Ice Cream in a Bag
Concepts:
Freezing point, energy transfer, chemical/physical changes
Description:
Today, we’ll be making the delicious snack, ice cream, while learning how salt can lower
the melting point of ice. Each student will make one cup of ice cream. Making the ice
cream involves lots of chemistry:
• Heat energy flows from a region that is hotter to a region that is less hot;
therefore, energy is transferred from the ice cream mixture (liquid) to the ice (a
solid).
o Solid: very little molecular movement
o Liquid: more molecular movement
•
•
The ice has to absorb energy to melt and goes through a phase change from
solid to liquid.
o On the other hand, the ice cream mixture losses energy and goes through
a phase change from a liquid to a solid.
When salt is added to the ice, it lowers the freezing point and makes the ice even
colder.
•
o The ice has to absorb even more energy before it can melt!
This is how the ice cream freezes at a faster rate.
The change of the liquid ice cream mixture to solid ice cream is a physical
change.
o Physical change = A change that is reversible by doing the opposite
procedure.
In this case, we froze the liquid mixture to a solid. So to reverse the
physical change, we melt the solid ice cream.
o Chemical change = A change that is NOT reversible.
Baking a cake or making scrambled eggs are examples.
What type of engineering?
Today’s project focuses on chemical engineering. Chemical engineers make use of
compound properties, such as freezing point, energy transfer, and chemistry all the
time.
• REAL WORLD EXAMPLE: Salt used on icy roads
o Ice starts to form when water reaches its freezing point at 32°F (0°C).
o When you add salt, the freezing point drops and therefore can stay a liquid
at lower temperatures.
Today, adding salt to the ice lowers its freezing point past 32°F. Since the temperature
in the big baggie is colder, the milk and cream (which are water-based) in the inside bag
are also cooled below the freezing point. This means they freeze, and you get ice
cream! The more salt you add, the lower the freezing point will drop.
Materials
• ½ cup milk
• ¼ cup heavy whipping cream
• ¼ cup sugar
• ¼ tsp vanilla
• ½ - ¾ cup rock salt
• 2 cups ice
• 1 quart size Ziploc bag
• 2 gallon size Ziploc bag
• Measuring cups/spoons
• Chocolate chips, Oreos, chocolate sauce, etc.
• Plastic cups and spoons
Procedure
Note for mentors: use an assembly line system
1. Add milk, cream, sugar, and vanilla to the SMALL, quart-sized Ziploc bag
2.
3.
4.
5.
Duct tape the SMALL, quart-sized Ziploc bag to make a good seal
Add ice to the LARGE, gallon-sized Ziploc bag
Add rock salt to the LARGE, gallon-sized Ziploc bag with ice
Place the small bag (with ingredients, sealed tight) into the large bag of salt and
ice. Seal the large bag carefully.
6. Place the large bag into another large, gallon-sized Ziploc bag and seal tightly.
7. Shake the bag gently for 10-15 minutes or until mixture is solid.
a. WARNING: The bag gets VERY cold! Pass it off to a partner or mentor
every few minutes so you don’t hurt your hands. When the contents in the
small Ziploc bag have solidified, you’re ready to eat!
8. Remove the quart-sized bag and wash it with clean water.
9. Cut a small corner off and squeeze it into a plastic cup.
10. Add sprinkles and other condiments
11. ENJOY!!!!
***ATTENTION MENTORS: Please wash all measuring cups! Thanks! ☺***