Evaporation and factors affecting evaporation

Evaporation :Notes for SSC UPSC Exams

When a liquid is left exposed to air, its volume decreases gradually. This is due to evaporation. 

In this Process some of the liquid at the surface turns into vapour (or gas) and mixes with the gases in the air.

Thus, when a liquid turns into vapour (or gas), it is said to evaporate. We can now define evaporation as follows : The process of a liquid changing into vapour (or gas) even below its boiling point is called Evaporation.

Evaporation 

Evaporation of a liquid can take place even at room temperature, though it is faster at higher  temperatures. Whatever be the temperature at which evaporation takes place, the latent heat of vaporisation must be supplied whenever a liquid changes into vapour (or gas).

The wet clothes dry due to evaporation of water present in them. And rain water puddles also dry up because of evaporation of water. Common salt is also recovered from sea-water by the process of evaporation.The process of evaporation can be explained as follows. Some particles in a liquid always have more kinetic energy than the others. So, even when a liquid is well below its boiling point, some of its particles have enough energy to break the forces of attraction between the particles and escape from the surface of the liquid in the form of vapour (or gas). Thus, the fast moving particles (or molecules) of a liquid are constantly escaping from the liquid to form vapour (or gas).

Factors Affecting Evaporation:Notes for SSC UPSC Exams

The evaporation of a liquid depends mainly on the following factors :

 (i) Temperature

 (ii) Surface area

 (iii) Humidity

 (iv) Wind speed

We will discuss all these factors in detail, one by one.

 1. Temperature

 The rate of evaporation increases on increasing the temperature of the liquid. In other words, the rate of evaporation of a liquid increases on heating. When the temperature of a liquid is increased by heating it,more particles of the liquid get enough kinetic energy to go into the vapour state (or gaseous state). This increases the rate of evaporation. Thus, the rate of evaporation of a liquid can be increased by heating it.

The rate of evaporation of a liquid becomes maximum at its boiling point.

 2. Surface Area of the Liquid

The rate of evaporation increases on increasing the surface area of the liquid. So, if the surface area of a liquid exposed to the air is increased, the rate of evaporation of the liquid increases. For example, if the same liquid (say, water) is kept in a test-tube and in a china dish, then the liquid kept in the china dish will evaporate more rapidly (because more of its surface area is exposed to air). In our daily life, we spread out the washed wet clothes while drying to increase their surface area for the rapid evaporation of water present in them (which leads to quicker drying of wet clothes). After rain, the wet roads dry quickly because the rain water is spread over a large area of road. This gives the particles of water a greater chance of escaping from the liquid. From this discussion we conclude that the rate of evaporation of a liquid can be increased by increasing the surface area of the liquid.

 3. Humidity of Air

 The amount of water vapours present in air is represented by a term called humidity. When the amount of water vapours present in the air is small, the air appears to be ‘dry’ and we say that the humidity is low.

On the other hand, when the amount of water vapours in the air is large, the air appears to be ‘damp’ and we say that the humidity is high. So, humidity of air tells us the degree of ‘dampness’ of air.

When the humidity of air is low, then the rate of evaporation is high, and water evaporates more readily. Under these conditions, sweat from our body evaporates readily and we feel cool and comfortable.

The wet clothes dry quickly under the conditions of low humidity of air. When the humidity of air is high, then the rate of evaporation is low, and water evaporates very slowly. In the later part of summer,the humidity of air increases. People sweat a lot in such weather. But the sweat from our bodies does not evaporate readily due to high humidity of air. Such weather becomes muggy (damp) and we feel hot and uncomfortable. This type of weather is experienced during cloudy days in the rainy season and in areas close to the sea (coastal areas). The wet clothes take a long time to dry when the humidity of air is high.

4. Wind Speed

The rate of evaporation of a liquid increases with increasing wind speed. When the speed of wind increases, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surroundings. This increases the rate of evaporation of water. The washed wet clothes dry more quickly on a windy day because evaporation is faster due to the high speed of the wind.

Notes for SSC UPSC Exams,Chemistry Notes for SSC UPSC Exams