Name:_________________________
Period:____________
Date:____________
Ice Cream Lab
(40 points)
Background:
Solutions have different properties than either the solute or the solvent used to make the solution. Those
properties can be divided into two main groups – colligative and non-colligative properties:
Colligative properties depend only on the number of dissolved particles in solution and not on
the compound being used as the solute.
Ex: vapor pressure lowering, boiling point elevation and freezing point depression
Non-colligative properties depend on the identity of the dissolved solute and the solvent.
Ex: taste, color, viscosity, surface tension, and solubility
Let’s look at the colligative property of “freezing point depresssion” which is the focus of this lab.
Adding a solute lowers the freezing point of the solution compared to the that of the pure solvent. One
way to think about the freezing point depression phenomenon without talking math is to consider the
freezing process. In order for a liquid to freeze it must achieve a very ordered state that results in the
formation of a crystal. If there are impurities in the liquid, ie. Solutes, the liquid is inherently less
ordered. Therefore, a solution is more difficult to freeze than the pure solvent so a lower temperature is
required to freeze the liquid.
This phenomena is the science behind:
 Using antifreeze in your car coolant system
 Windshield wiper fluid
 Using salt on the roads and walks to keep them from freezing
Ice cream, which is mainly a solution of sugar in milk, is really an aqueous mixture with many particles
in each liter. Because of this the freezing point of ice cream is lower than that of water, so to freeze ice
cream, or keep it frozen, you must keep its temperature significantly below 0 degrees Celsius. Adding
rock salt to ice produces a melted ice and saltwater solution with a depressed freezing point which
therefore allows the ice cream to be frozen; ie the saltwater solution is below the freezing point of the
ice cream mixture.
Heat of Fusion: Melting a solid to form a liquid is an endothermic process. The heat of fusion of a
substance is the amount of energy needed to change 1 gram of a solid to a liquid. The heat of fusion of
water is 334 joules/gram. Since both the room temperature and the milk mixture are warmer than the
freezing point of the ice mixture, energy will be transferred into the ice mixture on all sides. This
energy will be used in melting the ice (334 joules per gram melted) and the temperature of the ice
mixture will stay at its freezing point as long as there is still ice to melt. This process will effectively
draw energy out of the milk mixture, lowering its temperature and eventually freezing it!
Purpose:
 To investigate the effects of heat transfer on phase changes
 To investigate the effects of freezing point depression
Procedure: (14 points total)
1. Dissolve 2 tablespoons of sugar in ½ cup of milk (or cream), then add a teaspoon of vanilla
or other extract for flavor. Add any other additions you brought – nuts, fruit, etc.
2. Pour milk mixture in a small zip-close sandwich bag and close securely.
3. Half fill a larger 1 gallon zip-close freezer bag with ice cubes, then add a few tablespoons to
¼ cup of rock salt. (½ cup of rock salt works better).
4. Place the sandwich bag into the freezer bag and securely close the freezer bag.
5. Gently flip the bags back and forth by the corners. The outer bag will be quite cold, so you
want to grip only the edges. At some point, open the outer bag, when you have a salt
water/ice mixture and record the temperature again.
6. When you have ice cream (5-15 minutes), take the smaller bag out and rinse it off with cold
water. Be especially careful to rinse the seal area so you don’t get salt in the ice ceam.
Dump the excess salt and ice down the sink Throw the bag away in the trash.
7. Please clean up your area. (Leave it neater than you found it.)
Questions: Place your answers on the scantron form. (2 points each)
1. What state of matter was the milk when you began?
A. solid
B. liquid
C. gas
D. plasma
2. What state of matter was the milk when you were done?
A. solid
B. liquid
C. gas
D. plasma
3. In order to change the phase of milk, what had to be removed?
A. water
B. sugar
C. energy
D. salt
4. What happened to the majority of the heat energy that left the milk?
A. The heat energy flowed into the ice
B. The heat energy flowed into the room
C. The heat energy flowed into the bag
D. The heat energy never left the milk
5. Why was salt added to the ice?
A. salt was used to lower the temperature of the ice
B. to make the lab complicated
C. salt is a common ingredient in cooking
D. Because if it was all added to the milk it would taste disgusting
6. How does adding salt make the ice colder than plain ice water?
A. Ocean water is colder than fresh water
B. There is a chemical reaction that is endothermic
C. There is a physical change that is exothermic
D. Separating the sodium and chloride ion requires energy which is removed from the water.
7. If you did not add sugar would the ice cream have frozen faster?
A. Yes
B. No
8. Why did the outside of the larger bag get wet?
A. Water vapor lost enough energy to liquefy on the outside of the bag.
B. Students did sloppy work and got it wet.
C. Students mixed the ingredients so thoroughly that they worked up a sweat.
D. Water from the milk escaped through the pores in the bag.
9. How do you explain the rapid increase in the amount of the water in the large baggie?
A. Water from the milk escaped through the pores in the bag
B. Ice melted because the freezing point of the salt water mixture is lower than fresh water.
C. Humidity from the air went inside the bag
D. Energy from the milk was transferred to the large baggie increasing the temperature
above the freezing point of the salt/ice mixture.
10. What is a colligative property?
A. A property bought by somebody who went to college
B. A property that is based on chemical properties
C. A property that is based on physical properties
D. A property that is based on the number of particles dissolved in a solution
11. What do you think would happen if pure ice is used instead of the salt-ice mixture?
A. You would not have a low enough temperature to solidify the milk, sugar, flavor mixture
B. It would work just fine
12. Why is salt placed on icy patches on highways and steps in winter time?
A. To melt the ice
B. To make a mess
C. To create a commodity that can be sold in the winter time.
13. Why is salt rather than sugar placed in the ice?
A. Salt has more colligative properties because it separates into two ions when dissolved in
water
B. Sugar has more colligative properties because it doesn’t separate into multiple ions when
dissolved in water
C. Salt has less colligative properties because it separates into one ion when dissolved in
water.