The following FAQ contains frequently asked questions and answers about the "Chemistry of Ice-Cream Making: Lowering the Freezing Point of Water" project. If you are having trouble with the procedure, you may find assistance in the answers below.Q: I think something is wrong with my scale. It is not giving readings that make sense. How can I fix this? A:
If your scale seems to be giving inaccurate measurements, try calibrating it. The procedure for doing this may vary from scale to scale, so consult the documentation that came with your scale. For the scale in the Science Buddies kit, calibrate the scale by pressing the button that says "Cal/Mode." Wait until the screen flashes "CAL." Press the "Cal/Mode" button again, and weigh a 100 g item (100 mL of water weighs 100 g). Wait until the scale reads “PASS.” The scale is now calibrated.Q: My ice bath is not -10° C. What should I do? A:
As long as the ice bath's temperature is between -10° C and -8° C, the ice bath will be cold enough to get consistent results. To lower the temperature of the ice bath, try using crushed ice instead of ice cubes, and try adding more salt. Be sure to use the thermometer to check the temperature of the ice bath before testing your solutions. Q: Do I really need to stir the liquid in the test tubes with a thermometer? A:
Yes. If you do not stir the liquid in the test tube, temperature gradients will develop in the liquid. In other words, without stirring, different parts of the liquid will be different temperatures, so your thermometer might not read the real freezing point of the test liquid. In addition, it is important to leave the thermometer in the test tube so that the temperature of the thermometer does not skew your data.Q: I am confused about calculating molality. Can you explain how to do that?A:
Equation 1 in the Project Idea gives the definition of molality. The easiest way to calculate molality is to set up a table, like Table 1 in the Project Idea, that has the name of the solution, the grams of substance (how many grams of sugar or salt are in the solution), the molecular weight of salt and sugar, and the mass of water (in kilograms) in the solution. Molality is calculated by dividing the number of moles of solute by the mass of the solvent in kilograms. Begin by dividing the number of grams of the substance by the molecular weight of the substance. This answer should then be divided by the mass of water in the solution (in kilograms). This will give you molality of the solution.
If you have other questions about the procedure or need assistance troubleshooting your "Chemistry of Ice-Cream Making: Lowering the Freezing Point of Water" project, please post your question in the forum for this Project Idea at Ask an Expert: http://www.sciencebuddies.org/science-fair-projects/phpBB3/viewforum.php?f=72
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