Homemade Ice Cream Science

Who doesn’t like a nice, cold scoop of ice cream on a hot summer day?  This experiment will give you your own icy cold treat, and you get to learn some cool science along the way!  And, you don’t even need an ice cream machine - just a few simple items that you probably already have at home!

  What You’ll Need:
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• Whipping or Table Cream, or you can use ordinary milk, or Half and Half
• Ice cubes or crushed ice
• Salt (Ice Cream Salt or rock salt works best, but driveway melting salt or table salt will work)
• Sugar (optional)
• Ice Cream Toppings (optional)

Be sure to ask your mom, dad or another adult to help- you can share your ice cream with them!

Experimental Procedure:

1. Fill a gallon Zip-Lock bag about halfway full of ice.
2. Add 6 tablespoons of salt to the ice in the bag.  You might also add a little liquid water (just a couple tablespoons is enough).
3. Pour 1 cup of cream (or milk, a mixture of milk and cream, or Half and Half) into a quart ziplock bag.  If desired, you can add some sugar to make it sweeter (most ice cream that you buy has a LOT of sugar, but you don't need that much).  Carefully squeeze out most of the air and make sure the bag is tightly zipped all the way closed.
4. Shake vigorously for about 7 minutes.
5. Unzip the gallon bag.  Take out the quart bag and rinse it off with water, then wipe the top of the quart bag before you open it to make sure no salt water gets in your ice cream.
6. Add toppings if desired, and dig in!

What’s Happening:

The science behind this is actually rather complex, and is explained in much greater detail in our Instant Freeze Super-Cooled Water lab.  Basically, when salt (or anything else) is dissolved in water it lowers the freezing point of the salt-water solution below that of pure water alone.  In other words, when you put the salt and the ice together, you are making a solution that can get much colder than 32 degrees Fahrenheit (the normal freezing or melting temperature of pure water) and still remain liquid.  This is called freezing point depression, and it can be very useful when you are trying to get something else really cold (in this case, the ice cream).  Freezing point depression is also why the milk or cream needs to be much colder than 32°F before it will freeze; milk is mostly made of water, but with a lot of other stuff dissolved in it, and that lowers its freezing point too.  As you shake the big bag, sloshing around the smaller bag inside, the super cold salt-ice-water solution in the big bag takes heat away from the cream/milk inside the smaller bag, lowering the temperature enough for it to freeze.  You should also notice that this heat flowing out of the small bag melts even more of the ice in the big bag- it gets sloshier as you shake it.  Of course heat from your hands does the same thing (and you might want to wear gloves as you shake the bags).

It's very important that you shake the bags vigorously while the milk/cream is freezing.  This breaks up the ice crystals that are forming inside the smaller bag and keeps your final ice cream smooth and creamy- just the way you probably like it.  If you don't shake the bags, as these ice crystals form they will stick to each other and you'll end up with a hard frozen block of milk instead- a milk-sicle!  Try it by placing another Zip-Lock bag or cup of milk in your freezer to see what happens.  

Variations and Related Activities:

Here's another way to make ice cream, which uses the same ideas, but slightly different materials:
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1. Fill a large bowl about half full of ice cubes or cubes.  Add plenty of salt and a little liquid water, so that the ice cubes can move around much more freely in the bowl, and at least most of the salt dissolves.
2. If you have a thermometer, measure the temperature of this ice-salt-water bath.  You should try to get it down to 10-15°F.  You may need to add more salt or ice to achieve this.  If you don't have a thermometer, just make sure you have added plenty of salt.
3. Pour a small amount of your milk/cream mixture into a small cup or glass.  The less milk/cream you have, the faster it will freeze into ice cream.  2-3 ounces should be enough for a good experiment, and you can always make more after you eat it.
4. Nestle the cup deep in the middle of the ice-salt-water bath in the big bowl (but not so deep that any salt water might spill into the cup).
5. Gently move the cup around in the ice bath as you also stir the milk/cream inside with a spoon.
6. When your ice cream is frozen to your liking, enjoy!

You can play around with many different ways to make ice cream.  What if you used skim milk?  2% milk?  A combination of the two?  Does it take longer for them to freeze?  What if you used chocolate milk?  Do you prefer richer ice cream (made with pure cream), or not-so-rich ice cream (made with Half and Half)?  What about ice milk (which is ice cream made with milk and no cream at all)?  Which tastes the best?

Another way to do this is to get two metal cans (coffee cans work well).  One should be able to fit inside the other, with room for the ice and salt.  It should also have a very tight-fitting lid.  Put the milk/cream mix inside the small can, and put the lid on it.  You might want to use duct tape to make sure that the lid stays on.  Then, put the small can inside the big can.  Put the ice, salt and a little liquid water inside the big can, around the small can.  Put the lid on the big can, and duct tape it shut.  Then, play soccer with the can for about 15-20 minutes!  Untape the cans, wipe off the little can, and enjoy the ice cream!

Links to more information and activities:

More versions of this activity:

• https://www.scientificamerican.com/article/scrumptious-science-making-ice-cream-in-a-bag/
• https://www.stevespanglerscience.com/lab/experiments/homemade-ice-cream-sick-science/

The science of ice cream:

• www.icecreamnation.org/science-of-ice-cream/

​More homemade ice cream recipes and techniques:

• https://goldcoasticecream.com/how-to-make-ice-cream-from-scratch/
  
• https://www.allrecipes.com/recipe/258841/easy-ice-cream-in-a-bag/

Turn this activity into a real experiment:

• Make Ice Cream in a Bag | STEM Activity

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​Subject Tags
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• Food Chemistry
• Kitchen Science
• ​Milk Chemistry
• Phase/State Changes
  
• ​Water
  

All Subject Tags:

All
Acid/Base Chemistry
Bubbles
Crystals
Density
Food Chemistry
Heat/Cold
Kitchen Science
Milk Chemistry
Oils
Phase/State Changes
Polymers
Soap
Water