by Gretchen Noyes-Hull
Not all water freezes at the same temperature. Most ocean water begins to freeze at about 2 degrees below zero (Celsius), and very salty water can remain liquid as cold as 20 degrees below! We can use the physics of freezing to our advantage—to make ice cream, for instance. The recipe for scientific ice cream is given below, but its flavor is enhanced by understanding how it is formed! Read on.
Temperature is a measure of how fast atoms or molecules are jiggling around. In order for things to freeze, the jiggling must slow down to allow strong bonds to form between them. Heat energy must be removed before substances can change from a liquid to a solid “phase.” Just how much heat is determined by the substance, and for water, how pure it is.
As heat is removed from water, the temperature drops, and the molecules begin to move more slowly. Bonds between them are made and broken again, until, at zero degrees, the jiggling has slowed enough so that more bonds are made than are broken. Water begins to solidify. If more heat is carried away, the process continues without any more change in temperature until all the molecules are linked together in a crystal “lattice.”
Molecules have more difficulty “grabbing arms” of their molecular neighbors when particles get in their way. If water is not pure, the temperature must be even lower for them to push the particles out of their way and form ice. This is known as “freezing point depression.” The more salt in seawater, the colder the temperature must be before ice will form.
Freezing-point depression is the whole point of ice cream–making! A lot of heat has to be removed for the water in cream to solidify. Because of all the particles it contains, the temperature must drop far below zero. The only way this can occur is to send the heat somewhere else!
Here's what happens. When ice cream is made the old-fashioned way, rock salt (big chunks of salt crystals) is mixed with ice. Only a little water melts before some of the dissolved salt lowers its freezing point. Now when the ice wants to melt, it can absorb lots of heat from the water to do it, and the water still will not freeze. (In the recipe you will try, the temperature of the water may decrease to almost minus 20 degrees C and still be liquid! Pretty cool!) The water is very much colder than the cream mixture. Because heat flows from hot things to cold things, the cream now loses its heat to the water and rapidly cools down.
Such a deal! It's a chain reaction of freezing-point depression. The freezing point of cream is depressed by the milk and sugar particles, and the freezing point of the water is depressed by the particles of salt. That's the point of ice cream. Enjoy!
- What is freezing-point depression?
[anno: Freezing-point depression refers to the lowering of water's freezing point due to other substances, such as salt, in the water.]
- What is another situation in which a lowered freezing-point depression might be useful? Why? Write a few sentences to explain your answer.
[anno: Answers will vary but could include that lowering the freezing-point depression of water on road surfaces might help to prevent accidents. Students might make the connection to salting roadways as a means of lowering the freezing-point depression of water gathered on them.]