Name: Heather
Status: educator
Grade: K-3
Location: CA
Country: N/A
Date: February 2007
Question:
One of my students did a science fair project on
the evaporation of salt water and tap water. He used controls
throughout his experiment. He used the same plastic measuring cups,
14 ounces of water in each cup and than added 1 cup of salt to one
of the two cups. He put thermometers in each cup to make sure the
temperature was the same for both the salt water and tap water. He
also placed a heat lamp on top to speed up the evaporation process.
Each day he checked his experiment and the temperature for each cup
of water was 100 degrees. When he was finished with the experiment
the salt water had evaporated faster. I had thought the tap water
would evaporate faster because the vapor pressure of near-pure
water is higher than that of water with solutes. I noticed that his
cup with salt water had crusty salt deposits on the side of the
cup. I heard that hygroscopic could be the reason for the salt
water evaporating faster. Could you provide me more information
about this so I can help my student.
Replies:
So try testing your theory: the cup and the salt needs to be
weighed before and after to see if there is water in the salt
crystals.
Jeannine M. Durdik
Professor of Biological Sciences
Unversity of Arkansas
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