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Q & A: Evaporating saltwater

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Most recent answer: 01/09/2012
Q:
Does Salt water evaporate faster than plan water and does the salt to water ratio make a difference in the evaporation?
- Katlyn (age 10)
Blue Mountain Elem, Pine Grove, PA
A:
I would expect the salt water to evaporate more slowly, all other things being equal. Evaporation requires 540 calories per gram to turn the liquid water into vapor, and it also requires that the water vapor be carried away by the air. If you can keep the temperatures and airflow rates and humidity the same in a controlled experiment (not too hard, just put two identical glasses filled water next to each other, one with saltwater), you can do the controlled experiment to see which one evaporates faster.

The vapor pressure of saltwater is less than that of pure water at the same temperature. At the air-water boundary, water molecules are constantly flying off of the surface and falling back. Some manage to diffuse away through the air. The partial pressure of water molecules right above the surface should be higher for pure water, and therefore they should diffuse away at a higher rate.

Why don't you do the experiment, with different salt concentrations? Your differences in evaporation rates may depend on other things, like the temperature and the humidity.

Here's a possible exception to what I just said. Salt lowers the freezing point of water, so at some temperatures (just below 0 degrees Celsius), your saltwater may be still liquid while your pure water will be frozen. Then the saltwater should require less heat input to evaporate (taking only around 540 calories per gram instead of 540+80 calories per gram). If the slowest part of the process is for heat to leak into the water from outside, then the saltwater could evaporate faster than the ice. If the slowest part is for the water molecules to diffuse away in the air, then the ice will evaporate faster because it still has a higher vapor pressure.

Tom (and mike)

(published on 10/22/2007)

Follow-Up #1: quickly evaporating saltwater

Q:
I did this experiment several times but saltwater evaporated faster can you please explain why?
- suzan (age 39)
rowlett, tx usa
A:
We've heard this several times. The best explanation we know of is as follows.

As the saltwater starts to evaporate, it leaves a salty layer on the sides of the glass. This layer stays moist, and that leaves a bigger area for the water to evaporate from. So that speeds up the process, even though the evaporation from the main surface is slower for the saltwater.

We're not sure that's right, but it does make sense.

Mike W.



(published on 01/09/2012)

Follow-up on this answer.