Reef Chemistry Question of the Day #187 Dr Who and Evaporation!

[Cory]

Dr Who is a good show. I dont know the answer.

But using my psychic abilities i think its 4.

 

[SROPE]

I'm thinking #3.

 

[nervousmonkey]

what Habib said

My thoughts as well.

 

[Randy Holmes-Farley]

And the answer is ... 4. The aquarium has a small amount of water and some solids in it. All of the smaller containers are full to the brim. The fan has stopped for no apparent reason. There is water on the floor.

At least that's my prediction.

Here's the rationale:

Water will move around in the gas phase and distribute itself to lower its energy as much as possible. Water is "happier" in salt solutions than in pure water, and in this sense, the saltier the better.

One clear bit of evidence of this, and it matches our experiment quite well, is the vapor pressure lowering effect of salts in water. The more dissolved salt in the water, the lower the vapor pressure of water in equilibrium with it.

The gas phase water molecules are moving around and equilibrating with the various "places" in the room. Water will come out of the aquarium (lowering the level), go into the gas phase, and dissolve into the bags and bucket.

The bags and bucket have so many ions in them that the water will keep coming out of the aquarium and into these util the salt concentration in the water about matches (in the sense of vapor pressure lowering) in the different locations. That won't finish until the bags and bucket overflows.

So water spills onto the floor, carrying a lot of excess calcium, magnesium, and sulfate.

While slowly filling, the salt bucket will get precipitates of calcium carbonate that will not redissolve, but will stay in the bucket. When the overflow puddles encounter one another, calcium sulfate likely precipitates and stays precipitated as the water on the floor grows and grows at the expense of the aquarium water.

Eventually, the water level in the aquarium has dropped to the point where the salinity equilibrates with the same vapor pressure as the water on the floor. At that salinity in the tank, calcium carbonate and calcium sulfate have precipitated. Some other chemicals may precipitate in the tank as well.

The purpose of the fan is to equilibrate the water faster by blowing air with water vapor around (which is much faster than diffusion). Eventually, however, it just wears out or corrodes and fails to work (I have many fans that have failed in only a few years... ).

Here's a description of how the vapor pressure is lowered by dissolved salts:

http://www.chem.purdue.edu/gchelp/solutions/colligv.html

Here's a page that calculates the vapor pressure of seawater relative to freshwater:

http://butane.chem.uiuc.edu/pshapley/genchem1/L17/2.html
from it:

"The vapor pressure of pure water at 20 deg C is 0.0231 atm while the vapor pressure of seawater with the composition listed above is 0.0226 atm."

 

[Randy Holmes-Farley]

None of the answers fit to what I think will happen.

#4 comes closest.

The calcium chloride will liquify and since it is in an open plastic bag, and is getting more liquid like, the bag will flip. So brim full is not a possibility.

Forgot to mention they were special stand up bags.

 

[beaslbob]

Forgot to mention they were special stand up bags.

oh now you tell us.

 

[Habib(Salifert)]

Forgot to mention they were special stand up bags.

LOL