Salt water evaporation science fair project

[TMM]

My son just finished up an evaporation project for his science fair that is due friday. He placed 3 glasses on the counter and watched them for 2 weeks. The 1st glass had plain tap water. The 2nd glass had tap water with salt in it. The 3rd glass had tap water with sugar in it. For the final result, the salt water is the one that evaporated the fastest. I am trying to figure out why this happened in order for him to complete the research part of his project. Everything I can find online says that tap water should evaporate at a faster rate than salt water.. Any help would be greatly appreciated. Thanks

[Amber_MIT]

You are right in that the salt water and sugar water should have evaporated more slowly. The reason for this can be explained with some advanced chemistry, but I'll try to explain in simpler terms so your son can understand it.
You can think of it this way: Dissolving the salt requires energy, so the evaporation temperature increases with the amount of salt added (you need to add more energy to get the water to "boil"). Salt is normally made up of a molecule like sodium chloride (NaCl). In water the salt crystals break apart into molecules and then each molecule form ions which are positively and negatively charged (Na+ and Cl-). You can think of the ions like magnets with opposite polarities. The water molecules are neutral overall, but one end is negative and the other is positive, so they are attracted to the salt ions. You have to add energy to break their attraction to the salt ions. Something similar happens to the sugar molecules (each sugar molecule does not break apart into ions, but they have charged groups that attract the water molecules). Maybe this could be explained to your son with some magnets? In order to get the water magnets free from the salt magnets, he would have to pull them apart (i.e. use energy).

There are many factors that could influence the rate of evaporation:

-Surface area (were all the cups the same size?)
-Measurement of the water (how accurate was your son when he measured the water?)
-The color of the cups could have affected the results (were they all the same color? Dark colors absorb more light energy than lighter colors do)
-Were all the cups on the same type of surface and in the same location with the same amount of sunlight?
-Were some cups close to air conditioning or a heater?
-How much salt/sugar was added? You'd probably have to add quite a bit to see an obvious difference.

Natural variation in the environment would also make it almost impossible to have the water evaporate at exactly the same rate. How different were the rates of evaporation? If the rates were close together, there might not have been enough sugar or salt to see the difference.

Hope that helped. Let us know if you have more questions!

[Amber_MIT]

I forgot to mention that in his write up, he should explain what he think happened and why. It is perfectly fine that the experiment didn't turn out as expected (this happens in science all the time ). The teacher should be impressed that he tried to troubleshoot the experiment and really think about his results.

[TMM]

Thank you for all your help. That was very good!