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Name: Julie
Status: student
Grade: K-3
Location: N/A
Date: June 2007

Question:
We just did a 2 week experiment on evaporation of fresh versus salt water and the fresh water (from the tap) evaporated more than the salt (from the ocean) water? We kept the two bottles side by side in a room with a constant temperature of 82 degrees and 47% humidity (air conditioning) and kept the temperature gauge next to it to ensure that. What is the reason that the FRESH water evaporated more?



Replies:
Think of the water as a crowd. The fresh water is a crowd of people with no boundaries, and the salt water is a crowd of people surrounded by a bunch of poles. If you want to get out of the crowd, it is easier if you are in the fresh water because there is nothing blocking you. In the salt water, you have to navigate through people and avoid poles, so it takes longer to get out of the crowd. The salt "gets in the way" of water molecules trying to evaporate, so the fresh water evaporates more than the salt water. (this example is not exactly scientifically accurate -- because the salt can move around but poles can't, but the basic idea is what I am going for, here)

Also, imagine if the poles had delicious foods on them. The crowd would tend to cluster around the poles. The closer together the poles are (or the more salt there is), the less space there is for people to move through. Water "likes" salt -- salt dissolves easily in water -- in contrast, things like oil or peanuts or rocks do not dissolve easily. Individual "pieces" of water (the smallest piece of water possible is called a "molecule") tend to cluster around salt molecules. Some water is not near a salt molecule, so it is in its normal state. As the water evaporates, the salt stays there. (do not forget: tap water has salt too, just not as much). The concentration of the salt goes up as it evaporates, which is like having the poles get closer and closer together. The more salt there is for a given amount of water (this is called 'concentration'), the less evaporation can occur. So, the salt water will evaporate more slowly and more slowly the longer you let it sit there. Because there is less salt in the tap water, tap water stays evaporating at a higher rate for longer.

So there are two reasons why the salt water evaporates more slowly!

I hope this helps,

Burr Zimmerman


You are quite fortunate that you got the "right" answer. We are deluged with inquiries about various "rates" of dynamic process. The problem is that in most cases the "rate" depends upon variables that uncontrolled, maybe not even controllable, variables unknown, i.e. We are accustomed to thinking about how fast something happens, but when you want to attach a number to that "how fast" life gets complicated, because of the usually desired implicit (often unstated) condition: "all other things being equal or constant".

The answer to your observation is that water has a higher vapor pressure than. Vapor pressure is the amount of water in the vapor phase when the vapor and liquid are in equilibrium with one another (in your case this means the temperature is constant). The higher the vapor pressure the more water can be carried away by air current. If the vapor is depleted, more water evaporates because the water "wants" to keep a constant amount of water vapor in contact with the liquid phase. The sites below give good info and explanations you can use. Specifically, the boiling point of sea water is 103 C. compared to 100 C. for pure water, so water has the higher vapor pressure (Remember: higher vapor pressure means lower boiling point).

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html

http://ijolite.geology.uiuc.edu/02SprgClass/geo117/lectures/Lect15.html

Vince Calder


Julie,

The difference between salt water and tap water is that there are more things dissolved in the salt water. Whenever something gets dissolved in any liquid (not just water) it becomes harder for that liquid to evaporate. One way to imagine why this is so is that the water needs energy in order to evaporate, but when things are dissolved in the water, more energy is required for the water to evaporate. Since the water from the ocean has more things dissolved in it, the ocean water requires more energy to evaporate. However, you are supplying the same amount of energy to both the tap and salt water by keeping them at the same temperature, as a result the water from the ocean does not evaporate as much.

Greg (Roberto Gregorius)



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