I always wonder why this happens...

 

if I place a cup of water on the table, after a very long time, the water is gone...

 

why's that? why doesn't it remain liquid water?

because there is energy in the air. It takes 1 cal. of energy to evaporate 1 gram of water. If you're not adding heat, then it takes longer to evaporate, energy is being transfered much slower.

????!!

 

then why we call 100c the evaporating point? since even room temperature can evaporate as you say....

100c is the boiling point not the evaperating point

????!!

 

then why we call 100c the evaporating point? since even room temperature can evaporate as you say....

 

it's the boiling point... look at the link http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html

well,.

 

boiling point indicates the amount of heat to change water from liquid into gas...

 

I think evaporating is the same....

 

any help here?

Nope. The boiling point is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure.

this is abit confusing...

 

if water is evaporated in room temp, then, its pressure does not equal to the ATM?

At room temperature vapor pressure is much lower than atmospheric presuure, thus the liquid (call it water ) evaporates only on the surface. Once the liquid is heated to the boiling temperature, it can evaporate not only on the surface, but also inside of its volume, That's the difference between evaporating and boiling.

 

Best,

Borek

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thanks Borek....

The molecules have a distribution of energies. Some fraction will have enough to escape; this lowers the average energy of the remaining molecules, which is why evaporation is a cooling process.