Science Questions

Why is there no salt in sea ice?

Mon, 5th Sep 2016

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Marie Anne de Nys asked:

Dear Chris,

I manage an aftercare in Cape Town South Africa, and I listen to your show on the Redi Hlabi show, Cape Talk 567, on Friday mornings. One of my students, Zac, wants to ask the question: "If the ice caps melt, they will dilute the salt water because they are made from fresh water, so if the ice caps are made from fresh water, how did they form from salt ocean water?" Zac has asked all of his subject teachers, and myself, and he still doesn't have an answer. Please advise us. Thank you


Caroline Steel cooled down with this one...Iceberg

Kat - That is a great question.


Caroline - Yeah, so I learnt something new when I was looking into this. So, it turns out that icebergs themselves are actually just glacier fragments. They arenít ice that forms from the sea freezing. Thatís called sea ice. So, icebergs are freshwater because they come from glaciers and glaciers are compacted snow and so therefore, they're freshwater. But when the sea freezes and form sea ice, this ice is also fresh, because freshwater freezes at a higher temperature than saltwater. For kind of similar reason, thatís why we put salt down on the roads when itís cold to stop it freezing over and our cars skidding in the morning.


Kat - So the ice forms and pushes the salt out of it.


Caroline - Yeah, the salt ions are kind of forced out of the lattice.


Kat - Andrew


Andrew - So, this distillation process is actually quite interesting because if you get cider and you throw it on the backyard in Canada, the alcohol gets pushed into the core as the ice freezes around the outside. So you could distil alcohol in your backyard. Itís not going to be a good product but itís the same effect.


Kat - I'm sure in Terry Pratchett books, they do that and itís called ďscumbleĒ if I recall correctly.


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Polar ice caps are formed from precipitation, like snow, that compresses over time to a hard ice pack. The precipitation comes from clouds which are formed by water evaporated from the ocean. The evaporation leaves the salt behind, just as distillation does.

There is a separate process, though, where freezing salt water will separate the salt and water. The mixture of water and salt with the lowest freezing point is the eutectic mixture. If you have more water than the eutectic, as you cool the mixture toward the eutectic temperature, crystals of pure water will precipitate out and float up. If the mixture has excess salt, salt crystals will precipitate out.

But, the ice cap thingy is distillation. AndroidNeox, Thu, 1st Sep 2016

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