Evaporation

CLASS-9

1. Where does the water from the clothes go when they are spread to dry?

3.I kept a glass of water on my window it disappeared in a week where did it go?

2.Why do rivers dry up in summers?

Water from the clothes and rivers is lost to the atmosphere i.e it has been converted to vapour or gas.

The conversion of liquid into vapour either by exposing to air or by heating it below it’s boiling point is EVAPORATION.

Particles evaporate very slowly or are not able to evaporate from unexposed surface

Evaporation is the escape of fast moving particles from the surface of the liquid to the atmosphere

OR

Particles of matter are constantly in motion and posses kinetic energy besides they are attracted to each other by the force of attraction between them .

Particles in liquid also keep on moving and colliding with each other. The particles at the surface are attracted only from three sides and due to collision with air gain extra kinetic energy and break away from the bulk of liquid and move to the atmosphere.

Vapourisation

Often evaporation and vapourisation are thought to be same because in both cases liquid is changed into vapour state

DIFFERENCE

EVAPORATION

• Takes place at all temperatures
• Temperature changes during evaporation
• A very slow process
• Takes place only at the surface.
• Boiling does not take place

VAPOURISATION

• Takes place only at the boiling point.
• Temperature does not change during vapourisation.
• A very fast process.
• Takes place in the entire body of the liquid
• Occurs only when boiling takes place

EVAPORATION VS VAPOURISATION

FACTORS AFFECTING EVAPORATION

1.NATURE OF LIQUID

Lower is the boiling point of a liquid more is the rate of evaporation. Ether (B.P34⁰C) evaporates faster than water(B.P100⁰C)

More is the temperature of the liquid more is the rate of evaporation.

​

Evaporation slow

Evaporation fast

2.TEMPERATURE OF THE LIQUID

Evaporation is slow at night due to low temperature. Clothes take a longer time to dry.

Clothes dry faster during the day

Larger is the surface area of the liquid higher is the rate of evaporation.

Water in tank will evaporate faster as compared to that in glass.

3. SURFACE AREA OF THE LIQUID

Folded towels take maximum time to dry.

Lesser time to dry.

Fully spread towels take least time to dry due to large surface area

In an airy place rate of evaporation is faster. Wind carries away the particles of vapour faster.

4. WIND VELOCITY

The amount of water vapour present in air is called humidity. In humid air the rate of evaporation decreases

In the rainy season the rate of evaporation is slow as air is already laden with moisture and cannot take more

5. HUMIDITY

VARIOUS FACTORS AFFECTING EVAPORATION.

When we are wet with sweat and sit under the fan we feel cool and sweat disappears.

Does evaporation has any relation with cooling?

COOLING EFFECT OF EVAPORATION

Particles at the surface of the liquid take heat equal to it’s latent heat from the surrounding and get converted into vapour thus resulting in the decrease in temperature of the surrounding and hence cooling.

APPLICATIONS

1.Cooling by desert coolers– moist air from the cooler touches our skin, water takes heat from our skin and evaporates thus causing cooling.

2.Cooling of water in an earthen pot—particles at the surface take heat from the bulk of the liquid, become more energetic and escape into air as gas. This results in the fall of temperature of the liquid. The liquid is thus cooled.

Fill in the blanks

1. Evaporation is a ______________phenomenon whereas vapourisation is a bulk phenomenon.
2. ______________ involves attainment of boiling point.
3. Increase in humidity ___________ the rate of evaporation.
4. __________ is the amount of heat used to convert 1kg of liquid into vapour state.
5. Alcohol will evaporate _____ than water.

e

surface

Vapourisation

decreases

Latent heat

faster

1. A Sample of water under study was found to boil at 102⁰C at normal temperature and pressure. Is the water pure? Will this water freeze at 0 ⁰C. comment.
2. Water as ice has a cooling effect whereas water as steam may cause severe burns explain these observations.
3. Why do leaves acquire more leaves during summer?
4. Why should we wear cotton clothes in summer?

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