The Science of Ice Cream


To make ice cream, the ingredients—typically milk (or half and half), sugar and vanilla extract—need to be cooled down. One way to do this is by using salt. If you live in a cold climate, you may have seen trucks spreading salt and sand on the streets in the wintertime to prevent roads from getting slick after snow or ice. Why is this? The salt lowers the temperature at which water freezes, so with salt ice will melt even when the temperature is below the normal freezing point of water. This is an easy way to teach phase change.


Ice cream is delicious, but it is also a really cool example of phase change. Technically, the temperature that the salt lowers is called the freezing point.When a freezing point is lowered, such as by adding salt to water, the process is called freezing-point depression.  When the cream changes phases it will change form a liquid to a solid. However, because we will use salt to make this more difficult, the ice cream will stop before it freezes into a solid. As we’ll see in this activity, freezing-point depression is not unique to solutions made of water and salt; it also happens with other solutions. (A solution is made when a substance, such as salt, is dissolved and becomes a solute. The medium into which it is dissolved is a solvent—typically a liquid, like water.) . The secret to making ice cream is to lower the freezing point of ice so it can freeze the cream. How? The scientific secret is plain old salt! Here’s a simple recipe you can follow right at home to make your own ice cream. Try this one with your class.



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