The evaporation:

Evaporation is the changing of a liquid into vapors (gaseous state) from the surface of the liquid without heating it.

Evaporation causes cooling:

Evaporation plays an important role in our daily life. Ours clothe dry up rapidly when spread. During evaporation; fast-moving molecules escape out from the surface of the liquid. Molecules that have lower kinetic energies-l are left behind. This lowers the average kinetic energy of the liquid molecules and the temperature of the liquid. Since the temperature of a substance depends on the average kinetic energy of its molecules. Evaporation of perspiration helps to cool our bodies.

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Evaporation takes place at all temperatures from the surface of a liquid. The rate of evaporation is affected by various factors.

Factors affecting the rate of evaporation:

1. Temperature:

Why do wet clothes dry up more quickly in summer than in winter? At a higher temperature, more molecules of a liquid are moving with high velocities. Thus, evaporation is faster at high temperatures than at low temperatures.

2. Surface area:

Why does water evaporate faster when spread over a large area? The larger the surface area of a liquid, the greater number of molecules have the chance to escape from its surface.

3. Wind:

Wind blowing over the surface of liquid sweeps away the liquid molecules that have just escaped out, this increases the chance for more liquid molecules to escape out.

4. Nature of the liquid:

Do spirit and water evaporate at the same rate? Liquids differ in the rate at which they evaporate and spread a few drops of ether or spirit on your palm. You feel cold, why?

Mini Exercise

1. How does specific heat differ from heat capacity?

2. Give two uses of the cooling effect of evaporation.

Ans:   Uses of cooling effect by evaporation:

1. During hot summers, the water is usually kept in the earthen pot to keep it cool. Water is cooled in the pot since the surface of the pot contains large pores and water seeps via their pores to the outside of the pot. This water evaporates and takes the latent heat for vaporization hence retaining the water inside the pot to be cooled.
2. Especially in villages, people often sprinkle water on the round in front of their homes during hot summers.
3. Water vaporization from leaves of trees also cools the surroundings.
4. A desert cooler cools better on a hot and dry day.
5. It is a common observation that we can sip hot tea (or milk) faster from a saucer than from a cup.
6. Wearing cotton clothes on summer days to keep the body cool and comfortable.
7. Put a little spirit on your hand and wave around, the spirit evaporates rapidly and our hands feel cooler.

3. How does evaporation differ from vaporization?

Ans:    Difference between vaporization and evaporation:

Vaporization:

Vaporization is a transitional phase of an element or compound from a solid phase or liquid phase to a gas phase. It changes matter from one state or phase into another without changing its chemical composition.

Vaporization has three types:

(i) Boiling ii.    Evaporation iii.   Sublimation

Evaporation:

Evaporation, wherein the transition from the liquid phase to the gas phase takes place below the boiling temperature at a given pressure and occurs on the surface.

COOLING IN REFRIGERATORS

Cooling is produced in refrigerators through evaporation of liquefied gas. This produces a cooling effect. Freon, a CFC, was used as a refrigerator gas. But its use has been forbidden when it was known that CFC is the cause of ozone depletion in the upper atmosphere which increases the number of UV rays from the Sun. The rays are harmful to all living matter. Freon gas is now replaced by Ammonia and other substances which are not harmful to the environment.