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Name: Sherrie
Status: teacher
Grade: K
Location: NV
Country: USA
Date: January 2009

Question:
I would like to demonstrate how to separate salt from water by freezing it. How would I go about this?



Replies:
Sherrie,

As you already know, when most solvents (in this case water) freezes, they do not incorporate any dissolved solute (in this case salt). However, this is likely to happen under optimum conditions: very slow freezing process and undisturbed. I'm not sure that such conditions can be replicated in a classroom.

Since you are working with kindergarten students, perhaps a good way is to have the students prepare some fairly concentrated salt solutions, split the amounts freezing one and keeping the other unfrozen in sealed containers. When the ice has formed, you can then ask the students to quickly wash off the outer layer (as the salt which were not incorporated in the ice will deposit on the outside - hopefully this is something that is observable and students can already make some conclusion), and then allow the remaining ice to melt in a glass. The students can then compare the taste of the salt water that they kept unfrozen, to that of the water that came from the melted ice. The latter should taste less salty.

You might want to try this on your own first to make sure that a real distinction in saltiness can be observed.

Greg (Roberto Gregorius)


Sherrie

If you get it cold enough even salt water will freeze, so ideally you would want a way to do a partial freeze, separate the ice chunk from the unfrozen water, and then compare the salinity of the two after thawing, by taste test, or salinometer, or whatever means you have at your disposal.

The ice (after melting) will be slightly salty, but if you freeze and remove a fairly large chunk (say about half the water), the change in salinity should be pretty large (i.e. almost double) in the remaining unfrozen water.

An ordinary kitchen freezer can probably do for an overnight freeze, although you may have to test and tweak the settings, too cold and the whole sample freezes, not cold enough and you will not have much ice formed. If in a hurry you might try dry ice or something similar, although with a rapid freeze you are more likely to incorporate salt into the ice.

Don Yee



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