Thermodynamics

Evaporation and boiling

Evaporation is different from boiling besides this in both processes evaporation and boiling change of state involved. Evaporation is the change of state of liquid into vapours.

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 molecules of water are in constant motion and possess kinetic energy. Fast-moving molecules escape out from the surface of the water and go 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 the surface but also within the liquid. These vapours come out of the boiling liquid as bubbles which break down on reaching the surface.
 
 

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 a fixed or constant temperature, this process of changing state is called boiling. This temperature at which liquid change into vapours is called the boiling point.

Distinguish between evaporation and boiling

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

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