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Why is ice cream smooth instead of like ice?

Ingredients List

½ cup sugar

¼ teaspoon salt

1 cup milk

3 beaten egg yolks

1 tablespoon vanilla extract

2 cups chilled whipping cream

2 cups fresh strawberries and extra ½ cup sugar

Supplies List

Double boiler OR a metal bowl that will fit into a pan or pot


Salt, about 1 cup

Ziploc-style bags, both sandwich and gallon size

Paper or cloth towels



1.          make sure your space is as clean and uncluttered as possible.

2.         assemble the list of supplies and ingredients

3.         wash your hands

4.         put the sugar, salt, and milk into the top pan of a double boiler (this would also be your metal bowl if you do not have a double boiler). The water in the bottom part of the double boiler will boil, and the temperature will never rise above the boiling point of water. This ensures that we don't overcook the mixture, even if we get distracted.

5.         cook the mixture over boiling water until you can see bubbles forming around the edges of the mixture. The mixture is done when it is somewhat thick, and coats the spoon.

6.         let the mixture cool to room temperature.  

7.         when the custard has cooled, stir in the vanilla extract and the heavy whipping cream.

8.         pour a cup of the ice cream mixture into a plastic Ziploc sandwich bag.

9.         zip the bag closed, and put it into another sandwich bag for safety, and zip that one closed as well.

10.     fill a gallon size food storage zip-lock bag about one third full of ice cubes. Add a cup of salt.

11.       zip the large bag closed, and wrap it in a towel to keep fingers from getting too cold.

12.     squish the little bag around in the salt and ice, making sure that the ice contacts the little bag as much as possible, and that the little bag gets lots of kneading, to keep the ice crystals tiny, so the ice cream is very smooth.

13.     knead for 10 minutes. You can let the ice cream sit in the ice for another 5 minutes if you like harder ice cream, although continuing to knead it for the extra 5 minutes is also OK to do if you're having fun.

14.     You will know the ice cream is done by feeling the mixture become a paste instead of a liquid. When you take the little bag out of the ice, wipe off the salt water, and then remove the outer bag carefully, so you don't get salt in the ice cream. The little bag will stand up in the bowl, because it is a frozen paste.

You can spoon the ice cream into a bowl if you like, or just eat it out of the bag.

If you like strawberry ice cream, mash 2 cups of strawberries with a half cup of sugar, and add a half cup to each small bag before closing it up and putting it in the ice.

The result should be an amazingly delicious homemade ice cream.  

   Have you ever frozen milk?  (If you haven’t try it!)  What you get is a messy glob on the bottom and whitish water on the top.  What happens is the milk separating out into its natural components: milk solids on the bottom, fat particles in the middle and water on top. So why doesn’t that happen with ice cream?  

There are two different science principles involved here.  The first principle relates to the separation of the milk in our question above but not the ice cream.  The reason the cream doesn’t separate out is because you are keeping it in a SUSPENSION.   A suspension means that all the molecules are kept from naturally settling.  This is done by squeezing the bag of ice cream.  In ice cream freezers this is done by turning a paddle. Because the ingredients are continuously mixed they are literally frozen in a suspension.  The milk that is allowed to freeze breaks the bonds that keeps the solids and the water together letting them fall out of their suspension.

The next principle is actually a physics principle called THERMODYNAMICS.  In this case, that is fancy word to explain the transfer of heat or cold from on thing to another.

For ice to melt, it has to get heat from something. In our ice cream project, it gets the heat from the ice cream mixture (and from your hands, which is why they get cold while holding the bag). When the ice is melting, it is at 0 degrees Celsius (32 degrees Fahrenheit).


When ice is melting, the surface of the ice is wet. At the surface, there is solid ice on one side, and liquid water on the other. The surface is exactly at the freezing point. This means that some water molecules are leaving the ice and moving into the water, but it also means that some liquid water is refreezing onto the ice. We say that the system is in equilibrium when the rate of melting is equal to the rate of freezing, and this happens at 0 degrees Celsius.

At equilibrium, the heat lost by the water as it freezes is equal to the heat gained by the ice as it melts.

Because plain ice can only barely cool something to the freezing point of water, we will need to do something to make it much colder than that, since our ice cream mixture freezes at a lower temperature than water.

The ice cream freezes because the salt and the ice mix to make a substance with a lower freezing point than ice alone. This means that the ice and salt mixture must get even more heat from somewhere in order to melt.

Salty water freezes at a lower temperature than plain water. But the ice is made of plain water, so it melts at 0 degrees Celsius. Since the ice keeps melting, but the water no longer freezes (because there is only salt water, which doesn't freeze at 0 degrees), the temperature goes down.

The heat gained by the ice as it melts is no longer offset by the heat given up by freezing water (since the water is no longer freezing back onto the ice). The heat gain has to come from somewhere else. It comes from the ice cream and your hands.

The sodium and chlorine in the salt split apart into charged ions, and these ions attract water molecules to form weak chemical bonds.

The resulting compound has a freezing point of -21.1 degrees Celsius (-5.98 degrees Fahrenheit). This is 21.1 degrees colder than ice (37.98 degrees Fahrenheit colder than ice).

When people put salt on the ice on a sidewalk or a road, the ice mixes with the salt, and the mixture of the two solids (ice and salt) produces a liquid, but the sidewalk actually gets colder than it was before.

If we add a different chemical to the ice, such as calcium chloride, we can get an even lower temperature (-29 degrees Celsius, or -20 degrees Fahrenheit).


   How did your ice cream turn out?  If it wasn’t great, it probably was because of a mistake made in the preparation of the ice cream mixture.  Try mixing it again!

Did salt water get into your bag causing bad taste? Be more careful sealing next time.

If you eat ice cream, this one is a must do experiment.  It is a great science fair project because it involves two different science principles and you can share your results at the science fair.  Have extra bags ready and have the judges and/or the spectators mash up the samples for you!