Kids, the scientific concept to be learned in this experiment is lowering the freezing point. The fun to be had is in making and eating your very own ice cream. The recipe is actually more like a well-known Wisconsin treat called frozen custard.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
Into a bowl add 1 cup heavy whipping cream, 1 cup half & half, 1/3 cup sugar, 1 egg, and 1 teaspoon vanilla extract. After whisking for a few minutes, pour half of the mixture into a sandwich-size ziploc baggie and seal. Into a gallon-size ziploc baggie add four good handfuls of rock salt and eight good handfuls of crushed ice. Seal this baggie and gently shake the ice-salt mixture around until a good amount of water is formed. Rock salt is a chunkier version of common table salt (sodium chloride), but it is not clean enough to eat. Place the sandwich baggie into the gallon-size baggie and seal. Mix it around so that the smaller baggie is in contact with the cold water as much as possible. The material in the little bag should thicken in 10-15 minutes. When it is thick enough, take the small baggie out of the large one and scoop the contents into a cup for your very own edible treat.
The milk solution becomes thicker because it is freezing. When a liquid freezes it turns into a solid form. The salt-ice mixture should feel very cold, and though it will be hard to feel a difference, it is colder than the ice by itself. The experiment would take much longer if you used ice alone. The ice would melt quickly and need to be replaced often. So, how do you make the ice colder? The trick is adding the salt. It is a well-known phenomenon to chemists that a solution, such as salt-water, will freeze at a lower temperature than a pure liquid, such as water. So your salt-ice water solution is colder than just ice water alone. The same principle is being used when salt is sprinkled on icy roads in the wintertime.
The ice-salt baggie will in fact get so cold that you should wear gloves or use a towel. Try another experiment with the remaining unfrozen cream mixture. Add a few teaspoons of preserves for flavor, or some food coloring for looks. You can even try fooling around with the ingredients a little bit and use some chocolate milk!
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Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
January 1995
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References: Chemical & Engineering News 10/31/94, p. 48; Downer's Grove Friends of the Gifted and Talented: "Weird Science" Program, Fall 1994.
