University of Virginia
Physics Department

## Tasty Phase Change - The Ice Cream Lab

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.2
• PS.5
• PS.7

Objectives

Students will:

• Investigate the effects of temperature change on phase changes;
• Investigate the effects of changes in freezing point;
• Utilize these concepts in making ice cream.

Motivation for Learning

For homework the night before have students bring in an old towel, a pair of water-proof warm gloves, and a plastic (grocery store or Wal-Mart) bag to take everything home. Evoke prior knowledge of making homemade ice cream and ice cream makers. (I like to use the following true/false question. Ice cream makers use salt in the outside cylinder to decrease the friction as the cylinder turns around. The answer is false because the salt is used to lower the freezing point of the mixture inside.)

Background Information

In order to have a phase change in matter heat must be either gained or lost. Phase changes occur all around us in everday life. For instance, ice melts when a drink is left in a room at normal temperature; conversely, water freezes when place in a really cold temperature (the freezer). In this experiment we see how heat is lost in order to change the milk from a liquid state to a solid state. This is also an example of a physical change in matter.

Students will also be able to observe how adding solute (ice cream salt) to a solvent (ice) changes the physical properties of that solvent. In this case the freezing point of the ice is lowered allowing for the milk to turn into ice cream.

For a group of about 150 students this lab cost around \$35- \$40.

### Student Activity

Materials

• 240 mL milk
• 45 mL sugar
• cups
• 80 mL ice cream
• salt
• 2.5 mL vanilla or chocolate flavoring
• 50 mL beaker
• ice
• 400 mL beaker
• 3.8 L zipper bag (gallon); freezer quality
• gloves
• 0.95 L zipper bag (quart); freezer quality
• dish towel
• spoons
• Celsius thermometer

Procedure:

1. Find a partner to work with.
2. Place a dishtowel over your work area. Keep your work on the towel.
3. Pour 240 mL milk, 45 mL sugar, and 2.5 mL vanilla or chocolate flavoring into the 0.95 L zipper bag. CAREFULLY seal the bag and shake up the mixture thoroughly.
4. Put this small zipper bag inside the much larger 3.8 L zipper bag.
5. In the 3.8 L bag add enough ice to cover the 0.95 L bag and add 80 mL of ice cream salt. Take the temperature of the ice:______ C
6. CAREFULLY SEAL THE BAG!
7. Put your gloves on and get ready to make a phase change!
8. Take turns flipping the bag. Hold the bag by its corners. Keep the bag flipping over and over. Remember to keep the bag over the towel at all times. It should take 10 to 15 minutes to freeze. Take the temperature of the ice/water mixture again: ________ C
9. When you have ice cream, take the smaller bag out and rinse it off with cold water. One partner needs to take the larger bag and it's contents to the trash bag. DO NOT DUMP IT DOWN THE SINK!!!
10. Dish out the ice cream equally into the cups, and ENJOY! (You may rinse the cup out and use it for water if you are thirsty.)
11. Please clean up your area. (Leave it neater than you found it.)

Student Questions

Student Questions are included on the Student Copy of the activity. To print out the Student Copy only, click here.

Answer the following on a separate sheet of paper:

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 phase of the milk, what had to be removed?
4. What happened to the heat energy that left the milk?
5. Why was the salt added to the ice?
6. If you did not add sugar would the ice cream have frozen faster? Why?
7. Why did the outside of the bag get wet? (Assume that your bag did not spring a leak.)

Students with Special Needs

Each student should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

Assessment

Did the student make ice cream? Was the student able to answer the lab questions and comprehend the scientific principles behind the lab?

Take time to go over the answers to the questions on the students' sheet:

1. The milk was in the liquid phase of matter when we started.
2. The milk was in the solid state of matter when we finished.
3. In order to change the phase of the milk we had to remove heat.
4. The heat energy that left the milk went into the surroundings.
5. Salt was needed in order to lower the freezing point of the mixture inside.
6. Yes, the ice cream would have frozen faster without sugar because the sugar lowers the freezing point.
7. The outside of the bag was wet from condensation.

Extensions

If time allows have the students convert the recipe from the metric system to the standard system.