Learning Lesson: We all Scream for Ice Cream

Overview

Water freezes at 32°F (0°C). Adding salt to water lowers the freezing point. How low the freezing point goes depends upon the amount of salt in water. The students will make homemade ice cream but the "freezing times" will vary using different amounts of salt to lower the freezing point of water.

TOTAL TIME	30 minutes.
SUPPLIES	For each pair of students: One sandwich-size and one freezer-size zip-seal plastic baggy; 4 ounces of milk, cream, or half-and-half; A dash of vanilla extract or 1 teaspoon of chocolate syrup; 2 teaspoons of sugar;2 plastic spoons For the classroom: Several bags of ice; Rock salt
PRINTED/AV MATERIAL	None
TEACHER PREPARATION	None
SAFETY FOCUS	Cold Weather Safety

Procedure

1. For each pair of students, combine the milk, sugar, and vanilla/chocolate syrup into a sandwich-size baggy and seal closed.
2. Have the students shake/squish their baggy for one minute to thoroughly mix the contents.
3. Place the baggy inside the larger freezer-size zip-seal plastic baggy and fill that bag one half full with crushed ice.
4. In 2-ounce increments, up to 10 ounces, place varying amounts of rock salt in each large baggy. Include a pair or two of students with 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.
7. Record their times.
8. When finished, dispose of the large bag and eat the ice cream.

Discussion

The baggies with the most salt should "freeze" first with the bags containing decreasing amounts of salt taking longer. The greater the salt content, the lower the freezing point of the water and therefore the colder saltwater/ice solution becomes generating ice cream quicker.

Body of Water	Freezing point
Baltic Sea	31.3°F (-0.3°C)
Black Sea	30.2°F (-1.0°C)
The Oceans	28.5°F (-2.0°C)
Red Sea	27.9°F (-2.6°C)
Great Salt Lake	11.8°F (-11.2°C)
Dead Sea	-6.0°F (-21.1°C)

A common misconception is salt makes the ice melt faster. Salt has nothing to do with how quickly the ice melts - it just determines what temperatures it will melt (or freeze). 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.

The baggies without any salt will not freeze. Pretty much anything that dissolves in water (or milk) will lower the freezing point; such as sugar. Salt is used on roads because it's inexpensive. Adding sugar to the milk lowered the freezing point to below that of the plain ice (32°F) and therefore will not freeze.