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Q & A: evaporating salt water

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Q:
Ok I am doing a scince fair project and I am wondering why when the water from the ocean is evaporated the salt doesn’t go with it. Isn’t 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)
Austin
A:
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 molecules away.

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 any conclusions.

Mike W.

(republished on 07/24/06)

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