Re: Why does ice melt faster in diet sodas than regular sodas?

Hello Megan!

It's kind of hard to diagnose a difference of results without seeing the
details of the experiment.  You need to control as many variables as you 
can, and some of may not be obvious.

In general, heat is transferred by three phenomena: 

     a. Radiation.  All objects emit infrared radiation (invisible to our
eyes) at a rate proportional to the fourth power of the temperature. This is 
unlikely to be a major contributor in your case, so ignore this for now.
     b. Conduction.  Heat goes through different materials at different 
rates, by the jiggling of the atoms and/or electrons within the material.
     c. Convection.  Fluid moving past ice will be cooled down (by 
conduction) when it is in contact, and as it moves away, is replaced by more 
warm fluid.  Since this keeps warmer fluid next to the ice, the heat 
transfer from fluid to ice is enhanced because the temperature difference
is higher.

All three mechanisms depend on temperature differences. The larger the dif-
ference, the more rapid the heat transfers.

Let's look at your case.  You have two different fluids around the ice, and
a container around the fluids.  So we need to consider the heat as it moves
from ice to fluid, fluid to container, container to air, and also fluid to
atmosphere (at the upper surface).  We also have to control some other less
obvious variables. For instance, does your parent tend to hold his glass 
more than you do?  If so, there's a conduction mechanism (container to hand) 
that is different for the two of you. Do you use exactly the same type of
container?  Do you add exactly the same amount of ice?  Is the ice very 
close to the same shape (ice shards will cool a drink faster than a single
chunk of ice of the same weight, because the surface area to transfer heat
from the ice to the soda is much higher for the shards). Are the sodas 
stored in the same place (for instance, if dad's are on the top shelf of a 
cabinet, and yours are near to the floor on a lower shelf, there may be an 
initial temperature difference of the fluids when the ice is added.  Are the 
two types of sodas equally fizzy (caution: this is hard to quantify!)?  The 
bubbles do tend to stir the fluid as they rise and thus may change the 
convection.

Try making a list of all the factors you think could influence the rate that
the ice melts. I've listed some above; are there others?

Once you have that list, then we have to make some guesses (hypotheses) 
about which ones are responsible for the observed difference in melting rate
of the ice. For each guess, you must design an experiment to test whether 
or not that factor (there may be more than one) is a contributor.  Here's
an example.  We hypothesize that it is the fluid making the difference. So
we do the following experiment.

      a. Get two ostensibly identical glasses
      b. Weight out two equal weights of ice with about the same surface
        area.
      c. Pour in the sodas, which make sure were stored together in the same 
        location and thus known to be the same temperature as poured into  
        the glasses.
      d. See how fast the ice melts
      e. Repeat several times (so that small random fluctuations in the
       amount and surface area of the ice are averaged out). Switch
       which fluid goes in which glass, too (the glasses may not be of
       equal thickness, for example).
	 f. Remove the human handling by not allowing anyone to touch the
       glasses while the ice is melting.
      g. Keep careful records of each run of the experiment, such as

Trial   Glass     Fluid      Weight         Time to
Number   Used    in glass   of ice        full melting
---------------------------------------------------------
  1        A      diet       2.0 oz.      3 min, 20 sec
  2        A      regular    2.1 oz       3 min, 45 sec
  3        B      diet       1.9 oz.      2 min, 38 sec
  4        B      regular    2.1 oz       3 min, 30 sec
  5        A      diet       2.9 oz.      5 min, 27 sec
  6        A      regular    3.0 oz.      5 min, 17 sec
  7        B      diet       2.9 oz.      4 min, 48 sec
  8        B      regular    3.1 oz.      5 min, 35 sec 

    etc, etc

You may need to have quite a number of trials unless the differences are
obvious.  

Next, you have to analyze the data.  You can draw graphs of melting time
vs. ice weight added, for instance, and see if the regular soda data points
are consistently higher or lower than those for the diet soda data points. 
Or, average the melting time for each soda type in each glass, to see if the 
glass had any effect. And so on. 

Once you have done that, each hypothesis will be either confirmed or shown
to be false. 

Let's say, for the sake of example, that you confirm that the fluid does
indeed make a difference, and that other items are either unimportant (the
glasses have no effect) or behave in a very predictable manner (melting time
of the ice in any given soda is directly proportional to the amount of ice 
added).  The story doesn't end there.  What is it about the soda that does
make the difference?  Form a new hypothesis and test it.  Say you think that
the fizziness is the issue.  So you devise a new experiment:

      a.  Open the sodas and let them sit around for 24 hours 
      b.  Repeat the melting time tests, using same amounts of ice as     
            before.

Let's say for the sake of example that the melting times are slightly 
longer, but that the differences in melting time for the two sodas are aobut 
the same, and there was no discernable fizziness in either glass while the
ice was melting.  This leads us to the conclusion that fizziness is not the 
answer for the time difference, because the two sodas still exhibit the same 
differences in melting rate whatever amount of fizziness is in them, from 
as-opened to fully defizzed. Note however, that we did find that fizziness 
does have some effect on the melting rate.

Now what?  We've sort of burned up our initial list of variables. So we have
to think of what other property of the fluids might be causing the melting 
time difference. That in turn leads to new hypotheses, and new experiments
to test them, until you narrow down to the answer. 

Design your first experiment carefully, and good luck with it.