Countertop Chemistry Experiment 15
Ice Cream

Adding a solute to a solvent lowers the freezing point of that solvent. This change in freezing point is referred to as a colligative property. In this experiment, you will use the lowered freezing point of water to chill another mixture (ice cream) to the solid state.

Materials	Subsitutions
1 quart Ziploc™ bag
1 gallon Ziploc™ bag
1/2 cup milk
1/2 cup whipping cream
1/4 cup sugar
1/4 teaspoon vanilla flavoring
sodium chloride	rock salt
ice
thermometer
measuring cups (1, 1/2, and 1/4 cups)
Styrofoam™ cups
plastic spoons

Procedure

1. Into a one-quart Ziploc™ bag, place 1/4 cup sugar, 1/2 cup milk, 1/2 cup whipping cream, and 1/4 teaspoon vanilla (4-hydroxy-3-methoxybenzaldehyde). Securely seal the bag and mix well.
2. Into a one-gallon Ziploc™ bag, place 2 cups of ice.
3. Using the thermometer, measure and record the temperature of the ice.
4. Add between 1/2 and 3/4 cups of sodium chloride to the gallon bag.
5. Place the sealed quart bag into the gallon bag. Close the larger bag securely.
6. Holding the large bag by the top seal, gently rock the bag from side to side. Do not hold the bag in your hands—it will be cold enough to cause tissue damage to your hands.
7. Continue rocking the bag until the contents of the quart bag have solidified (10-15 minutes).
8. Measure the temperature of the salt/ice mixture in the gallon bag and record the temperature.
9. Remove the frozen contents from quart bag into Styrofoam™ cups. Consume the contents of the cups.

Data and Observations

Initial temperature of ice _____

Final temperature of ice mixture _____

Change in temperature _____

Questions

1. Why is sodium chloride added to the ice?
2. Why are large crystals of sodium chloride used instead of small crystals
3. Why is sodium chloride placed on icy patches on highways and on steps in the winter?
4. Why is sodium chloride used rather than sucrose?

Teacher's Notes

When a substance freezes, the particles arrange themselves into an orderly pattern. This arrangement is called a crystal. When sodium chloride is added to the water, a solution is formed. The forming of the solution interferes with the orderly arranging of the particles in the crystal. Therefore, more kinetic energy (heat) must be removed from the solvent (water) for freezing to occur. This results in a lower freezing point. Furthermore, the more particles of solute (salt) added, the more kinetic energy must be removed. The greater the concentration of solute, the lower the freezing point of the solvent.

Answers to Questions

1. Sodium chloride is added to the ice to lower the freezing point of the ice.
2. Large crystals dissolve more slowly than small crystals. This allows time for the ice cream to freeze more evenly.
3. When sodium chloride is placed on the highway or on steps, the freezing point is lowered, and the ice melts.
4. Sodium chloride is used for three reasons. First, some solids such as sugar do not dissolve in ice water as well as salt. Second, salt is an abundant mineral in the form Halite and is not expensive. Finally, when sodium chloride dissolves, it separates into two particles (Na+ and Cl-), lowering the freezing point further. Only advanced students would need to know this concept. It is called ionic dissociation.

Disposal

The ice/salt mixture can be poured down the sink. Ziploc™ bags can be washed and reused.

Credit: The formula for the ice cream mixture is from Mr. William M. Black of Kewanee High School in Kewanee, IL.

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