Chemical Engineering - Lowering the Freezing Point


Activity 5.1.1 Let’s Make Ice Cream

Introduction
Suppose you have a great recipe for chocolate ice cream. You like to make it at home for your family and friends. You make it in a little one-gallon machine that goes into your freezer. But what if you sell your recipe to a big food company? Now they have to be able to make thousands of gallons a day. Each gallon of ice cream needs to look and taste exactly the same. What kind of equipment could the company use? How would the recipe change? How can the factory make the ice cream at low cost? These are all questions for the chemical engineer.
Ice must absorb energy in order to melt, changing the phase of water from a solid to a liquid. When you use ice to cool the ingredients for ice cream, the energy is absorbed from the ingredients and from the outside environment (like your hands, if you are holding the baggie of ice). When you add salt to the ice, it lowers the freezing point of the ice, so even more energy must be absorbed from the environment in order for the ice to melt. This makes the ice colder than it was before, which is how your ice cream freezes.
You could use other types of salt instead of sodium chloride (NaCl), but you couldn't substitute sugar for the salt because (a) sugar doesn't dissolve well in cold water, and (b) sugar doesn't dissolve into multiple particles. Compounds that break into two pieces upon dissolving (e.g., NaCl breaks into Na+ and Cl-) are better at lowering the freezing point than substances that don't separate into particles. This is because the added particles disrupt the ability of the water to form crystalline ice. The salt causes the ice to absorb more energy from the environment (lowering the freezing point), so although it lowers the point at which water will re-freeze into ice, you can't add salt to very cold ice and expect it to freeze your ice cream or de-ice a snowy sidewalk (water must be present). This is why NaCl isn't used to de-ice sidewalks in areas that are very cold.
In this activity you and a partner will make and eat ice cream. Notice how the salt affects the melting point of the ice.

Equipment
  • GTT notebook
  • Pencil
  • Measuring cups and measuring spoons
  • Thermometer
  • Napkins
  • Spoons
  • Cups
  • Bucket or sink to dump ice in when ice cream is made

The following ingredients are needed for each group of 2 students:
  • ½ cup milk
  • ½ cup whipping cream (heavy cream)
  • ¼ cup sugar
  • ½ - ¾ cup NaCl (sodium chloride) as table salt or rock salt
  • 1 quart-size bag
  • 1 gallon-size bag
  • 3 cups ice
  • ¼ teaspoon vanilla for vanilla ice cream
  • 1 tablespoon strawberry syrup – omit ½ of the sugar for strawberry ice cream
  • 1 tablespoon chocolate syrup – omit ½ of the sugar for chocolate ice cream

Procedure
You and your partner need to determine which flavor ice cream you will be making. If you are making chocolate or strawberry ice cream, use only 2 tablespoons of sugar.
  1. Pour the milk, whipping cream, sugar, and (vanilla, chocolate, or strawberry) into the small Ziploc bag. Squeeze as much air out as possible and seal the bag.
  2. You may want to save adding the sugar for last because it varies depending upon the flavor of ice cream chosen.
  3. Make sure the bags aren’t leaking. Double bag if necessary.
  4. Fill the gallon size bags a quarter full with ice (about 3 cups).
  5. Use a thermometer to measure the temperature of the ice in the gallon bag and record the temperature in Conclusion Question 1 below.
  6. Add the salt to the bag of ice
  7. Place the sealed quart bag into the gallon bag and seal securely.
  8. Your teacher will assign your group the mixing method that you will use to make your ice cream. Examples of mixing methods include:
  • Gently knead
  • Shake violently
  • Flip over and over
  • Rock back and forth
  • Carefully toss (make sure you catch)
  • Leave on table and push back and forth on sides
  1. Continue to mix the contents of the bag until the contents of the quart bag have solidified into ice cream.
  2. Record your mixing method and the time it took in Conclusion Question 2.
  3. Open the gallon bag and use the thermometer to measure and record the temperature of the ice/salt mixture in Conclusion Question 4.
10.Once done, put your ice bags in the bucket or sink
11.Divide your ice cream bag into two cups and eat your ice cream!
Conclusion
1. What is the temperature of the ice?
2. What mixing method did your group use and how long did it take to make ice cream?
3. How does the texture of your ice cream compare to other students in the class who used a different mixing method?
4. What is the temperature of the ice and salt after you have made ice cream?
5. How can melted ice be colder than an ice cube?
6. What does the salt do to the ice?
7. How does your ice cream taste compared to what you can buy in the store?
8. How could a chemical engineer help you if you wanted to sell this recipe so you could make thousands of gallons of ice cream a day?