What direction does thermal energy travel during evaporation and condensation?

Question

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Answer 1

It will generally travel in the direction of lower temperature. When steam evaporates it leaves the hot surface/area and is quickly replaced by a colder mass of water.

Answer 2

'Heat Transfer' takes place from a Hot substance to a Cold substance and results in temperature changes.

However, in the change of state, the Heat Exchange Transfer takes place with no Temperature Change.

The Heat DOES pass into or out of the substance but, it only increases or decreases the molecular motion (Kinetic Energy) of the molecules  without changing the temperature.

In Evaporation, the Latent Heat is passing INTO a liquid and the energy goes into breaking the molecular bonding of the liquid enabling the molecules to pass into the Vapour Phase without change in temperature..

In Condensation, the Latent Heat is passing OUT OF the vapour into the surroundings or into the cooler condensing medium having the opposite effect to Evaporation but, the vapour and condensate temperatures remain the same..

Some people like those who love giving thumbs down to correct answers, obviously know NOTHING about Heat Transfer Principles and Equipment....something I've work with in the Oil & Gas Industry for most of my working life of 51years....

If you're thinking that Temperature changes when Condensation or Evaporation (Vaporisation) takes place.... Think again...

Answer 3

If you mean heat when you say "thermal energy" then heat goes out of the liquid during evaporation and into the liquid during condensation.  This is why the flux of water vapor into or out of the ocean is called latent heat.  

Just to be pedantic, temperature gradients between the liquid and the gas phase aren't the central issue in evaporation and condensation.  (This is where I will get confusing and possible get you angry, but that last sentence is true and if you assume it is strictly temperature that controls evaporation and condensation and get into an argument with someone like me, who has suffered (and I mean suffered) through Denbigh's book "Thermodynamics of Phase Equilibria" without appreciating why temperature isn't central, they will take you apart.)

All liquids have what is called a vapor pressure.  That vapor pressure is determined by temperature.  But even though the vapor pressure is a function of temperature of the liquid, it is still not temperature that controls whether the liquid will gain or lose heat through condensation or evaporation, respectively.  What does control whether the liquid will evaporate or condense is the difference between the vapor pressure of the liquid and the partial pressure of those same molecules in the gas phase above the liquid.  If the vapor pressure of the liquid is higher than the gas-phase partial pressure, evaporation occurs and the fluid loses heat through evaporation.  If the vapor pressure of the liquid is lower, condensation will occur and the fluid gains heat.  

So, while temperature controls the partial pressure of the liquid, it is not necessarily true that the partial pressure of the molecules of the liquid in the gas phase above the liquid is a function of the temperature of the gas phase.  Confusing, I know, but what that means is that you can have situations where you have a cold liquid with warmer gas above it that contains no molecules of that liquid in the gas phase.  In that case, you would have sensible heat going from the gas phase to the liquid phase (since the gas is warmer than the liquid) but the liquid would be evaporating since its vapor pressure is greater than the partial pressure in the gas phase.  So while heat goes from hot to cold because of the temperature gradient, latent heat from evaporation is going from cold to hot.  

This situation can occur in the ocean, when warm dry air blows out over a colder ocean.  The sensible heat flux is into the water, while the latent heat flux is into the air.  Freaky but true.

edit:

pssssst Norrie:  

http://www.colorado.edu/physics/2000/bec...

http://en.wikipedia.org/wiki/Evaporative...