Re: Which gets cooler, fresh or salt water?

Elizabeth,

I am going to answer this question based on an assumption.  Since the coolest
temperature most things can attain is absolute zero, and water (salty or
otherwise) is no exception, I am going to assume you mean "which gets
cooler before freezing".

Both sea- and freshwater can lose heat (or get cooler) until it reaches
absolute zero.  However, seawater behaves differently along the way.  As
you may know, freshwater displays a "maximum density anomaly" as it cools.
The water molecules get closer and closer together, increasing the density
until it reaches a maximum at 4oC (4 degrees Celsius). This is called an
anomoly because other liquids continue to increase in density as they cool.
Water is different because of the structure of the ice crystals that begin
to organize below 4oC.  These 6-sided crystals are highly organized, and
contain some empty space that pushes the liquid water molecules apart to
form the crystal lattice.  This crystal formation results in a DECREASE in
density as water cools further, below 4oC. That is why ice floats.  It is
also why fish in a pond that is covered with ice can find unfrozen water
at the bottom of the pond.  It will be 4oC there.  True ice crystal
formation occurs at approx 0oC, after enough heat is removed (this is
latent heat, that is the temperature of the water will not change, while a
state transition takes place from liquid to solid)

So now we see what happens to pure water, let's add some solids. In seawater
these solids are primarily salts, sodium chloride, magnesium sulfate and
so on, which dissociate into ions in water.  These ions interfere with the
formation of those highly organized ice crystals -- they get in the way.

To overcome this, more heat needs to be
lost from the seawater.  The average salinity of seawater is 35o/oo.  That
is shorthand for 35 parts salt per thousand parts seawater.  So seawater is
96.5% water.  But that 3.5% of salt has some significant effects on the
behavior of seawater as it cools.  Seawater does not display a maximum
density anomaly, and it does not freeze at 0oC.  The maximum density
anomoly disappears at about 25o/oo where the maximum density occurs at
the same temperature as ice formation.  The freezing point for 35o/oo 
average seawater is -1.9oC.  This extra heat loss is needed to over-
come the interference of ions with ice crystal formation.  It is at this 
temperature that ice crystals will form, despite the presence of the salts.
These are crystals of pure water in salty water that becomes saltier as 
more crystals form, leaving behind ions.

This is an important process in the formation of sea ice.  And the unusual
properties of water are also important for the circulation of the world's 
oceans.  As ice crystallizes out of seawater, the liquid that is left
behind becomes saltier (as well as cooler).  This dense, cold, salty water
sinks and is replaced by slightly fresher, warmer water drawn from the area
around the ice formation.  This is called "thermohaline circulation".

Sea ice, then is fresher than seawater.  If the ice forms very fast, a small
amount of salt ions will be trapped in the sea-ice.  But if it forms slowly
or is old, the salt ions will escape the crystal structure as a salty brine
leaving behind nearly pure water in the form of ice.