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Q & A: Quicker evaporation of saltwater?

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Q:
Tom I am doing a science project about the evaporation rate difference between distilled, purified, spring and salt water. In your Q & Aís, salt water was said to evaporate slower than other water, but in my 3 week experiment the salt water evaporated the quickest. Can you explain this? I placed 4 quart jars side by side and filled each with 1 cup of each type of water and marked the level each day in a notebook. I also started with a fresh batch each week. I did place wire screening on top to prevent bugs and animals from getting into the water, and left them inside the house. Thank you.
- Victoria (age 10)
Millbrook Elementry, Aiken SC
A:
Hi Victoria,

I have an idea of what may be going on with your saltwater experiment. Itís a bit messy, though.

If you dissolved lots of salt in the water, you probably coated the sides of your jar with crusty salt deposits. The ions in the salt attract water molecules very strongly (we say they are \"hygroscopic\"). Iíll bet what was happening is that the salt residue was acting as a wick, causing water to creep up the side of the jar where the salt was, making it damp. This effectively increases the surface area available for evaporation. Sorry I didnít think of this effect.

The effect of salt residues on evaporation rates is mentioned in the ownerís manual of a humidifier I just had installed. It talks about the white crust of calcium salts that will inevitably form over the humidifier parts involved in the evaporation, and how this actually helps the humidifier operate by increasing the wet surface area.

Tom

(republished on 07/24/06)

Follow-Up #1: evaporating saltwater

Q:
Hi,
According to my experiment i placed 2 cups and i labeled them a and b the one that says a has only water in it tha one that says b has water and salt i left it for 4 weeks. The result was that Water with salt evaporated faster.
- Olushola (age 10)
Greenbelt,MD 20770
A:
Thanks for your results!
Mike W.

(published on 12/14/07)

Follow-Up #2: rates and levels

Q:
does,nt water without salt evaportat faster then water with salt?
- Annalise Robidoux (age 11)
Auburn, ma,us
A:
It seems that according to some typical experiments, no. It's true that in a humid enough atmosphere the salt water won't evaporate at all and the pure water will. In fact, if you put two cups together in a sealed container, one with salt and the other with water, after a while the water will all evaporate from the container without salt, and some will condense into the container with salt.

That's a fact about thermal equilibrium, the stable state that a system reaches if it isn't disturbed, which is something we can understand based on simple principles. It's closely related to more familiar facts, like water managing to trickle downhill until it settles in the lowest place.

However, the rates at which thermal equilibria are reached depend on all sorts of complications, like the ones Tom discussed above. For example, knowing that one pond is higher up a hill than another doesn't tell you which will drain faster- that depends on all sorts of details.

Mike W.


(published on 03/02/09)

Follow-Up #3: evaporation from solutions

Q:
Hello, I am doing a few experiments on the evaporation rate of salt solutions. I have four solutions (50 mOsm/kg H2O, 300 mOsm/kg H2O, 1000 mOsm/kg H20 and 1500 mOsm/kg H2O.) According to what I've reasoned out, I would expect the higher salt concentration solutions to evaporate the slowest. My reasoning is that the salt in a way "holds onto" the water in solution via hydrogen bonding. Is this correct? Also, if I have equal volume samples of each different solution under the same conditions and allow them to evaporate, then measure their osmolality, would I expect the lower one to creep up the most over time in terms of mOsm/kg. Thanks for all your help!
- Nick (age 22)
Chicago, IL
A:
The actual evaporation rates can be tricky. Sometimes the salt crust formed on the sides of the container help wick liquid up, speeding evaporation. So you can get the somewhat paradoxical result that the saltier solution evaporates faster for a while.

Your basic idea that the salt 'holds onto' the water is right though. At some fixed temperature the water won't all evaporate, but will leave behind a little stuck to the salt. The more salt, the more water left. They should all end up with the same concentration of salt.

As for how the salt holds on to the water, your idea makes a lot of sense. The tendency of the positive parts of water (H) to stick to negative ions and negative parts (O) to stick to positive ions can contribute to this effect, mainly by making it possible for lots of salt to stay dissolved in water. However, the biggest contributor to the tendency of solutes to lower the vapor pressure of water is something else, shared by even neutral solutes. It's that as liquid volume changes, the room available for the solutes to move around in changes. The formal way to say that is that as the liquid evaporates the solute entropy goes down. Nature always heads toward a maximum net entropy condition, so this favors keeping the water liquid, other things being equal.

Mike W.

(published on 06/13/09)

Follow-Up #4: Saturated salt solutions

Q:
I am currently researching on the salt-water evaporation information for an experiment in my Earth Science class, and was wondering what ratio, salt : water, can an average drinking glass hold if 1 cup of water is present?
- Dakota (age 15)
Morris, Il, U.S.
A:
Hi Dakota,
The maximum salt you can dissolve in water at room temperature is about 30% by weight.  This amount varies a bit with temperature, the higher the temperature the more salt can be dissolved.   So for a cup of water, about 130 grams, you should be able to dissolve 130 x 0.3 = 39 grams of salt. 

LeeH

(published on 05/06/11)

Follow-up on this answer.