Pure Substances and Freezing Rates
Name: Atika R.
My 7-year old has a project "Do all liquids freeze at the same rate?" I know
that different liquids have different freezing point and that all liquids don't freeze at
the same rate. For e.g, among water, Gatorade and milk, which will freeze fast. I would
say water, because it has less ingredients than others. But, our results show that water
takes more time to freeze than others. I would like to know why it took more time to
freeze water than Gatorade and milk since these have more ingredients.
Newton BBS receives many inquiries about rates of "freezing" and "evaporation" of
various solids and liquids. While it appears that the answer to these inquiries should
be straightforward, in fact, rates of cooling and rates of freezing or evaporation are
very complicated. They depend upon many variables that are difficult to control -- shape
and material of construction of the vessel containing the substance which alters
convection of the liquid and/or vapor, the thermal conductivity of the vessel, air
flow over the surface, relative humidity, the presence/absence of dust particles in the
vapor or liquid... The list is very long, and this list does not even take into account
differences in the substances upon which the measurements are being made!! So it is not
possible to make "simple" comparisons of various complex fluids that would have their
own list of variables that could influence the outcome.
Just a guess, but this may be because of supercooling. Pure liquids can be cooled below
their freezing point for awhile before they freeze. The lack of impurities makes it so
there are not enough crystallization sites for the solid to form at its usual freezing
Perhaps other NEWTON scientists will provide other
points of view. Good luck!
How quickly something cools will depend on the heat capacity. The more complex the
molecule the greater the heat capacity, meaning the more heat it can store -- which
means it takes longer to cool it down.
Since Gatorade and milk are mostly water with other things added I would expect them to
have higher heat capacities than water and to take longer to cool by the same number of
degrees as compared with pure water.
A second effect that will effect your experiment is that dissolving substances in water
has the effect of lowering the freezing point. Again, this would mean that it would take
Gatorade and milk longer to freeze because they would have to cool to a lower temperature
in order to freeze.
If this is not what you observed, I can think of three possibilities:
1. You did not start with all the liquids at the same temperature. Perhaps the Gatorade
and milk had been stored in the refrigerator and were already at about 40 degrees while
the water came out of the tap at around 60 to 70 degrees. In this case the water had to
cool an additional 20 to 30 degrees in order to start freezing. Make sure that all three
liquids have been stored in the same refrigerator (very close to each other) for at least
4 or 5 hours before starting the experiment (assuming you are using volumes of about 1
2. You had a greater volume of water than you did of milk or Gatorade. The more you have
the greater the heat capacity and the longer it will take to cool. Make sure you use the
same volumes of liquids for the experiment.
3. The liquids were in containers of different types. Heat transfer to the freezer will
occur only at the surfaces of the liquid (sides, tops, or bottom). How rapidly the heat
transfer occurs will depend on the material the container is made of, whether it is
closed, and what it is in contact with. For this experiment, try to use three containers
of exactly the same type -- a plastic cup would be ideal, with the top uncovered.
There is a fourth factor that might also impact this experiment but I think it is unlikely
to be the most important in your case. That is the heat transfer rate through the liquid
itself. In order for the liquids to cool on the interior of the liquid the heat there
must be able to get through the liquid around it to the surfaces where heat is exchanged
with the freezer. This would normally occur through conduction or convection. If, after
ensuring you have used the three suggestions above, you still do not have the expected
result, you might try enhancing conduction by giving each of the containers a quick stir
every 10 minutes in the freezer. Make sure you treat all three liquids the same...
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Update: June 2012