Definition of Evaporation
The spontaneous conversion of a volatile liquid into its vapors is called evaporation. The process of evaporation can be easily understood on the basis of kinetic molecular theory according to which a liquid consists of a large number of molecules which remain in a state of constant motion.
Process of Evaporation
A liquid is composed of many molecules in rapid movements. The temperature of the liquid is depends upon to the average kinetic energy of the liquid molecules:
Although kinetic energy of liquid molecules can be represented by average value but there is a range of velocities to make up that average. On the basis of kinetic energy the molecules in a liquid are divided into three types (i) small fraction of molecules having very small (low) kinetic energy (ii) small fraction of molecules having very high kinetic energy (iii) large number of molecules which have average K.E.
The high energy molecules (high velocity and kinetic energy) rise to the surface of the liquid and if their K.E. overcomes the force of attraction of neighboring molecules then they will escape from the liquid and enter into gas phase. Such escaping molecules from the surface of the liquid into vapors state are considered evaporated.
When a liquid is evaporated rapidly, high energy molecules (Hot) escape from the liquid, leaving behind low energy molecules. Due to the loss in energy which the molecules have taken along with them, the temperature of liquid falls. Hence, it is a cooling process. It absorbs heat from the surroundings and thus temperature of the surrounding gets lowered.
Factors affecting evaporation
Evaporation occurs at all temperatures. However at higher temperature evaporation is more rapid than at lower temperature. Temperature is a measure of average kinetic energy of the molecules, so, at high temperature molecules have higher kinetic energy which helps more molecules to overcome the intermolecular attraction of neighboring molecules and evaporate out. Thus rate of evaporation of a liquid increases as the temperature increases.
It is a surface phenomenon. Only those molecules which come to the surface of the liquid will evaporate. If the surface area of a liquid is increased then more molecules of high energy will reach the surface, thus, leading to rapid evaporation.
Different liquids evaporate at different rates even if the temperature is kept the same. Thus petrol evaporates more quickly as compared to water at room temperature. It is so because evaporating tendency of molecules at the same temperature depends on intermolecular attractions. Water molecules are strongly associated through hydrogen bonding while intermolecular attractions in petrol are small which increase their evaporation ability and thus petrol evaporates rapidly as compared to water.