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Ok I am doing a scince fair project and I am wondering why when the water from the ocean is evaporated the salt doesnt go with it. Isnt when the salt has dissolved the salt just becomes like part of the water and is kind of somewhat a chemical change?
- Taylor (age 10)
You're right that when the salt is dissolved in liquid water, it really
is changed. It falls apart into positively and negatively charged ions.
The positive ones (e.g. Na+) tend to be surrounded by contacts to
negatively charged parts of water molecules (the oxygen part). The
negative ones (e.g. Cl-) tend to be surrounded by contacts to
positively charged parts of water molecules (the hydrogen part). Those
contacts greatly reduce the electrostatic energy that would be required
to pull ions off a salt crystal. Another way to picture this is that
the water molecules tend to line up so that their electrical fields
reduce the electrical fields from the ion. The lined-up water molecules
include not only those in contact with the ion, but also those several
When water evaporates, its molecules form a gas, in which they are
almost always far apart. An ion in that gas would be in trouble,
because it could only be in contact with one water molecule, with no
other neighbors. The water couldn't lower the ion's electrostatic
energy much. The ions almost all stay behind in the liquid, where their
energy is low.
The key to understanding this process is to think in terms of
physically meaningful quantities like 'energy'. Sometimes words that
are taught, like 'chemical change', are vague and hard to use to draw
(republished on 07/24/06)
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