Experiment #5/ Unit 7
An Application of Colligative Properties: Making Ice Cream
Introduction:
The making of homemade ice cream relies on the colligative properties of a solution to lower the freezing point of water. Cream will readily freeze if you can lower the temperature surrounding it to below 0 *C. When rock salt (NaCl) dissociates in water, the particles (Na + and Cl ions) cause a freezing point depression and a boiling point elevation in the resulting solution. By adding enough ice, the endothermic change of melting will draw heat from the solution until it is below 0 *C. How far below? Freezing point changes can be quantified by knowing the molality of the solution and a special constant called the molal freezing point depression constant (K f
).

T f
= K f m K f
for water = 1.86 *C/m
In Advance : Prepare your ice cream recipe. This can be done the night before and stored in the refrigerator. However, if you plan to add mix-ins, keep these separate until you freeze it. Standard recipes for ice cream can be found in cookbooks. Recipes for different flavors of Ben & Jerry's ice cream are available.
You will also need to bring cups, spoons, and gloves. You will need one large metal spoon for dipping the ice cream and individual spoons (remember, it is flu season) for eating. I will need to have your ice cream mixture before school so they can be kept cool until we freeze them. It would be preferable to have the mixture already in the small coffee can and labeled with names.
Materials:
 ice cream recipe (1 qt will do)
 two coffee cans, 1 and 3 lb. sizes (or two zip-loc bags, 1 qt and 1 gal sizes)
 rock salt, 2 cups
 crushed ice, 2 qt.
 masking tape
 thermometer

250 mL beaker

400 mL beaker
Procedure:
*I want you to get a good estimate of the total masses of ice and salt that you
use to make your ice cream. You will need to mass a 400 mL beaker that will
be used to obtain all ice and a 250 mL beaker that will be used to mass all salt.
I have a high capacity, top-loading, electronic balance that should allow these
masses to be obtained quickly.
Day 1
1. Pour the cooled ice cream mixture into the small can until the can is 3/4 full
(why not full?). Replace the lid and tape it securely.
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2. (Remember to record masses) Add a small amount of ice and salt to form the
bottom layer in the large can. Place the small can inside the large can and add
alternate layers of ice and salt so that the amount of ice is about 4 times the
amount of salt.
3. Roll the can back and forth. The frozen cream will be smooth if it is kept in
motion as it freezes.
4. After about 10 min., you will need to add more ice with a little more salt.
5. After 20 min. of rolling, open the outer can and record the temperature of the salt
water solution. Rinse the thermometer before it is returned.
6. Determine the mass of a clean, dry 250 mL beaker. Pour approximately 75 mL
of the solution into the beaker without getting any undissolved salt crystals or
unmelted ice cubes in with the solution. You want a sample of homogeneous
solution. Mass the beaker containing the solution, label it, and set it aside for
day 2.
7. Decant the rest of the salt solution. Collect and mass any undissolved salt.
Return undissolved salt to a recycle bucket.
8. Rinse all the salt solution from both cans and the lab top .
9. Open the inner can and enjoy the fruits of your labor.
Day 2
1. Remove the tape from the beaker. Heat the solution. We have two objectives
here: (1) we want to measure the boiling point of the solution and (2) we want to
obtain the dry solute mass by evaporating the water. After you reach the BP, you
can add a watch glass in order to avoid spattering of the salt.
2. Allow the beaker to cool then mass the beaker and salt residue.
Alternative Freezing Procedure I: Place ice cream mixture in a quart size
zip-loc bag and zip tightly. Place bag inside a gallon size zip-loc bag and
surround with ice/salt mixture. Zip tightly and shake for 10-15 min.
Data:
Day 1
 mass of empty 250 mL beaker for salt masses
 mass of empty 400 mL beaker for ice
 mass of each addition of salt (250 mL beaker + salt)
 mass of each addition of ice (400 mL beaker + ice)
 temperature of solution upon opening the can
 mass of empty beaker for solution sample
 mass of undissolved salt
 mass of beaker and solution
Day 2
 mass of watch glass
 mass of beaker, watch glass, and salt
 boiling point of solution
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Calculations and Questions:
1. Write thermochemical equations for the two processes that formed your
solution. The

H values must have correct subscript, sign, value, and units
of Kcal/mol. These can be found in textbooks and the Handbook of
Chemistry and Physics.
2. Determine the total volume of water molecules that you used both before and
after they melted. What percent reduction in volume did the water experience?
*For calculations #3-6, use two methods (a) running totals of salt and ice
from day 1 and (b) data from day 2 solution sample
3. Calculate the mass of solute.
4. Calculate the mass of solvent.
5. Calculate the molality of your salt solution.
6. Predict the freezing point that could have been obtained based on the
concentration of your solution.
7. How close did you get to the predicted FP of your solution? Remember, we
did not actually freeze our solutions.
8. If you had _____ g of water in your solution, how much heat energy did the
water have to lose to cool from 0 *C (melted ice) to its final temperature?
9. If you used ____ g of total salt to make your solution, how much heat energy
was required to dissociate it?
10. If you used ____ g of total ice to make your solution, how much heat energy
was required to melt it?
11. Your solution got very cold. First, explain where the kinetic energy went as
energy was stolen from the water molecules. Second, explain how the
water was able to lose so much energy and not become a solid.
12. Why is the predicted FP depression usually greater than the actual FP depression?
13. Predict the boiling point for your solution.
14. Calculate a percent error based on the predicted change in boiling point vs. the
actual change. Calculate one in *C and one in absolute K.
15. Calculate the difference between the total amount of heat absorbed (#9 + 10) and
the amount of heat lost by the water. Where did the rest of the energy come from?
16. Draw a diagram that shows all the heat transfers that took place in making the
ice cream.
Lab Report #7.5:
 title page
 abstract
 procedure sheet
 data
 calculations and questions
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