For my daughter's science experiment, she placed six beverages (cola, diet cola, milk, chocolate milk, apple juice, and water) in the exact same amount in the exact same type and size of plastic cups, and placed all of the cups in a refrigerator at the same time and allowed them to remain in the refrigerator for the same period of time (4 hours). When the beverages were taken out and set on the same counter, in a room at approx. 70 degrees, a cooking thermometer was then used to measure the temperature of each of the beverages at different time intervals (the intervals were the same for all beverages)---i.e., the temperature of each was measured at 15 minutes, 45 minutes, 1 hour, and 90 minutes. The results were that all but two of the beverages held the same temperatures throughout the measuring period. The two beverages with different (colder) temperatures were diet cola and (white) milk. Of course, my daughter is supposed to cite to references that support (and explain) the basis of the test results/conclusions, and we are finding nothing that addresses this. "Viscosity" is the closest principal that we have found, but we cannot find anything that provides the viscosity levels for the separate beverages (hence, no conclusion can be drawn). Any ideas on where to find research references related to this? thanks,
The specific heat of milk is considerably lower than that of water: http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html
So for a given amount of heat gained (through radiation and convection across the glasses surface) the temperature would increase more for milk than solutions that were more "water-like". But you saw that the temperature of milk was lower which suggests a different mechanism playing hte dominant role.
I would have thought that the ordering of solute concentration in these solutions to be chocolate milk > milk > cola > apple juice > diet cola > water. If anything I would have expect that a sugar rich solution would be less like water and so cola and apple juice have a different specific heat than water or diet cola. I would have guessed that evaporative losses would be greatest for the water-like solutions and would have expected chocolate milk, milk and regular cola to have less evaporative loss because solutes tend to attract water molecules and hold them in solution. So I would have guessed that the evaporative losses would be greatest for tap water and correspondingly less on the scale I outlined. None of that exactly fits your observation. I would suggest repeating the experiment with a longer time for equilibration at refrigerator temperatures and with the addition of glasses across which you place plastic wrap to prevent evaporative losses. This will be more "scientific" in that you will a) be repeating the experiment and b) be constructing a hypothesis to be tested.
The rate of loss of liquid and the amount of cooling that would cause will be directly related to vapor pressure of the liquids. Since the cooling of milk after its removal from the cow is done at least partly by evaporation, you should be able to search out data on that physical attribute. Likewise I suspect you can find out the concentration of sugar in cola and find citations for the effect of sugar on vapor pressure.