Evaporation

• The process of a liquid changing into vapour (or gas) even below its boiling point is called evaporation.
• Amount of energy equal to the latent heat of vapourisation is absorbed by water during its transformation from liquid to gaseous state.

Factors Affecting Evaporation:

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

1. Temperature

2. Surface area

3. Humidity

4. Wind speed

1. Temperature:

• The rate of evaporation increases on increasing the temperature of the liquid.
• 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.
• This increases the rate of evaporation.

2. Surface Area of the Liquid:

• The rate of evaporation varies directly with the surface area. More is the surface area more is the rate of evaporation. So, if the surface area of a liquid exposed to the air is increased, the rate of evaporation of the liquid increase.
• We spread out the washed wet clothes while drying to increase their surface area for the rapid evaporation of water present in them.

3. Humidity of Air:

• The amount of water vapour present in air is called humidity.
• Rate of evaporation varies inversely with the humidity. The more is the humidity the less is evaporation. When the humidity of air is high, then the rate of evaporation is low, and water evaporates very slowly.

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.

Cooling Caused by Evaporation:

• The cooling caused by evaporation is based on the fact that when a liquid evaporates, it draws the latent heat of vapourization from ‘anything’ which it touches. By losing heat, the substance gets cooled.
• If we put a little spirit (ether or petrol) at the back of our hand and wave it around, the sprit evaporates rapidly which cools our hand.
• This is due to the fact that spirit absorbs energy equal to its latent heat of vapourisation from the surface when it changes from  liquid to the vapour state thus , cooling it.

1. During hot summer days, water is usually kept in an earthen pot to keep it cool.

• The earthen pot has a large number of extremely small pores (or holes) in its walls.
• Some of the water continuously keeps seeping through these pores to the outside of the pot.
• This water evaporates continuously and takes the latent heat required for vapourization from the earthen pot and the remaining water.
• In this way, the remaining water loses heat and gets cooled.

2. In villages, people often sprinkle water on the ground in front of their homes during the hot summer evenings as water evaporates by taking the large latent heat of vapourization from the ground and surrounding air.

By losing heat, the place becomes cool and comfortable.

3. We should wear cotton clothes in hot summer days to keep cool and comfortable.

This can be explained as follows:

• We get a lot of sweat on our body in hot summer days.
• This sweat formed evaporates absorbing energy equal to the latent heat of vapourisation from the body thus cooling it.
• Cotton absorbs sweat from our body and exposes it to the air (or atmosphere) for evaporation.
• The synthetic clothes (made of polyester, etc.) do not absorb much of sweat. So it is advisable to wear cotton clothes in summer season.

4. A desert cooler cools better on a hot and dry day because the higher temperature on a hot day increases the rate of evaporation of water, and the dryness of air also increases the rate of evaporation of water. And due to increased rate of evaporation of water, a desert room cooler cools better on a hot and dry day.