When salty water freezes, why is the ice not salty?

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Offline chris

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When salty water freezes, why is the ice composed of fresh water and not salt? How are the salt ions excluded when the ice forms?
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline chiralSPO

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Re: When salty water freezes, why is the ice not salty?
« Reply #1 on: 26/08/2016 02:43:13 »
Crystallization is a very good way to purify substances (as a chemist, I take advantage of this quite frequently!) With the exception of compounds that co-crystallize (in which case the crystal will have a very well-defined ratio of the components), almost all crystals are nearly pure substances. The crystal structure has little room for deviation from perfect repetition of its unit cell.

Essentially, you can think of it like so: as the crystal is growing, dissolved (or molten) molecules or ions bump into the side of the crystal. If they happen to fit just right, they will stick and be incorporated into the crystal, and if not, they will just bounce off and go back into solution (or melt). Even water molecules will most bounce off of the growing ice crystal--they have to hit at just the right angle and just the right position with just the right speed to stick, but non-water molecules or ions have no chance of being incorporated in the crystal. If the crystal forms too quickly, it can have defects, and potentially contain impurities, but if it is still crystalline, then it must be quite close to pure and perfect.

A corollary to this is that mixtures of substances are much harder to crystallize. This is essentially the reason behind why salt melts ice (salt+water is harder to freeze than pure water, and therefore salt+ice is easier to melt than pure ice). Ice will also melt very quickly if exposed to absolute alcohol (and will get very cold!) and vodka won't freeze until it gets below 25C.

Hope this helps!

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Offline chris

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Re: When salty water freezes, why is the ice not salty?
« Reply #2 on: 26/08/2016 08:56:37 »
Wonderful answer! Thank you!
I never forget a face, but in your case I'll make an exception - Groucho Marx