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Name: Debbie
Status: other
Grade: other
Location: CT
Country: N/A
Date: March 2006

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!



Replies:
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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