Ice Cream in a Bag - Hands on Lesson about the Changing States of Matter

 

No doubt, one of the most popular presenter topics that I get asked to do is to have a lesson that ends with Ice Cream.  The sweet treat is a good pay off for students that have a hard time listening to a guest speaker AND making it by hand has some great lessons to teach about Matter changing Phases and Energy Transfer (depending on the age level).  

The opportunity to repeat my Ice Cream in a Bag demo came up again this week, so armed with better photo's this round - I'm excited to share with you the basic chemistry of Ice Cream making!

How does it work and what does the Salt do?

Solids, liquids, and gases are the three most common states of matter on earth (Although we could also add plasma, and Oobleck if we wanted too!).  All matter is made out of atoms.  The speed at which the matter is vibrating is what causes matter to change into different forms.  If the atoms are vibrating faster and faster - matter will melt from a solid to a liquid and evaporate from a liquid to a gas.  If the atoms begin to slow down, matter will condense the opposite direction from gas to liquid and freeze from a liquid to solid.   

NASA - Salt Water Freeze
One of the most common substances on earth is water.  We know that water freezes at 32°F and boils (and also evaporates into water vapor) at 212°F.  During winter, we can sprinkle salt on our sidewalks to help the snow and ice melt off.  The reason this works is because adding salt changes the temperature that water would normally freeze at to a lower temperature.  Take the ocean as a great example - out in the open ocean, the salt water will freeze at 28°F, or 4 degrees colder than the typical freezing point of water.



When we add rock salt into our ice bag to help make ice cream, some of the ice melts, but heat is also transferred from the cream into the melting ice, which actually makes the cream colder.  This transfer of heat (energy) from cream to ice results in the cream freezing, and the inner bag developing that sweet treat we are all now craving! 

Note:  For older audiences, expand this lesson into Energy Transference as energy is used to help transfer heat and therefore change matter from state to state.  


Steps to Making Ice Cream in a Bag

Supplies Needed (Per Student):

  • 1 gallon zip-lock bag
  • 2, 1-quart zip-lock bags
  • 1/2 cup Half & Half
  • Crushed ice (or snow) - Enough to fill half of your gallon zip-lock bag
  • 1/2 cup Rock Salt
  • 1/2 Tea sp. Vanilla (Imitation ok!)
  • 1 Tabl sp. Sugar
  • Roll of Paper Towels
  • Measuring utensils 
  • A spoon for enjoying!

Optional:

> Add your favorite flavors like mint extract and mini chocolate chips or fresh strawberries!

Directions:

  • Fill the gallon zip-lock bag half full with crushed ice.
  • Add 1/2 cup of rock salt to the ice. Seal the plastic bag and wrap it in a towel. Shake the ice and salt for five minutes. The temperature of the rock salt and ice mixture gets down to about 14 degrees F (-10 degrees C)!
  • Add the following ingredients to the 1 quart-zip-lock bag and shake to mix:
    • 1/2 cup of half & half
    • 1 tablespoon sugar
    • 1/2 teaspoon vanilla extract
    • Note: if you are adding other flavors, add them here.  Leave toppings out until ice cream is solid.
  • Zip the lock closed, and just before you finish the seal, try to squeeze out as much of the air in the bag as you can. Double check that your 1 quart zip-lock bag is completely sealed. Seal tightly, allowing as little air to remain in the bag as possible.
  • Place this bag inside the other quart zip-lock bag, and as you zip the second bag closed, remove as much air as you can. Double check the seal.
  • Place the two bags inside the gallon zip-locked bag with the ice and seal the bag. Wrap the bag in the towel again. GENTLY! Shake, rock, roll, and mix! Your ice cream should be ready after about 15-20 minutes.
  • Once mixed, remove the inner bags from the gallon zip-lock bag and rinse them well with COLD water. You don’t want any salt water accidentally getting into your ice cream. 
  • Add any additional toppings now and Eat and enjoy! 
Ice Cream with Berries and Mini Chips

Final Thoughts 

Of course, there are several ways this lesson and experiment can be varied, but you get the basic bones of the process above.  For ease, I've included the 1 page printable version of this activity below from CSU Extension.  Hopefully you've enjoyed this lesson and until next time - Keep on STEMin! 

Downloadable PDF Version - Ice Cream in a Bag
Links to National Ag in the Classroom age group versions - Ice Cream Lessons
I am a STEM/k12 Educator with Colorado State University Extension. I love working with youth and encouraging them to participate in many forms of hand on education! Overall, our goal is to promote positive youth development and encourage youth to pursue higher education after high school.

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