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Water, Salt Water Evaporation
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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Update: June 2012
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