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Name: Heather
Status: educator
Grade: K-3
Location: CA
Country: N/A
Date: February 2007

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.

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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