Salt, Fresh Water Evaporation Experiment /h3>

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

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?
---------------------------------------
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)
====================================================================