EVAPORATION
The phenomenon of change of a liquid into vapours at any temperature below its boiling point is called ‘evaporation’. Evaporation is a surface phenomenon i.e. only the particles present on the surface are involved.
*FACTORS AFFECTING EVAPORATION
*Surface Area
Greater is the surface area more is the rate of evaporation. This is because only the particles on the surface of the liquid get converted into vapours.
For example, we often spread the wet clothes in air to dry them. By doing so, the surface area available for evaporation of water increases and hence the clothes get dried up soon.
*Increase in temperature
The rate of evaporation increases with increase in temperature due to increase in K.E. of liquid particles. This is because, due to increase in K.E., the liquid particles can more easily overcome the forces of attraction of neighbouring particles on the surface of liquid and hence can more easily get converted into vapours.
For example, evaporation of a liquid occurs at a faster rate in summer than in winter.
*Decrease in Humidity
By humidity we mean, the amount of water vapour present in air. The air around us can hold only a certain definite amount of water vapours at a particular temperature. Now in case, humidity of air is already high i.e. the amount of water vapours in the air is already high, then air can hold only a little more amount of vapours to reach that optimum level (as air can hold
only a certain definite amount of water vapours). Therefore the rate of evaporation decreases.
For example, we sweat a lot in hot and humid weather than in dry weather because, air already has high amount of water vapours in humid and hot weather. Therefore, the sweat liquid that comes out of our skin does not evaporate and remains sticking to our body.
*Increase in the speed of wind
The rate of evaporation increases with increase in wind speed. This is because, due to increase in wind energy, the liquid particles move away with the wind and thus decreasing water vapours in the surroundings.
For example, wet clothes dry faster on a windy day due to increase in wind speed and thereby increasing the rate of evaporation. Similarly we usually sit under the fan during summer days (when we sweat a lot) because fan increases the wind speed around us, thereby increasing the rate of evaporation and making us feel more comfortable (since evaporation causes cooling).
*Nature of Liquid
The rate of evaporation also depends upon the nature of the liquid. In other words, lesser is the boiling point, more is the tendency of the liquid to evaporate or to change into vapours.
It can be explained more clearly by the following example :
Alcohol with a boiling point 351K or 780C evaporates much more quickly than water with a boiling point 373K (or 1000C). This is because the inter particle force of attraction are weaker in alcohol than in water, so that the particles of alcohol can leave the liquid surface to form vapours more easily than the particles of water and thus rate of evaporation of alcohol is faster than that of water.
Hence the liquid with less boiling point will evaporate more quickly than the liquid with more boiling point.
The effect of factors like surface area, temperature, humidity and wind speed on the rate of evaporation of liquids can be explained more clearly by performing the following experiment :