Freezing Ocean Water and Salt
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.
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.
PG Research Foundation, Darien, Illinois
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Update: June 2012