[thedoc (OP)]

Marie Anne de Nys asked the Naked Scientists:
   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
What do you think?

[AndroidNeox]

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.