As an undergraduate I majored in chemistry
and biology. I am currently working on a
graduate degree in chemistry.
I'd just like to clarify the difference
between freezing point and freezing rate before
I start answering your question. When we add
salt or any solute (including carbon dioxide) to
water, we lower its freezing point, or the
temperature at which it freezes. When we talk
about rates, however, it is ultimately a
question of thermal conductivity, or how quickly
we can add/remove thermal energy (note that I
don't say heat! -- heat is a process, not
a "thing") from the system. The higher the
thermal conductivity, the faster we can cool or
heat a substance.
As a disclaimer, I would like to say that the
answer to your questions is somewhat involved.
Let's start with the simplest case: distilled
(pure) water. Thermal conductivity depends on
temperature. I believe the trend is that at
higher temperatures thermal conductivity is
higher. But I would like to say that thermal
conductivity only changes very slightly with
temperature. What does this mean? As an example,
if we start cooling one cup of water at 90F and
another cup starting at 40F, the rate at which
the 90F water cools will be slightly greater.
Note that if we were trying to freeze the two
cups of water the 40F would probably still
freeze first, but only because it was closer to
the freezing point. You can think about it in
terms of the slopes of the lines that you would
get from plotting temperature vs. time. The
slope of the 90F water would probably be
slightly steeper (i.e. have a faster rate) than
the 40F water.
However, we need to be careful here. At a
certain point, water will boil and become
steam/vapor. The thermal conductivity of a vapor
will be much lower than that of a liquid. In
this case, the difference is not slight at all --
it is about an order of magnitude (10 times)!
You can think about it like this: the molecules
in the liquid are much closer together, so they
can transfer energy (and thus heat or cool) more
easily. But the molecules in the vapor are far
apart, and will not "bump into each other" as
often, so energy won't be transferred as quickly
as in the liquid.
That said, let's think about what this means.
When you say "how does carbonation, salt and
fresh water affect cooling rate in different
environments?" I understand "different
environments" to mean different temperatures.
You should be very careful here, because a good
experiment usually only changes one variable at
a time. Here, you're talking about two
variables: (1) type of solute (salt, carbon
dioxide, or none), and (2) temperature. If we
only consider liquid distilled water, then at a
lower temperature, thermal conductivity is
lower, and cooling rates will probably be
marginally slower. Does this make sense?
Remember the definition of thermal conductivity!
OK. So now let's discuss the other two cases,
where we include solutes (carbon dioxide or
salt). When we add impurities to water, we very
slightly decrease the thermal conductivity of
the water. I believe this is true if we keep all
other things constant. So what does that mean?
Compared to pure water, cooling rates would
probably be slower. If we stay within the
constraint of liquid water and starting the
cooling/freezing process from different
temperatures, I think the cooling rates of the
water with solutes will still be slower, due to
lower thermal conductivity. But remember, it's a
small decrease in thermal conductivity, and
thermal conductivity does not change drastically
with temperature -- so the differences may be
hard to discern!
I think it would also be a good idea to try
the experiment if you haven't already. Take
three cups, fill all three with the same amount
of water, but one will be distilled water, one
will be salt water, and one will be carbonated
water. Put them in the freezer at the same
distance from where cool air comes into the
freezer. Be very sure to try and make all the
conditions equal, except the fact that they have
different solutes -- having other conditions be
different could introduce experimental errors!
Check on them periodically to see which freezes
first. And do it a few times to verify that
you're getting the same results.
One other important thing to note is that I
think the experimental conditions to observe
what I think would be very slight differences
would have to be very pristine and precise. Most
household environments are probably not ideal
for this. Furthermore, your definition of when
water is considered "frozen" would have to be
very consistent. This is difficult, even in an
ideal experimental setting. To give you an idea:
do we consider it frozen when the first few
molecules on the outside begin to have a
crystalline structure? Or is it something else?
It can be complicated ... But please don't let
this discourage you! Try the experiment out,
just to see. There are so many amazing
discoveries and advancements in science that
happened because people were curious and
tenacious about it.
I hope this helps. I realize it was a rather
involved answer, so if you still have questions
feel free to ask.
Click Here to return to the search form.