0.1 Colligative properties and ice cream  (Page 3/3)

NOTE: The tert-butanol must be distributed by your TA to avoid impurities that will cause tremendous errors in the experiment.  You will need a very clean and very dry test tube for each of your experiment runs. If any impurities (especially water) mix with the tert-butanol, your data will be severely affected.

ANOTHER NOTE: Make sure that your solution in your test tube is below the level of the water in the water bath

• Warm the test tube to 30-35°C by placing it in a warm water bath. Start your data acquisition program, and place the test tube in an ice/water bath. You must constantly stir the tert-butanol to prevent supercooling.
• The temperature of the tert-butanol should steadily drop, then level off, as the liquid freezes. When the tert-butanol is completely solid and the temperature starts decreasing again, you may stop your experiment. This will be the freezing temperature you use as Tpure when you calculate Δ T. Record this value in your data sheet. If your cooling curve does not flatten out very well and it is difficult to determine the freezing temperature, warm your sample with the hot water bath, and measure the freezing point again.

Note: If you do not get an acceptable curve (your TA can verify if it’s acceptable) on your second try, then you should ask for a new sample of tert-butanol.  The rest of your results for this lab depend on this measurement being accurate.

Part 2: Freezing point depression

• After finding the freezing temperature of pure tert-butanol you will make a solution of tert-butanol and antifreeze. Set your test tube inside a 50 or 100 mL beaker and then place the beaker on the balance to weigh your sample.
• First, weigh only the empty test tube and record this value in your data sheet.
• Fill the test tube half full of tert-butanol (again from your TA), weigh again, and record this value.
• Finally, add a few drops of antifreeze (3 or 4 drops is sufficient), weigh the beaker/test tube combination a third time, and record this value. Use the same balance for all three weighings. Use subtraction to find the masses of the tert-butanol and the added antifreeze. It is not critical how many drops you add, but the mass that you measure is the important value.
• Find the freezing temperature for your solution in the same way you found the freezing temperature for pure tert-butanol. The flat portion of your temperature curve will be smaller and more difficult to see for your mixed solution than for the pure tert-butanol. If you are running a live graph in your program, you should be able to tell where the freezing point of your solution occurs. As before, you may re-warm your solution and run it again if your cooling curve does not show a clear freezing point. If time permits, you may want to perform more than one run on each solution to confirm your freezing temperatures.
• Subtract the freezing temperature your solution from the freezing temperature of pure tert-butanol. Record this Δ T value in your data sheet.
• Use the information recorded in your data sheet to calculate the molar mass of ethylene glycol. Calculate the percent error for your experimentally determined molar mass. See your TA if you are unsure how to make this calculation.
• Repeat Steps 1 through 5 for the unknown solutes. After calculating the molar mass of the unknowns, identify them using the following information:

 Part 3: Chemistry of Life: Ice Cream

As we found above, adding a solute to a solvent lowers the freezing point of that solvent.  This occurs because as a substance freezes, a crystal is formed, but if a solute is added to the solvent more kinetic energy must be removed from the solvent in order to freeze, since it’s harder for the solvent molecules to form the regular pattern of the crystal.  Therefore, the more solute molecules you add, the lower the freezing point becomes.  We can use this to our advantage to lower the freezing point of water by enough to freeze ice cream, since ice cream is mostly water. 

This is a recipe that you could use at home:

Put 59.15 ml (¼ cup) of sugar, 118.29 ml (½ cup) of milk, 118.29 ml (½ cup) of whipping cream, and 1.23ml (¼ teaspoon) vanilla (4-hydroxy-3-methoxybenzaldehyde) into a one-quart Ziploc ™ bag.  Seal the bag and mix well by carefully shaking.

Put this one-quart Ziploc ™ bag into a one-gallon Ziploc ™ bag that has 2 cups of ice..

• However, we are going to cheat by using 6 packets of Junket ice cream mix + 7 ½ cups of whole milk + 4 ½ cups of whipping cream in a one galloon jug.
• Measure and record the temperature of the ice with your thermometer in the one-gallon Ziploc ™ bag .
• Weigh and pour 177.44 ml (¾ cup) of sodium chloride into the gallon bag.
• Place the smaller bag inside the larger bag and seal the large bag securely.
• Holding the large bag by the zipper seal, carefully shake the bag back and forth. 

NOTE: Do not touch the part of the bag with the ice as it could cause tissue damage.

• Continue until your ice cream is solid, approximately 10 – 15 min.
• Measure and record the temperature of the salt/ice mixture.
• Remove the frozen ice cream and place into a Styrofoam cup and enjoy!