Re: will sea water desalinate when frozen,why or why not?

Dear Matt,

Yes, freezing salt water will indeed turn it into fresh water. This process accounts for the fresh water ice floating on the Arctic Ocean. It takes several years for the process to make the sea-ice salt-free enough for people to melt and use for drinking water, but they can and have done this for centuries.

The process happens at the molecular level. When water forms ice crystals, the water molecules go together in an ordered manner, pushing the sodium and chloride ions (dissolved salt), out of the way. On a single freeze, only part of the salt is eliminated. If the ice is removed from the salt water partly through the freezing process and is melted and refrozen, the ice gets less and less salty with each repeat.

This purification process not only works with water and ice, but some metals are purified in this way also. In this case, a long bar of the impure metal is slowly heated, starting at one end. The heat is moved slowly along the bar, melting the metal in that area. The molten metal cools as the heater moves on, leaving a solid and more refined, bar of metal behind. The impurities tend to stay in the melted part of the metal and when the heater reaches the end of the bar, it is allowed to cool off. Once cool, the impure end of the bar is cut off, leaving a more pure metal bar behind.

It should be an interesting experiment. Just remember, the ice must be removed from the salt water, rinsed off quickly, and then melted to detect the improvement in water quality. You will probably want to repeat this freeze / remove / thaw cycle several times and determine the salt content after each cycle. You might use a volumetric flask and a triple-beam balance to determine the density of the water solutions. This would make a very interesting graph!

If you let the entire block of ice freeze, it will be more difficult to remove the purified water from the more concentrated salt ice because the salt will be pushed to the center of a freezing block of ice where it will be trapped.

I hope this helps.

Reference: The Book of Popular Science
Grolier Incorporated
New York, NY 1967