Why is there no salt in sea ice?

06 September 2016

Question

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

Answer

Caroline Steel cooled down with this one...

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 aren't ice that forms from the sea freezing. That's 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, that's why we put salt down on the roads when it's 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. It's not going to be a good product but it's the same effect.

Kat - I'm sure in Terry Pratchett books, they do that and it's called "scumble" if I recall correctly.

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