Learning Lesson: We all Scream for Ice Cream

Overview

Water freezes at 32°F (0°C). Dissolving salt in water lowers the freezing point (the temperature at which water turns to ice) and also lowers the temperature. How low depends upon the amount of salt in water. The students will make homemade ice cream, but the freezing times will vary due to different amounts of salt in the ice bath.

Procedure

1. For each pair of students, combine the milk, sugar, and vanilla/chocolate syrup into a sandwich-size baggy and seal it closed.
2. Have the students shake/squish their baggy for one minute to thoroughly mix the contents.
3. Place the baggy inside the larger zip-seal plastic baggy and fill the larger bag one half way with crushed ice.
4. In 2-ounce increments, up to 10 ounces, place varying amounts of rock salt in each large baggy. One or two pairs of students should have no salt added.
5. Have each pair estimate how long it will take for their mixture to freeze.
6. With all students beginning at the same time, have them mix and churn their baggy at a fast pace until the contents have solidified. Students should wear gloves or wrap the baggy in a towel to protect their hands.
7. Record their times.
8. When finished, dispose of the large bag and eat the ice cream (Have extra or share with the no-salt group as theirs likely will not freeze.)

Discussion

The baggies with the most salt should freeze first while the bags containing decreasing amounts of salt take longer. The greater the salt content, the lower the freezing point of the water. At the same time, dissolving salt in the liquid water causes the water temperature to drop, generating ice cream quicker.

A common misconception is that salt makes the ice melt faster, but salt has nothing to do with how quickly the ice melts – it just determines what temperatures it will melt (or freeze). By lowering the freezing point, the ice will melt into liquid water without becoming warmer (instead, it actually becomes slightly colder). The table (right) provides the average freezing points of water for various bodies of water based on their salinity.

The lowest freezing point for a salt solution is -6.0°F (-21.1°C). At that temperature, the salt begins to crystallize out of solution, along with the ice, until the solution completely freezes. Below -6.0°F (-21.1°C), the frozen solution is a mixture of separate saltwater crystals and fresh water ice crystals, not a uniform mixture of saltwater crystals.

Pretty much anything that dissolves in water will lower the freezing point. Milk is naturally made up of water with a variety of proteins, sugars, and other dissolved substances; therefore, its freezing point is below that of the plain ice (32°F). As a result, the ice cream in the plain ice baggies will not freeze, especially with the extra sugar added to the milk.