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Name: Marilyn
Status: Educator
Age: 30s
Location: N/A
Country: N/A
Date: N/A 

Is it true that when ocean water freezes the salt is left behind, only the pure water freezes. If this is true, why does this happen?

Within rather wide limits what you say is true, i.e. until the concentration of brine becomes very large. The reason for the exclusion of ions is that ice has a very unique, complex crystal structure -- remember it expands upon freezing. So for the most part where the freezing surface is washable with salt water the ions cannot fit into the occlusions.

By the way that is also how restaurants and commercial establishments are able to make "clear" ice cubes. They trickle the water across the freezing surface washing away dissolved impurities.

Vince Calder

It is largely true. It happens because it is hard to make crystalline mixtures. In general, a crystal consists of molecules in a very precise, repeating array. To insert another type of molecule into the lattice would disrupt the lattice, which requires work. In ice, the water molecules are arranged so that the hydrogens of each water molecule point to the oxygen atoms of other water molecules, making a very stable array. It is not as stable when you add salt to it.

That said, it would be irresponsible for me not to point out that 1. Real crystals are never perfect, and 2. Real solids actually consist of many crystals, with boundaries between them. What this means is that in a crystal of ice, there will be places where the lattice is disrupted for one reason or another. If the crystal forms by freezing of a saltwater solution, small amounts of salt can be included in defects in the lattice. Also, in the boundaries between crystals, the fraction of salt can be higher than in the crystals themselves, elevating the overall salt content. Overall, however, the fraction of salt in ice made by freezing saltwater will be lower than in the original swaltwater.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

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