repeat the experiment with different volume of the liquid in a warmer room, it is observed that in all such cases the liquid eventually disappears and the time taken for complete evaporation depends on (I) the nature of the liquid, (II) the amount of the liquid and (III) the temperature.
When the watch glass is open to atmosphere, the rate of evaporation remains constant but the molecules are dispersed into large volume of the room.
