Ice cream experiment

Ice Cream Experiment!

Megan, the brilliant mind behind the blog Coffee Cups and Crayons, has a super fun recipe for Ice Cream in a Bag.  It is literally hand-made ice cream.  You (or the kids, depending on how good they are at pouring) add the ingredients to a zip-top bag, then place that bag inside another bag, in which they add ice and ice cream rock salt.  After a lot of shaking, the ice cream ingredients turn to into solid ice cream!

You may notice that I have this tagged as “tech.”  Yes, it’s pretty low-tech, but it’s still tech.  This activity is a way to introduce (or reinforce) a few scientific principles and get young minds to apply STEM to their life.

  • Discuss States of Matter.  What does solid, liquid, and gas mean?  Talk about how ice is solid water, and how ice cream starts out liquid (the ingredients you added), but turns into a solid food as it gets colder.
  • Why do we add salt to the water?  Impure water (such as water with salt) has a harder time freezing and making the solid crystalline structure required to be solid.  The impurities get in the way of water making perfect crystals.   To quote from, “Only a little water melts before some of the dissolved salt lowers its freezing point. …it can absorb lots of heat from the water to do it, and the water still will not freeze. … The water is very much colder than the cream mixture. Because heat flows from hot things to cold things, the cream now loses its heat to the water and rapidly cools down.” (There’s much more about the science of ice cream making at the link above.)
  • Discuss thermodyamics.  No, we’re not going into a massive treatise on heat transfer, but you can talk to your kids about how heat travels from hot things to cold things.  When they hold the bag of ice, the heat from their hands leaves and is absorbed by the cold ice.  Likewise, the heat from the ice cream ingredients transfers to the cold salt-ice.  They aren’t absorbing cold; the cold item is absorbing heat, and the lack of heat makes them feel cold.  Also, the reason why the cold absorbs the heat is that things want to be in a state of equilibrium.  The cold thing wants to be at the same temperature as the warm thing.  Once they’re the same temperature, they transfer of heat between them will stop.
  • Discuss the concept of insulation.  Have the kids put gloves or mittens on when their hands get cold from handling the bag of ice.  Notice now that their hands aren’t transferring as much heat?  It’s because the heat has to get through the gloves.  The gloves will slowly transfer their heat to the bag of ice as they receive heat from the hands inside of them.  It takes more energy to transfer that heat, so it takes longer for the hands to feel cold.

Of course, keep the lessons age appropriate.  Your middle schooler might get more out of the insulation and thermodynamics than your preschooler.  My daughter is still at the states of matter stage.  And the share-with-your-baby-brother stage.  Both are important lessons.

Here is the recipe for the ice cream, thanks again to Coffee Cups and Crayons!

  • 1/2 cup of half and half (also called single cream and 10% coffee cream)
  • 1 tablespoon sugar
  • 1/2 teaspoon of vanilla
  • quart size plastic bag (good quality)
  • gallon size plastic bag
  • ice
  • 4 tablespoons ice cream rock salt

Mix together the half and half, sugar, and vanilla in the quart size bag and seal it well.  Then put the quart size bag inside the gallon bag and fill it with ice.

Pour the ice cream salt on top of the ice and seal the gallon bag.  Then shake!