The sea near McMurdo Station, Antarctica, is -2° C, but the water is not frozen at that temperature. Why? In this activity, students can explore some of the properties of water and ice by making their own ice cream.

MATERIALS: (per pair of students)

2 heavy duty ziploc bags per student—1 large and 1 small
½ cup milk 1 Tablespoon sugar ½ teapoon vanilla
Ice salt 2 spoons 2 bowls


DIRECTIONS:

1. Place the milk, vanilla and sugar in the small Ziploc bag and carefully close it, being sure there are no leaks.
2. Place the small bag inside the larger one.
3. Surround the small bag with ice to ½ the large bag capacity.
4. Give each pair of students a different amount of salt. Students will add the salt to their ice, NOT to their ice cream mixture!
5. On the board, make a graph for collecting class data including amount of salt and time it takes to freeze.
6. Shake the bags until the ice cream freezes.
7. Students should check frequently and as soon as theirs freezes, add their time to the graph.
8. While eating the “experiments,” compare the amounts of salt and the times it took each to freeze.
9. Discuss the results.

SCIENCE BACKGROUND:

For students in areas where they live with ice and snow in the winter, the concept of melting ice with salt is not new. It is sprinkled on sidewalks and snowplows spread it on roadways to melt the ice. As salt is added to ice, the freezing point of the ice is lowered. Water will normally freeze at 32 degrees F (0°C). A 10% salt solution freezes at 20 degrees F, and a 20% solution freezes at 2 degrees F. By lowering the temperature at which ice is frozen, heat transfers out of the milk mixture into the brine solution and the milk freezes into ice cream.