evaporation

[ej45]

Evaporation, My next thinking question is....
If I have a plate of water sitting in a room uncovered, why does it evaporate?
If I have a sealed jar of water sitting in a room, why does it not evaporate?
ej45

[Dad]

The water on the plate is open to surrounding air that is humid. The humidity level in the surrounding air is less than 100%. The water in the glass is 100%. The surrounding air will draw the higher humidity and the current will carry it away until the water in the glass is gone, thus evaporation.

The top on the glass is a barrier to the lower outside humidity.

Higher levels of humidity will always travel to lower humidity like heat always follows cold.

[Dad]

It has been a while since physics class... and wayyyy to early in the morning..

[ej45]

I guess that was to easy.

[Carnak]

Water has a P/T relationship just like freon. If you go to Denver there is less atmospheric pressure on the water and the boiling point of water drops. Your car rad has a spring loaded cap which allows pressure to build up a bit, and raises the temperature at which water will boil.

The water vapour pressures in the table are absolute pressures and they are all below atmosheric pressure of 14.696 pais. These vapour pressures are like the "in HG" readings on your guages.

Do a Google Search on "Dalton's Law of Partial Pressures" it explains why water evaporates and how the propane fridges in RVs work.

Here is a quick summary of daltons law. If we had a container that was a vaccuum we could add oxygen to the container until the pressure was 3 psia. Oxygen has certain properties under this pressure and it will behave accordingly how it would under 3 psia pressure.

Now we add nitrogen to this container unitl the pressure in the container is 14.46 psia. Inside this container oxygen is making 3 pounds of pressure and nitrogen is making 14.46-3= 11.46 psia. The oxygen behaves as if it is still under 3 psia pressure and the nitrogen acts like it is under 11.46 psia pressure.

We could add a liitle bit of CO2, methane, ozone, neon, helium, argon etc and pretty soon we would have a cylinder of a mixed gas at 14.696 psia which would be the same thing as container of bone dry atmospheric air.

Below 212F, water can evaporate until the partial pressure of the water vapour in the air is equal to the vapour pressure of water at that temperature.

For example at 72F, water is free to evaporate until the partial pressure of the water in the air is 0.3887 psia. At this point air is 72F at 100% RH.

If the air was 72F, and the partial pressure of water vapour in the air was 0.1944 psia, it would be 50% relative humidity, because the partial pressure of water vapour in the air is half of the maximum 0.3887.

When water is heated to 212F, its vapour pressure equals atmospheric pressure. At this point, as long as it has enough heat, it can rapidly change to a vapour, it is no longer being held back by atmospheric pressure.

[speedracer]

That was a very good explanation.

[RichardL]

I was really wanting to see an explanation of how "Ice" can evaporate in a rrom much less than 32°f without ever changing to the water state.....

[ej45]

Originally posted by RichardL
I was really wanting to see an explanation of how "Ice" can evaporate in a rrom much less than 32°f without ever changing to the water state.....

thats called dynamic chomping

[Carnak]

Originally posted by RichardL
I was really wanting to see an explanation of how "Ice" can evaporate in a rrom much less than 32°f without ever changing to the water state.....

I guess we would have to search sublimation there Sir Richard, I never gave that much thought beyond the analogy of a mothball

[Carnak]

When Wilma dropped to 882 millibars of pressure, I think that was about 12.79 psi.

This would have brought the boiling point of water down to probably under 205F.

Without the effect of driving winds, this would have allowed SALT water to rise about 4'3" above sea level.

[Dad]

You guys have wayyyy to much free time..