harsh chemistry ppt

Name-HarshClass - 9th

dRoll no -

34Chemistry

Project

Evaporation

•Evaporation

Evaporation is nothing but a thermal separation process. Theoretically, evaporation means simply

vaporization from the surface of a liquid. Vaporization of a liquid below its boiling point is called

evaporation. Thus, no boiling occurs and the rate of vaporization depends on the diffusion of vapour through the boundary layers above the liquid.

During Evaporation

• Water turns from a liquid to a gas.

• Water moves from rivers and lakes into the atmosphere in the form of water vapor.

• Water vapor forms clouds.

• Water changes into solid.

•Factors Affecting

EvaporationThere Are 4 Factors To Affect Evaporation. 1) Temperature 2) Surface Area 3) Humidity4)wind Speed

•TemperatureIt is also affected by temperature. As the temperature of air is increased, its capacity to hold moisture also increases . The rate of evaporation increases on increasing the temperature of the liquid. The rate of evaporation of a liquid can be increased by heating it. Hence evaporation is directly proportional to the temperature of evaporating surface.

Warmer the evaporating surface, higher the rate of evaporation

Humidity

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 .

Wind Speed

Evaporation depends on the wind speed as well. When the winds are light, a thin layer of air just above the surface gets nearly saturated; under the circumstances the difference between the vapour pressure between ground and air is very small . This results in very low evaporation. On the other hand, when the wind velocity is high, turbulence is set up in the air. Moisture evaporated from the ground is mixed upward and the vapour-pressure difference between the atmosphere and the surface remains large.Thus, the rate of evaporation is accelerated. Wherever there is a combination of high temperature, very low relative humidity and strong wind, the rate of evaporation is exceptionally high.

Surface AreaThe rate of evaporation increases on

increasing the surface area on

the liquid.The rate of

evaporation can be increased by

increasing the surface of the

liquid.

Evaporation Cause Cooling?

• In an open vessel the liquid keeps on evaporating. The particles of liquid absorb heat energy from the surrounding to regain the energy lost in the evaporation. • This absorption of energy from the

surroundings make the liquid cool.

Cooling by evaporation

•Air is blown into the dichloromethane liquid.•This liquid rapidly evaporates.•The rapid evaporation causes cooling.•Eventually the water below the can freezes the can to the block of wood.

1 Cooling effect of evaporation

When we apply a perfume/alcohol to our body... We feel cool. Perfume/alcohol evaporates easily and takes the energy from your body.

Cooling of a Deodrant on hand

• When perfume is put on our palm, it feels cold because the particles of matter gain energy from our surroundings or from our palm and evaporate at a fast rate causing our palm to feel cold.

Water kept in an earthen pot

• Water kept in an earthen pot becomes cool after some time because of the tiny holes present in the pot, which helps the water to evaporate. And due to this the water becomes cool in a short period of time .

We can easily catch a cold

when we are wet. As we dry out, latent heat is

taken from our body. If it is

windy, we can cool down even

more.

2.Cooling effect of evaporation

3.Sweating• When we are hot , water is produced by our skin.• As this water evaporates it cools our skin.

Drying clothes•Evaporation of water from wet clothing is encouraged by:– warming (the Sun)– blowing air across

(wind)– maximizing the area

exposed (hanging from a line)

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