[chris (OP)]

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?

[chiralSPO]

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 –25°C.

Hope this helps!

[chris (OP)]

Wonderful answer! Thank you!

[Ru Ko]

Was the question answered? I'm sorry but I still don't know why salty water when frozen doesn't taste salty. When the ice melts doesn't the salt go back into solution? In order to taste it the ice has to melt a little doesn't it?

[evan_au]

I still don't know why salty water when frozen doesn't taste salty

It's because when you freeze salty water, the ice doesn't contain any salt (salt is excluded from the ice crystals).

When you taste the melted ice, it still doesn't contain any salt, so it doesn't taste salty.

[Ru Ko]

You say the salt is excluded during the freezing process. Where does it go? Is it at the bottom of the ice cube tray? Did it just disappear? Is it not in the ice cube anymore? What if you put the ice cube in glass and allowed it to melt and then tasted the water. Would it taste salty?

[alancalverd]

You will find concentrated brine below the ice, because ice is less dense than even fresh water.

Festive season tip: buy some cheap cider and put it in the freezer. Pure ice separates out and you can pour off the remaining liquid which now contains up to 40% alcohol: cheap and delicious.

[Petrochemicals]

How does salt melt ice therefore and is the crystalisation problem of ice why warm water freezes faster and superchilled water will not solidify in the upper atmosphere (always told it needs a seed particle) ?

[Bored chemist]

You will find concentrated brine below the ice, because ice is less dense than even fresh water.

Festive season tip: buy some cheap cider and put it in the freezer. Pure ice separates out and you can pour off the remaining liquid which now contains up to 40% alcohol: cheap and delicious.

Don't use a glass bottle.

How does salt melt ice

Essentially, the ions get in the way of the ice forming, so they lower the freezing point of the mixture.


It's also important to realise that, if you have a solution with a lot of salt in it and cool it you get pure salt crystals.
As the mixture is cooled more and more salt crystallises out and so the solution becomes more dilute.

If you take a dilute solution and cool it the first material to crystallise out is water (as ice) and the solution becomes more concentrated.

If you think about both of those you will realise that, at some point the two systems (one getting more dilute, and the other more concentrated) must "meet".
That point is the eutectic mixture.

https://www.researchgate.net/figure/Phase-diagram-of-salt-water_fig3_276999290

(it's actually slightly more complicated, at low temperatures the salt  crystallises as a hydrate.)

[alancalverd]

warm water freezes faster

This is a consequence of the anomalous convection of water below 4 deg C. It only works in conditions of very rapid cooling, which makes me wonder whether the famous M'pemba experiment was accurately reported - the folklore is that he demonstrated it with ice cream, which AFAIK doesn't convect, or have a density anomaly, or a well-defined freezing point.