Evaporation process


What is Evaporation?

Evaporation definition can be stated as “It is the process by which a liquid or solids change to gaseous state or vapors state at the free surface through transfer of heat energy”.

Evaporation Process

When the external thermal energy supplied to surface of water body, the kinetic energy of water molecules will be increased. When the molecules near the free surface attain enough kinetic energy, they escape from the water body they eject themselves in to the atmosphere.

Out of total atmospheric pressure on the free surface there will be some contribution from the vapour molecules present in the free surface. This partial pressure exerted by the vapor is called vapor pressure.

Continued supply of heat energy causes accumulation of more and more vapour molecules and thus gaseous medium can no longer accommodate and reject vapour molecules in the form of condensation at the same rate as vaporization. At this stage the air is said to be saturated. 

At saturation the partial pressure exerted by water vapour is called the saturation vapour pressure and denoted by (es) which increase with temperature.

Thus, if vapour pressure of air above free surface of water is already equal to the saturation vapour pressure (es) neither evaporation no condensation takes place and then it is called as equilibrium state.

From the above explanation for evaporation to occur it is necessary to have:

a. A supply of water

b. A source of heat

c. Vapour pressure deficit, i.e. difference b/w saturated vapour pressure of water correspond to water temperature.

Factors Affecting Evaporation

1. Vapour pressure difference: The number of molecules leaving or entering a water body depends on the vapour pressure of water body at the surface and also the vapour pressure of air. If  water has Higher temperature then it leads to high vapour pressure at surface and tends to increase the evaporation rate. High humidity in air tends to increase vapour pressure in air and in turn reduces rate of evaporation.

2. Temperature of air and water: The rate of emission of molecules from a water body is a function of its temperature. At higher temperature molecules of water have greater energy to escape. Hence maximum evaporation from water bodies takes place in summer. It has been estimated that for every 1o C rise in atmospheric temperature increases 5 cm of evaporation annually.

3. Wind Velocity: When wind velocity is more the saturated air (humid air) is drifted away and dry air comes in contact with water surface which is ready to absorb moisture. Hence rate of evaporation is dependent on wind velocity. It has been estimated that 10% increase in wind velocity increases 2 – 3% of evaporation.

4. Quality of water: The rate of evaporation of fresh water is greater than saline water. (Specific gravity of saline water is greater than that of fresh water. It is established that saline water has lesser vapour pressure and it is observed that evaporation from fresh water is 3 –4% more than sea water.

5. Atmospheric pressure and Altitude: Evaporation decreases with increase in atmospheric pressure as the rate of diffusion from water body into the air is suppressed. At higher altitude the atmospheric pressure is usually lesser and there by evaporation rate is higher.

6. Depth of water body: Evaporation shallow water bodies is greater when compared to deep water bodies as the water at lower levels in deep water bodies is not heated much and vapour pressure at lower levels is also reduced.

7. Humidity: If the humidity of the atmosphere is more the evaporation will be less because during the process of evaporation, water vapour, moving from the point of higher moisture content to lower moisture content and rate of this movement is grounded by this difference of their moisture content or moisture gradient existing in air.

8. Radiation: Since the evaporation requires continuous supply of energy which is derived mainly from solar radiation. The radiation will be a factor of considerable importance. Evaporation increases and the radiation increases and vice versa.

Evaporation Examples

Evaporation examples can be seen in your own house, where liquids changes into vapours or gaseous state. As it is a part of evaporation water cycle, some of examples like

1. Drying clothes under the sun: 

As you can see the wet clothes or laundry, placed outside under the sun, where evaporation of water takes place from liquid to vapours.

2. Pressure cooker or steam kettle:

Just have a glance at the kitchen when your mom is cooking, where the pressure cooker containing water turns into vapours.  

3. Sweating:

Your own body is an example for evaporation, When your body gets hot the sweat is released from the skin, and slowly the evaporation water takes place as body gets cool down.

4. Evaporation water bodies:

You can see water bodies like lake, river and well gets dried up in summer, that’s due to evaporation.

5. Glass of water:

Take some water in a glass or pan, Place it outside under the sun, by the evening half or full(Depends on evaporation rate)water is evaporated, thats whats evaporation. 

Also Read – Methods to Control Evaporation From Lakes

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