Question in the evaporation of water

[gavoh3d]

Suppose that I have a 1 m3 of air humidity ratio 20%, and put this amount of air with a small amount of water at a temperature of 30, of course, part of the water will evaporate, my question is: Is the size of the air will increase? Or is the air pressure will increase? Although it will increase the rate of increase
Thank you

 

[MortenA]

Think about it as a closed box of 1 m3 initially including the water (not evaporated)

You could find a density for each phase, but also an average density for the box not taking the two phases into account.

Now if all the water evaporates (assuming that the temperature is kept consatnt) will the average density change then? No, also the no. of moles in the entive volume is constant, and therefore the pressure will nto change considering PV=zVnRT

I think the change in Z is so small that it will not matter.

Best regard

Morten

 

[LostHippie]

Well it really depends on the units of temperature as to how the system behaves...30 degrees Celsius is very different from 30 kelvins.

 

[IRstuff]

or °F, for that matter. Also, what is the temperature of the air itself?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[gavoh3d]

Thanks to all,
@MortenA: I think so, the pressure doesn't increase
@LostHippie & : I means .30 degrees Celsius
@IRstuff: Suppose that the air box in 30 degrees Celsius also

Thanks a lot

 

[LostHippie]

Seems to me that if you are considering the gaseous mixture of Air and water vapor, and assuming that the temperature remains constant, with an RH that low you will certainly get evaporation, which in the case of the aforementioned system would act as a mass flow into the system, in the form of water vapor. To a very small extent this will increase the pressure of the vapor mixture (more molecules/unit volume @ the same temperature = more molecules impacting the container aka the water vapor will add a partial pressure to the system.

 

[IRstuff]

The partial pressure of water is about 4.2 kPa. Since the RH is 20%, there should be about 3.4 kPa increase in pressure from the extra water.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MintJulep]

Water will evaporate until the liquid water is all gone, or the partial pressure of the water vapor reaches the saturation pressure for water at 30C.

From there the mixture will obey Dalton's Law of Partial Pressures.