There is a science to making ice cream. Really. In order to make ice cream, you need to change the state of cream (and any other ingredients), making it go from being a liquid to becoming a solid. You know how to do this: by cooling it down to the freezing point.
One very effective way to cool the ingredients down is by using a mixture of salt and ice. The ice is cold, of course. But why the salt?
After a snowstorm, you have probably seen salt being spread on the streets. You might wonder why this is done. The reason is that salt lowers the freezing point of water, that is, lowering the temperature at which water freezes. So, unless it’s much colder than freezing outside, adding salt to the layer of snow on the road will help keep it from forming a slippery sheet of ice.
To sum it up, with the addition of salt, ice will melt when the temperature is below the normal freezing point of water. This process is called the freezing point depression.
Now you can take this knowledge into the kitchen and make ice cream. You’ll find a description of how to make it using simple materials here in the sidebar. This is a fun activity that requires no pots and pans and is very easy to clean up—it’s all done in a plastic bag!
As you make your ice cream, you’ll find you can’t help making observations and having questions. How do the ingredients change during the process? Why? And, well, how do the results taste?
For an in-depth description of this activity that includes instructions for doing a side-by-side comparison of making ice cream with ice alone to cool the ingredients and making it with an ice and salt mixture as your cooling agent, check out this article in the Scientific American.