Celebration of AP Chemistry
Freezing Point Depression and Ice Cream
Print a copy of this lab from my website.
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. We will use large crystals of
salt because large crystals dissolve more slowly than small crystals. This allows time for the ice
cream to freeze more evenly.
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.
Materials (per group of 2 or 3)
1 Glad™ bag (large)
1 Glad™ bag (medium)
½ cup of ice cream ingredients (vanilla, chocolate or orange sorbet)
coarse salt ( ½ cup)
ice (1 cup)
thermometer
plastic spoons
Styrofoam cups or plates
Measuring cups
Purpose: To study freezing point depression by making ice cream.
Procedure:
1. Into a medium Glad™ bag, place ½ cup of the ice cream mixture. Securely seal the bag.
2. Into a large Glad™ bag, place 2 cups of ice.
3. Using the thermometer, measure and record the temperature of the ice sample at the front
of the room.
4. Add ¾ cup of coarse salt into the large Glad™ bag.
5. Place the sealed medium Glad™ bag into the large Glad™ bag. Securely seal the larger
Glad™ bag.
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 medium bag have solidified (10-15
minutes).
8. Remove the medium bag and measure the temperature of the salt/ice/water mixture in the
large bag and record the temperature.
9. Pour the contents from the medium bag into Styrofoam cups or plates (1 per group
member). Consume the products.
Disposal:
The ice/salt mixture can be poured down the sink. Glad™ bags can be washed and reused.
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Data and Observations:
Data Table I: Temperature Changes of the Ice Water
Final temperature of the ice mixture
Initial temperature of the pure water/ice
Change in temperature
Ice cream observations (point form)
Questions
1. ¼ teaspoon of vanilla was added to the vanilla ice cream mixture. The IUPAC name for
vanilla is 4-hydroxy-3-methoxybenzaldehyde. Draw the structure of vanilla.
2. Why is sodium chloride placed on icy patches on highways and on steps in the winter?
3. Why is sodium chloride used rather than sucrose? (two reasons)
4. The freezing point depression constant of water is 1.86 oCkg/mol. To make the ice cream
we used ½ cup of water which is 118.3 mL and water has a density of 0.998 g/mL. What
mass of sodium chloride dissolved in the water?
Conclusion:
AP Chemistry 2006 (Baverstock)
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