EVAPORATION
Channel: SCHOLAR PATHSHALA
Date: 2021-05-08

Hello!
TOPIC: EVAPORATION
NCERT Class IX Science.
It proceeds like this,
The process of a liquid changing into vapour (or gas) even below its boiling point is called evaporation. 
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 evaporation of a liquid depends mainly on the following factors :
(i) Temperature
(ii) Surface area
(iii) Humidity
(iv) Wind speed

1. Temperature
The rate of evaporation increases on increasing the temperature of the liquid. Thus, the rate of evaporation of a liquid can be increased by heating it.

2. Surface Area of the Liquid
The rate of evaporation increases on increasing the surface
area of the liquid. Thus, the rate of evaporation of a liquid can be increased by increasing the surface area of the liquid.
Wet clothes are spread for drying. The spread wet clothes have large surface area. This makes the evaporation of water faster. Due to this, clothes dry up more quickly.

3. Humidity of Air
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. 
When the humidity of air is high, then the rate of evaporation is low, and water evaporates very slowly.
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. 
The washed wet clothes dry more quickly on a windy day because evaporation is faster due to the high speed of the wind.

Cooling Caused by Evaporation:
The cooling caused by evaporation is based on the fact that when a liquid evaporates, it takes the latent heat of vaporisation from ‘anything’ which it touches. 
If we put a little of spirit (ether or petrol) at the back of our hand and wave it around, the spirit evaporates rapidly and our hand feels very cold.

During hot summer days, water is usually kept in an earthen pot (called matka) to keep it cool.

At many places, especially in villages, people often sprinkle water on the ground in front of their homes during the hot summer evenings.

Perspiration (or sweating) is our body’s method of maintaining a constant temperature. 

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

A fan increases the rate of evaporation of sweat (or moisture) from our skin and makes us feel cool and comfortable.

A sip hot tea (or milk) faster from a saucer than from a cup.

To Show the Presence of Water Vapour in Air:
The formation of drops of water on the outside surface of a glass containing crushed ice, shows the presence of water vapour in air.

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