Thermodynamics

# Evaporation and boiling

If you are moping the floor in home you will observe that the wet floor or surface of floor becomes dry very early after a while.We can say that thin layer of water on floor surface has evaporated.Evaporation is the change the change of state of liquid into vapours. Evaporation is different from boiling besides this in both processes evaporation and boiling change of state involved.

## What does evaporation mean?

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

Take some water in a dish.The water in the dish will disappear after some time.It is because the the molecules of water are in constant motion and possess kinetic energy.Fast moving molecules escape out from the surface of water and goes into the atmosphere.This is called evaporation.

Unlike boiling,evaporation takes place at all temperature but only from the surface of a liquid.The process of boiling takes at a certain fixed temperature which is the boiling  point of that liquid.At boiling point,a liquid is changing into vapours not only from surface but also within the liquid.These vapours come out of the boiling liquid as bubbles which break down on reaching the surface.

## Factors effecting evaporation

• #### Temperature

At higher temperature,more molecules of a liquid are moving with high velocities.Thus,more molecules escape from its surface.Thus ,evaporation is faster at high temperature than at low temperature.

• #### Surface area

Larger is the area of a liquid ,greater number of molecules has the chance to escape from its surface.Therefore there will be more evaporation at sea than river and small reservoirs of water.

• #### Wind

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

• #### Humidity of surrounding air

When we say that air is humid it means there is a lot of water vapours present in the air.The rate of evaporation decreases with the increasing of humidity.For example wet clothes do not dry easily if surrounding air is wet and damp in home take a long time to dry on a rainy day.From this example we concluded that rate of evaporation increases with decreasing humidity.

In an other example,if we sleep in an air conditioned room where the humidity of air is very low,due to this our skin and throat feel very dry.

• #### Pressure

Evaporation increases by the decreasing of atmospheric pressure.For example wet objects dry rapidly on the mountain top where atmospheric pressure is less than sea level.

## How evaporation causes cooling?

Evaporation causes cooling because water on your body is evaporating.Evaporation needs heat and in this process or example heat comes from our body.Since our body loses heat when the water evaporates,we feel cold.

## Boiling

When the pure liquid is heated it changes to vapour at fixed or constant temperature,this process of changing state is called boiling.This temperature at which liquid change into vapours is called boiling point.

## Distinguish between evaporation and boiling

 Evaporation Boiling It occurs at any temperature. It occurs at fixed temperature. It is a slow process. It is a fast or quick process. It take place only on the surface of liquid. It takes place in the liquid. In this process no bubbles are formed in liquid. In this process bubbles are formed in liquid. Temperature of this process may change. Temperature remains constant in boiling process. Heat supplied by the surroundings. Heat  supplied by an energy source.

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