Water, Salt Water Evaporation

name         Debbie
status       other
grade        other
location     CT

Question -   My third grader was doing an experiment to see if 
salt or plain water would evaporate first.  We did much research 
(including your web site) and expected to confirm that the plain 
water would evaporate first (or that the difference would be 
undetectable).  However, our experiment (repeated to be sure) had 
the salt water evaporating first.  Is the difference in the ratio 
of salt to water?  (We used 1 part salt to 2 parts water and then 
heated 6 ounces of the salt water solution and compared it to 6 
ounces of plain water.)  After 50 minutes in the oven (at 400 
degrees Fahrenheit), the salt water had evaporated down to 2 ounces 
but there was still 4 ounces of plain water remaining.  The two 
samples were in the same oven and they were in identical containers 
(2 cup Pyrex measuring cups).  Can you explain?  Thank you for any 
light you can shed on this problem!
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Debbie,

Your results are contrary to my expectations as well. One of the 
most important properties of solutions is that its vapor pressure 
lowers (the number of solvent particles in gaseous form decreases) 
as more solute is added. Moreover, as the solvent evaporates, this 
effect becomes more and more pronounced since there will be more 
solute (the salt) in the remaining solution (because of the reduced 
amount of solvent (the water). Thus, one would expect that the 
solvent should always evaporate to dryness first.

The only thing that I can think of that might have caused the result 
you observed is that both containers were in the oven at the same 
time and, I am assuming, that the kind of oven that seals very 
tightly. Along with the colligative property mentioned above is 
another one called osmotic pressure. This expresses the phenomenon 
where two containers containing a solvent and a solution are sealed 
together in another container (such as your oven - I am guessing), 
will cause the transfer of solvent molecules from the container of 
pure solvent to the container of holding the solution. ---why this 
happens is directly related to the first paragraph. Since the vapor 
pressure of the solution is lower than that of the pure solvent, 
then the pure solvent is releasing more solvent molecules into the 
atmosphere. At the same time, some solvent molecules that are 
already in the atmosphere (which are trapped within the oven) will 
be captured by the liquids (at an equal rate for both the solution 
and the solvent). Thus, there will be an apparent transfer of 
solvent particles from the container of pure solvent (which is 
losing solvent particles at a faster rate) to that of the solution.

If you could, please try the experiment again, but this time, do it 
with one container at a time. If your results are now in line with 
what we both expected, then the 2nd paragraph is a good explanation 
of what happened. If you still get the same results - let us know 
and we will think about this some more.

Greg (Roberto Gregorius)
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