Thread: Thermodynamics-Evaporation of water

water can boil at room temperature itself if we decrease the pressure.So to boil water at room temp say 28 degrees C we decrease pressure from 1 bar to 0.035 bar(from steam tables).
My question is- when we start decreasing the pressure (at 28 *C) from 1bar,does the water wait until pressure reaches 0.03 bar to evaporate OR WILL THE PROCESS OF EVAPORATION START AS SOON AS THE PRESSURE STARTS DECREASING?

Does this have anything to do with the fact that the more altitude seemingly water takes longer to boil?

Or am I mistaken?

Originally Posted by satvik

My question is- when we start decreasing the pressure (at 28 *C) from 1bar,does the water wait until pressure reaches 0.03 bar to evaporate OR WILL THE PROCESS OF EVAPORATION START AS SOON AS THE PRESSURE STARTS DECREASING?

Water evaporates at normal temperature and pressure. The rate of evaporation will increase at lower pressures (I don't know how you would work out how it changes, though).

Originally Posted by babe

Does this have anything to do with the fact that the more altitude seemingly water takes longer to boil?

It should be quicker at altitude because you don't have to heat it so much. But then again, it may be colder at altitude so that could cause it to take longer.

Originally Posted by Strange

Originally Posted by babe

Does this have anything to do with the fact that the more altitude seemingly water takes longer to boil?

It should be quicker at altitude because you don't have to heat it so much. But then again, it may be colder at altitude so that could cause it to take longer.

I just remember cooking at a cabin and it took forever. Mahalo for your reply.

Originally Posted by babe

I just remember cooking at a cabin and it took forever.

Could also be the quality of the catering equipment in the cabin.

And cooking may take longer because the water is boiling at a lower temperature.

Originally Posted by Strange

Originally Posted by babe

I just remember cooking at a cabin and it took forever.

Could also be the quality of the catering equipment in the cabin.

And cooking may take longer because the water is boiling at a lower temperature.

CATERING? *howling* I'm a hell of a cook.....the equipment however, was not grade A, so possibly Sir Strange, the quality of the "catering" equipment could be part of the issue. Yes...when I throw fresh picked corn in ...it's minutes to perfect not Too many minutes! *chuckle*...it's ok....everyone loved it and no one dies....so that equals success...but I did notice the difference in cooking. The cabin was a friends. I had no control on the utensils.....

Originally Posted by PhDemon

Evaporation of water is constantly occurring at all temperatures and pressures (you even se water evaporating from ice surfaces), it just gets slower as the temperature is reduced or the pressure increased. The equation (which I'm sick of the sight of as I use it everyday for calculating vapour pressures from experimenal evaporation rate data from small droplets) is:




For a flat bulk surface this equation is:




In these equations M[sub]w is molar mass of water, and are the ambient pressure of water at the surface and at infinite distance from the surface, T is temperature, D is diffusion coefficient of water in air R is the gas constant and SA is teh surafce area of the flat water surface.

However, my leetle Sir Demon...does not altitude affect boiling water and times in cooking? I believe that is what I have always known is that it took LONGER the high the altitude.

Originally Posted by PhDemon

Yes because at higher altitude water boils at a lower temperature as described by the Clausius-Clapeyron equation. When you are boiling potatoes at altitude you are cooking them at a lower temperature than at see level so it takes longer.

Which was my understanding. Mahalo

The boiling will take less time. The cooking will take longer.

an old phrase about one's ability for/at boiling water is tugging the edge of my memory
...............
for one brief fleeting moment
......
gone now
sigh

Notice many people confuse the different concepts of boiling and evaporation.

Note the equations PHdemon's posted discuss partial pressures, those are the pressures for only the water vapor, not the lower total pressure for all gases being discussed for higher altitude and increased cooking times.

Thank you Harold, Sculptor, and Lynx_Fox!!!!!! I appreciate it!

Originally Posted by PhDemon

Evaporation of water is constantly occurring at all temperatures and pressures (you even see water evaporating from ice surfaces), it just gets slower as the temperature is reduced or the pressure increased.

So in space ice crystals are shrinking, very very slowly?

I think I read somewhere that much of the evaporated water immediately re-attaches to its parent in space... but this seems wrong. Wouldn't the vapour zing away since there's nothing to stop it?

Sorry....this just brought this song to mind.....Zip-a-Dee-Doo-Dah (Original) - YouTube

Originally Posted by PhDemon

Why would it zing away? The kinetic energy of the evaporated water molecules depends on temperature. As the temperature is very low in space the kinetic energy of the water molecules will also be very low. They will be moving very slowly and hence stay in the vicinity of the ice for a long time. This may be why most of it recondenses onto the the ice...

So, dah dew don't zing away anyday. Okay.

I'd imagined liberated molecules "pop off" regardless of temperature... that's what I get for trusting to imagination.