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Freezing Ice Cream (10 Favorites)

LAB in Freezing Point, Phase Changes, Freezing Point Depression. Last updated February 4, 2019.


Summary

In this lab, students will investigate changing states of matter, chemical reactions, and the properties of ice and salt while creating their own ice cream.

Grade Level

Middle and High School

Objectives

By the end of this lab, students should be able to

  • Differentiate between a solid, liquid and a gas.
  • Describe phase changes, such as freezing and melting.
  • Define the meaning of freezing point for a substance.
  • Explain the basic concept of freezing point depression.

Chemistry Topics

This lab supports students’ understanding of

  • States of matter
  • Freezing Point
  • Freezing Point Depression

Time:

Teacher Preparation: 15 Minutes

Lesson: 45 Minutes

Materials

(Per student)

  • 1 cup half & half or whole milk
  • ½ teaspoon vanilla
  • 2 tablespoons sugar
  • 4 cups crushed ice
  • ½ cup rock salt
  • 2 quart size zip-top plastic bags
  • 1 gallon size zip-top freezer bag
  • Plastic spoon
  • Bowl

(To share)

  • Thermometers
  • Timer
  • Gloves or hand towels (If students could bring their own towel, this would be helpful.)
  • Measuring cups
  • Measuring spoons
  • Paper towels
  • Crushed cookies or candies (optional)

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials.
  • Once the ice begins to melt, it will get messy. Please ensure students have enough paper towels at their station to clean up the water.

Teacher Notes

  • It is easier and much faster to set up all the materials at each station and allow students to add in their materials upon teacher’s permission. If not, the teacher will need to go to each student and add in every ingredient which could take more than the projected amount of time.
  • This is a very good article to have students read prior to the lab.

Background information:

  • The freezing point of water is 32⁰F or 0⁰C.
  • The freezing point of salt water is lower than pure water. The presence of salt in water reduces the freezing point of water. Therefore, the more salt in the water, the lower the freezing point will be. The freezing point of salt water can be as low as -21⁰C. This is called freezing point depression.
  • Salt molecules disrupt the normal solidifying of water molecules which causes the freezing rate to slow down. This is the reason salt is often used on icy roads to slow down freezing and to make them safer to travel upon.
  • States of matter: solid, liquid, gas.
  • Matter: Anything that occupies space.
  • Solid: A substance that has its own shape.
  • Liquid: A substance with the ability to flow easily and takes the shape of its container
  • Gas: A substance that does not have its own shape.
  • This article provides a good extension option.

For the Student

Lesson

Background

The freezing point of water is 32⁰F or 0⁰C. The freezing point of salt water is lower than pure water. That means that salt water will be a liquid solution at very cold temperatures; the same temperatures that cause pure water to freeze! The freezing point of salt water can be as low as -21⁰C.

Pre-lab Questions

  1. Describe the difference between a solid, liquid and a gas. Use sketches to help your explanation.
  2. What happens to a substance at its freezing point?
  3. What is the temperature for the freezing point of water?
  4. Is the freezing point of salt water warmer or colder than plain water? What is a possible temperature for the freezing point of salt water?
  5. Why is salt put on the roads in the winter time?

Problem

How can we use salt and ice to help us make ice cream?

Materials

  • 1 cup half & half or whole milk
  • ½ teaspoon vanilla
  • 2 tablespoons sugar
  • 4 cups crushed ice
  • ½ cup rock salt
  • 2 quart size zip-top plastic bags
  • 1 gallon size zip-top freezer bag
  • Thermometer
  • Timer
  • Gloves or hand towels
  • Measuring cups
  • Measuring spoons
  • Plastic spoon
  • Paper towels
  • Bowls

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow your teacher’s instructions for how to clean-up your materials.
  • Once the ice begins to melt, it will get messy. Please make sure to clean up the water.

Procedure

  1. Add 2 tablespoons of sugar, 1 cup of milk, and 1/2 teaspoon vanilla to the quart Ziploc bag. Seal the bag securely.
  2. Double check your bag to ensure that it is completely sealed!
  3. Put 4 cups of ice into the gallon Ziploc bag.
  4. Use a thermometer to measure and record the temperature of the ice in your data table at “0 minutes.”
  5. Add 1/2cup of rock salt (sodium chloride) to the bag of ice.
  6. Start your timer.
  7. Every minute record the temperature of the salt-ice mixture in the data table below.
  8. Place the sealed quart bag inside the gallon bag of ice and salt. Seal the gallon bag securely. NOTE: You are not pouring the mixture over the ice. Keep the bag containing the milk SEALED at all times!
  9. Gently rock the gallon bag from side to side. It's best to hold it by the top seal or to have gloves or a cloth between the bag and your hands because the bag will be cold enough to damage your skin.
  10. Continue to rock the bag for 10-15 minutes or until the contents of the quart bag have solidified into ice cream.
  11. Remember, every minute open only the gallon bag ad use the thermometer to measure and record the temperature of the salt-ice mixture in the data table below.
  12. When the ice cream bag has solidified, remove the quart bag, open it, serve the contents into cups with spoons and ENJOY!

Data

Time (minutes)

Temperature (C)

0 (no salt added)

1

2

3

4

5

6

7

8

9

10

Analysis

  1. What was the temperature of the ice in the bag before the salt was added? How did this compare to the temperature of the ice after 10 minutes?
  2. Attempt to explain what effect the salt has on the ice, and why it is important for making your ice cream.
  3. What is another situation in your life where adding salt to water or ice could be helpful?
  4. Often people will add salt to boiling water, for example when they are cooking a pot of spaghetti. Thinking about what you learned today—what impact do you think the salt will have on the boiling point of water and the spaghetti?