MadSci Network: Physics |
OK, first some initial conditions: 1) the fluids all start at the same temperature; 2) we're starting with an equal volume of each fluid; 3) the heat transfer to the fluid from the heat source is exactly the same in each case; 4) the fluids when boiling are vented exactly the same way; 5) we're starting with an appreciable volume of fluid; and 6) by "boil down" you mean drive off all the volatile matter in the fluid. The answer is milk. Why? The boiling point of water, 5% acetic acid solution (vinegar), and milk are all very close. So if you heat the same amount of each of them at the same rate, they'll all reach their boiling point at about the same time. The boiling point of water is 100 degrees Centigrade, or 212 degrees Farenheit; vinegar and milk are slightly higher, at least at first (the milk will actually start to boil at a lower boiling point after a certain amount of evaporation). So why does milk evaporate first? Simply because not all of the sample is volatile; that is, not everything in milk evaporates. So if the milk is, for example, 5% non-volatile, and you start the experiment with 1000 grams of each fluid, you need to make 1000 grams of the water and the vinegar evaporate, but only 950 grams of the milk.
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