Monday, June 11, 2012

Tasty Science: Homemade Vanilla Ice Cream in a Ziploc Bag

     Here is a FUN activity and lesson that I did a while back.  I'm sharing it with you to do with your children this summer -- along with some history and science!  The end result is yummy ice cream!

READ:  Should I Share My Ice Cream by Mo Willems

HISTORY:  No one knows when ice cream was invented.  About 3,000 years ago, the Chinese mixed rice, snow, and milk together to make a tasty treat.  In Persia, about 400 BC, people would pour grape juice on snow and eat it when it was hot outside.  The Emperor Nero had ice and snow brought to him in Rome to eat with fruit juice.  It seems that the Arabs were the first to use milk to make ice cream sweetened with sugar.  In the 10th century, many cities in the Arabic world such as Baghdad, Damascus, and Cairo produced ice cream with milk, cream, or yogurt and flavored it with rosewater, dried fruits, and nuts.
     Here in America, the British brought ice cream to the American colonies about 300 years ago, but only the wealthy could eat it because ice was very rare and expensive.  In the 1840s, Nancy Johnson from New Jersey invented a hand-cranked ice cream maker, and the rest is history!

How does it happen:
  • Milk can change from a liquid to a solid with the help of ice and salt.
  • It involves the exchange of heat.
  • Cold does not exist by itself. It is simply the absence of heat.
  • For example, a cold room is cold because some of its heat energy has escaped.
  • Heat energy goes from places with more heat energy to places with less.
  • The milk mixture has more heat energy than the salt and ice does.
  • Therefore, heat energy goes from the milk mixture to the ice and salt.
  • The milk mixture lost heat energy, which caused it to cool and freeze, eventually becoming ice cream, a solid.
  • The ice gained heat energy, which caused it to melt, eventually becoming salt water, a liquid.
  • When you add salt to the ice, it lowers the freezing point of the ice, so even more energy has to be absorbed from the environment in order for the ice to melt.  This makes the ice colder than it was before.
  • So the addition of salt lowers the freezing temperature, which causes the ice cream to freeze sooner.
  • If you do not add the salt, the mixture will get cold but only as cold as the ice – so it will be more like a thin milkshake.
TRY THIS:  Put ice and water in two cups.  Add salt or rock salt to one.  Stir.  Take the temperature of both.  Which is colder?

  • So….The ice cream mixture is about 32 degrees.
  • The salt-ice mixture is below 32 degress.
  • Heat flows from warmer to cooler objects, so the energy from the ice cream mixture flows to the salt-ice mixture.
  • The ice cream mixture gets colder, and the salt-ice mixture begins to melt.


Large gallon size ziploc/re-sealable bag
2 Quart sized ziploc/re-sealable bags
1 cup half and half
½ to ¾ teaspoon Vanilla
2 tablespoons Sugar
½ to ¾ cup Rock salt

STEP 1. Carefully pour the half and half, vanilla and sugar into one of the small bags. Seal tightly, squeezing out as much air as possible.
STEP 2. Place the bag with the mixture inside the other small bag and seal it, leaving as little air as pos­sible.
STEP 3. Place the ice inside the one-gallon re-sealable bag.
STEP 4. Place the two small bags containing the mixture inside the larger bag with the ice. Make sure the ice surrounds the bags with the cream mixture.
STEP 5. Sprinkle the salt over the ice in the larger bag.
STEP 6. Seal the larger bag, letting as much air escape as possible.
STEP 7. Wear gloves or wrap bag in a towel.
STEP 8. Shake and massage the bag for approximately 15-20 minutes.  
STEP 9.  Remove small bags from the large bag, open, and eat!  (Be careful not to get salt in the ice cream.  You may want to rinse off the bag in the kitchen sink before opening it.)

*History of Ice Cream,  Wikipedia 
*Ice Cream:  The Full Scoop by Gail Gibbons

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