Objective: Students should be able to describe how salt lowers the freezing point of water. The subject of latent heat is dealt with in this experiement.

 Background: Normally, water freezes at 32 degree F or 0 degree C. Adding salt to ice will lower that freezing point, so the ice will melt. This is the same method when people poor salt onto icy roads. Thus, when we pour the salt onto the ice, it will make the water/ice solution colder. The normal freezing point is not cold enough to make ice cream. Adding the salt forces the temperature of melting ice down below 10 degree F.

Materials Needed:

This recipe is enough for one student, so that each student can make their own.

1/2 cup milk
1/2 teaspoon vanilla
1 tablespoon sugar 
4 cups crushed ice 
4 tablespoons salt
2 quart size Zip-loc bags
1 gallon size Zip-loc freezer bag
a hand towel or gloves to keep fingers from freezing as well!

Activity #1:

1. Teacher can demonstrate this activity to the class as a whole.
2. To prove that salt melts ice and makes it colder we will conduct an activity that will show this happening.
4. Fill a cup with ice.
5. Place the thermometer into the cup. You should get the reading of about 32 degree F or 0 degree C.
6. Next, add 3 Tablespoons of salt into the cup of ice.
1. Have the students observe what is occurring to the ice.
3. Measure the temperature of the ice now.
7. The thermometer should read a colder degree then before salt. Also, the ice should be melting.
8. 

Activity#2

1. Mix the milk, vanilla and sugar together in one of the quart size bags.
1. Seal tightly, allowing as little air to remain in the bag as possible. Too much air left inside may force the bag open during shaking.
2. Place this bag inside the other quart size bag, again leaving as little air inside as possible and sealing well.
1. By double-bagging, the risk of salt and ice leaking into the ice cream is minimized.
3. Put the two bags inside the gallon size bag
4. Fill the gallon size bag with ice, then sprinkle 4 tablespoons of salt on top. Again let all the air escape and seal the bag.
5. Wrap the bag in the towel or put your gloves on, and shake and massage the bag, making sure the ice surrounds the cream mixture.
1. Five to eight minutes is adequate time for the mixture to freeze into ice cream.

Challenge Questions:

1. What happened when we added salt to the ice? What difference did the amount of salt make?
2. In what other ways do we use salt for controlling “ice energy?”
3. Why is sodium chloride used rather than sucrose?

• Teachnet.com
• Physics: A World View 6th Edition by Kirkpatrick/Francis