Experiment:
Step I : Take 5ml of water in a test tube and keep it under a fan.
Step II : Take 5ml of water in an open china dish and keep it under a fan.
Step III : Take 5 ml of water in an open china dish and keep it inside a cupboard.
Step IV : Repeat all above three steps of experiment on a rainy day or humid weather and record the time and days taken for evaporation process in all cases
Observation :
(i) The water taken in a test tube will evaporate slowly than the water taken in two open china dishes.
(ii) The water taken in open china dish placed under fan will evaporate more quickly then water taken in open china dish placed inside a cup-board.
(iii) The first three processes will take longer time for evaporation process on a rainy day or humid weather
Discussion: The surface area of water exposed to atmosphere is minimum in case of test tube, so, it takes a long time (2/3 days) for 5ml of water to evaporate. Although surface area of 5ml of water taken in two open china dishes is the same, yet water in the china dish placed under the fan evaporates more quickly than the water in china dish placed inside a cupboard. This is because wind speed increase due to fan and thereby increases rate of evaporation.
On the other hand, three processes will take longer time for evaporation process on a rainy day or humid whether.
This is due to the reason that
(i) on a rainy day, temperature is reduced and thus rate of evaporation is decreased
(ii) in a humid weather, the amount of water vapours in air are already high and thus rate evaporation is decreased.
Conclusion: From above discussion we led to conclude that, the rate of evaporation of liquid increases with
(i) increase in surface area exposed to the atmosphere.
(ii) increase in temperature.
(iii) increase in wind speed.
(iv) decrease in humidity (i.e. amount of water vapours present in air)
*HOW DOES EVAPORATION CAUSES COOLING ?
During evaporation, only the liquid particles having high K.E. leave the surface of the liquid and get converted into vapours. As a result, the average K.E. of the remaining particles of the liquid decreases and hence temperature falls, thus evaporation causes cooling.
It can be explained more clearly by the following example
Place some water in an open vessel. The water keeps on evaporating. For evaporation to occur heat energy is taken from water. The particles of water in turn, absorb energy from the surroundings to regain the energy lost during evaporation. This absorption of energy from the surroundings makes the surroundings cool. Hence evaporation causes cooling.