Pour milk, whipping cream, sugar and vanilla into a 1-qt zip-lock freezer bag, and seal tightly with duct tape.
Place the quart zip-lock bag with the ice cream ingredients inside a 1-gallon zip-lock bag.
Pack ice around the small bag.
Add the rock salt and water.
Seal the larger bag with duct tape.
Have students shake or toss the bags between them while protecting their hands with cloth towels or hot/cold mitts.
Continue until ice cream is frozen.
Open the outer bag, and discard the ice and salt.
Rinse the bag containing the ice cream.
Divide the ice cream into sundae cups with the wooden spoon.
Physical Science Activity
After ingredients have been put together in the bag, students will observe and describe their mixture.
After the ice cream is made, students repeat the observation and describe the ice cream.
Ask students what changes they have observed, and what caused the changes. Was motion involved in the change?
What does freezing do to foods?
Most foods are made from living or once-living material and all such material is made of cells. Water is one of the few substances that expands on freezing, so if you put something made of cells into a freezer, the water inside them turns to ice and expands, rupturing the cells and damaging other structures. Slow freezing gives large ice crystals time to form. For ice cream, commercial freezing is done rapidly to prevent large ice crystals from forming. Constant churning is also used, as in homemade ice cream and the ice cream made in this activity. This breaks up larger ice crystals as they form. When melted ice cream is put in the freezer, it freezes slowly, allowing large ice crystals to grow and making the ice cream unpleasantly crunchy.