Separating Salt by Freezing Water
Date: January 2009
I would like to demonstrate how to separate salt from
water by freezing it. How would I go about this?
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
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)
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.
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Update: June 2012