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Our own kitchens provide a perfect setting in which to explore a variety of scientific activities. Making homemade ice cream is a wonderful opportunity to learn about how liquids can take on the properties of a solid, or the phase change of matter. There's also the added benefit of having it for tonight’s dessert! The best ice creams are still made with fairly simple ingredients: milk, cream, sugar, and maybe eggs. You can make your own ice cream at home using simple ingredients and no special equipment, and enjoy a “cool” science experiment at the same time!

Concept:

Ice cream is a colloid, an emulsion where two substances are suspended within each other rather than being chemically bonded together. This is why many ice creams also have an emulsifier to prevent the fat molecules from separating from the rest of the ice cream. This makes the texture smoother. Ice cream also uses a stabilizer like gelatin or guar gum to help hold air, which contributes to its light texture.

Objective:

Observe the creation of an emulsion when combining different ingredients and exposing them to a change in temperature.

Materials:

1. 1/2 cup milk

2. 1/2 cup heavy cream

3. 1/4 cup sugar

4. 1/2 teaspoon of vanilla, peppermint or another flavoring of your choice

5. Quart-size Ziploc™ bag

6. Gallon-size Ziploc™ bag

7. Ice

8. Rock salt

9. Thermometer

Procedure:

1. Stir the milk, cream, sugar and flavoring together in a bowl, then pour the mixture into a quart-size freezer Ziploc bag.

2. Stick this bag inside a gallon-size Ziploc, half-filled with ice and rock salt - about 2 cups of ice and 1/2 cup of salt. Salt lowers the freezing point of water, which causes the ice to melt at a lower temperature. The lower freezing point provides the temperature difference needed to transfer heat between the freezing ice cream ingredients and the melting ice. Use a thermometer to measure the temperature in the outer bag. Next, begin shaking the bag so that the ingredients are whipped together. After five minutes of shaking, let the bag sit for a few minutes. Now take the temperature inside the gallon bag again. Has it changed? What happens if you don't shake it?

Questions:

What happens if you use table salt instead of rock salt?

Rock salt doesn't lower the freezing point as much as table salt does (which results in smoother ice cream, because it freezes more gradually), however for this activity you can try table salt. Take the temperature.

What physical change happened to the mixture?

An example of a physical change would be a solid (ice), when heated becomes liquid (water) and when heated to the boiling point becomes a gas (water vapor). The mixture of milk, cream and dissolved sugar begins as a liquid. Lowering the temperature causes this mixture to begin a physical change by taking on the properties of a solid.

Can you make a list of your favorite foods that might go through similar physical changes?

Example: pancakes, Jell-O, candy...

All Buttered Up!

Butter is made in a similar way. Pour some heavy cream into a small jar, screw the lid on tightly, and shake the jar briskly for 10-20 minutes. The butterfat molecules in the cream collide as you shake the jar and they begin to stick together, forming butter and leaving the watery liquid of the cream separate. You'll notice a change in the sloshing sounds and how heavy it feels. Regular milk has a lower fat content than cream. Can you still make butter using whole milk? Skim milk has had so much fat removed that it can't be used for butter. 

Who knew Science could taste good!

If you enjoyed this experiment, check out our Tasty Science Chemistry in the Kitchen! Mmmm, who knew science could taste this good? Learn the chemistry behind what makes cakes rise, candy crystallize, and more!

Go there now!

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