Classroom Ice Cream

Target Curriculum: Science

Target Grade: 8

SOLs: SCI.PHS.2   SCI.PHS.5   SCI.PHS.6   SCI.PHS.7  
Time: 45 minutes

Objective:
To demonstrate that a solute will lower the freezing point of a solvent, that shaking or stirring will make a solute dissolve faster, and that energy must be removed in order for a liquid to freeze.

Purpose:
To reinforce concepts relating to solutes and solvents, solubility and how to increase it, freezing point depressions, states of matter, energy changes involved in changes of states of matter, and the property of water which causes it to expand when it freezes.

Materials:
milk
sugar
vanilla
ice
ice cream salt
teaspoon
measuring cup
measuring spoons
1 quart zipper bag
1 gallon zipper bag
towel
paper towels
spoon

Procedure:
STUDENTS ARE GIVEN THE FOLLOWING INSTRUCTIONS:
1. Place a folded towel on the lab table. Because bags sometimes leak and condensation always forms on the outside of the bag, the towel will absorb any liquid. Make your ice cream ONLY on the towel.

2. Into your small zipper bag, pour 1 cup of milk, 1/2 teaspoon vanilla, and 3 tablespoons of sugar. Carefully seal the bag and swish it around to mix thoroughly. This makes 2 servings of ice cream.

3. Place the sealed small bag into the large bag.
4. In the large bag, add one scoop of ice and one scoop of salt. Seal the large bag.

5. Lay the bag on the towel. Take turns flipping it over and over until the ice cream is frozen. Be sure to hold the large bag only by the corners as it will be very cold. DO NOT LIFT THE BAG COMPLETELY OFF THE TOWEL OR TOSS IT INTO THE AIR!

6. When the ice cream is frozen (about 10-15 minutes), open the large bag and remove the small bag. Place the large bag and all of its contents into the large trash can. DO NOT PUT ANYTHING DOWN THE DRAIN.

7. Quickly rinse the small bag in cold water to remove the salt from the outside of the bag. Cut off one corner of the bag and squeeze ice cream into 2 cups.

8. EAT with spoons!

9. If you are thirsty when you finish eating your ice cream, wash out your cup at the sink and drink water from the faucet.

10. Clean up by throwing cups and spoons into trash cans. Wipe up any spills and throw away paper towels.

Observations:
Students will observe that the ice cream freezes in 10-15 minutes, or less depending on the temperature of the milk mixture when they started.

The volume of the semi-solid ice cream will be greater that the volume of the liquid milk.

Conclusions:
1. It is necessary to "swish" the sugar in order to make it dissolve in the milk.

2. The salt causes the the freezing point of the ice to be lowered, and consequently, the temperature of the melting ice is below 0 degrees Celsius.

3. It is necessary to remove energy (heat) from the milk mixture to freeze it or change it to a solid. The energy is used to melt the ice and the ice mixture becomes colder.

4. Water expands when it freezes. One cup of milk makes more than one cup of ice cream.

Extension:
1. Try inserting a thermometer into melting ice which has no salt added and into the ice/salt mixture and compare the two temperatures.

2. Discuss the possibility of changing the flavor of the ice cream by adding various flavorings.

3. If you use rock salt, there will be pieces of rock (probably granite) present. Take this opportunity to explain that the salt used on our food is the same sodium chloride, but has been refined to remove the pieces of rock and other impurities.

Class Discussion Questions:
1. What state of matter was the milk when you began?

2. What state of matter was the milk when you were done?

3. In order to change the milk to a solid, what had to be removed?

4. What happened to the energy that left the milk?

5. Why was salt added to the ice?

6. If you had left out the sugar, would the ice cream have frozen faster or more slowly?

7. How could you make your ice cream taste better?

8. Assuming the bag was not leaking, why did the outside of the bag become wet?

9. Why is salt spread on icy roads in the winter?

10. What would happen if you didn’t add salt to the ice?

11. Why do ice cream makers have two containers, one of which fits inside the other?

Cautions and Concerns:
1. Make sure that students keep their plastic bags on the table; burst bags make a big mess!