During evaporative cooling (sweating), will the temperature of sweat reaches 100 deg Celsius (for before phase change) or is it the phase transformation temperature is much lower than 100 deg Celsius due to the Partial pressure ?
Hi Rajesh,
during evaporation, the fastest molecules from the liquid will enter the gas volume, and the gas phase. At that point in time, the kinetic energy distribution of those molecules is a strange one: all lower energies have been cut off. They are not in thermal equilibrium, so they (technically) don't have a temperature at all.
The (probable) reason you are asking about 100°C may be that you are under the impression that the phase change from liquid to gas happens (for water) at that temperature. It does not. 100°C is merely the temperature at which the pressure of water vapour reaches 1 bar.
@ Thorwald, thanks for the info.
So you mean to say the partial pressure plays the role.
Hi Rajesh,
I guess I misunderstood your question, sorry. I would say partial pressure does not play any role, it follows from the temperature of the liquid.
What do you mean by "phase transformation temperature"? The temperature of the liquid, or the minimum temperature for which one will find water molecules in the gaseous phase? That will be a very low (cryogenic) temperature indeed. I am not sure how well this point has been determined experimentally.
@ Thorwald, thanks for the info.
I mean, the latent heat is made used during sweating, right. For which temperature of sweat should increase and finally its state change to vapor. Does temperature of sweat reach 100 for phase change or is it some other phenomenon happening like mass transfer.
When the small fraction of "fast" molecules escape, then what's left over are the slower molecules. This means the average speed of the leftover molecules is a bit slower than before, which means the puddle is now a tiny bit colder than it was. That's why you may hear people say, "evaporation is a cooling process." That's why we sweat! When water evaporates from our skin, it cools us off.
I dont know whether sweat temp would reach 1000C
https://answers.yahoo.com/question/index?qid=20100107104133AAOfHlT
Hi Rajesh
Evaporation on skin does not require 100°C ; it is a matter of mass transfer as you suggest.
If you leave a recipient with water in the room and come back a few days later the water will have evaporated - at room temperature. There is a difference between evaporating and boiling.
Evaporation is what happens at the surface of the liquid. As you mention, the driving force is the water vapour pressure : on the surface it is the saturation pressure at temperature of the water psat(Twater) ; in ambient air that is the water vapour pressure corresponding to temperature and humidity RH*psat(Tair). Multiply the difference by the convection coefficient and you have the mass flux evaporating. The latent heat for evaporation being provided both by water (which cools down) and air.
Boiling is what happens if the water evaporates not from surface but INSIDE the liquid. In this case the medium for evaporation is not air as for the surface, but liquid water. And the medium being 100% water, the water vapour pressure is 100% of the local pressure, meaning atmospheric pressure (plus added pressure due to water column). That means about 1 bar ; and the saturation pressure of water vapour is 1 bar when its temperature is 100°C : that means you need to warm up water to 100°C if you want water to turn to vapour INSIDE the liquid.
Evaporation is mass tranfer driven by water vapour pressure ; the heat transfer and cooling is the consequence.
For boiling, the heat you provide and the resulting temperature is the driving factor, the evaporation is the consequence.
Going back to sweating, the sweat comes up as liquid at body temperature through the sweating duct, and arrives as liquid on the surface of the skin, from which it evaporates in air. No boiling in the duct (no 100°C), just evaporation from surface.
I hope that addresses your question.
Bernard
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