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)
(republished on 07/24/06)