Amazing Salt

Ruby Bell                      Park Manor
                               7037 South Rhodes
                               Chicago IL 60637
                               (312) 535-3070

Objectives:

1.  To see what effect salt has on ice.
2.  To have students get experience with the concept of freezing.
3.  To see which way heat travels.
4.  To have students manipulate variables.
5.  To see the societal impact of freezing with ice and salt.

Materials needed:

(Equipment per group: 4-6 students)
First experiment
salt                     ice                     water
2-styrofoam cups         2-thermometers          2-test tubes
a measuring cup or graduated cylinder            2-stirrers
timer or minute-hand watch or clock              1-teaspoon

Team Equipment
2-clean coffee cans (1 and 3 lbs.)
2 cups of Rock salt (NaCl)
2 quarts of crushed ice
masking tape or duct tape
mittens
thermometer (that measures down to -20oC)

Ice Cream Recipe
(Mix the following in the 1 lb. coffee can)

3 to 3 1/2 cups of half and half
3/4 cup of sugar
2 tsp. of vanilla extract
Heat mixture to dissolve the sugar.  DO NOT BOIL.  Let the mixture cool.

Strategy:

Fill two styrofoam cups 1/2 full with crushed ice.  Place into each cup a test
tube filled with 10ml of water.  Place the thermometers in each cup.  In one
cup place two teaspoons of salt.  Observe the temperature of each cup and 
record.  For the next seven minutes, record the temperature in each cup each
minute.

Students should have observed that the test tube with the salt lowered the 
temperature of the salt water causing the water in the test tube to freeze.

Team Ice Cream Procedure:  Pour the cooled ice cream mixture described in the
materials section into the small can until the can is about 3/4 full.  Put the
lid on and tape around the top securely.  Place the smaller can into the larger
one.  Put alternate layers of ice and salt so that the amount of ice is about
four times the amount of salt.  Put on the lid.  Place tape around it.  Put on
the mittens.  Have students sit in a circle and roll the can back and forth.
The ice will melt.  When very little ice is left, you may add more ice with more
salt.  After 20-30 minutes open the can and enjoy your ice cream.  Check the
temperature of the water-ice-salt mixture with the thermometer.

When the salt dissolves, its particles interfere with the freezing process which 
forms ice.  Even though the temperature is 0oC, the ice can melt.  Energy is 
needed to melt the ice.   This energy comes from the water.   As a result the 
ice and the water solution becomes colder, because the heat goes to the ice.  
The mixture could go to about -15oC.  As energy (heat) is removed from the ice 
cream mixture, it gets very cold and the ice cream is formed. 

Road salts (CaCl2) helps to melt ice by the same principle. The addition of salt 
to ice lowers the freezing point of water. 

REFERENCES:

AIMS Education Foundation, 1987.

Gardner, Robert, Kitchen Chemistry, 1980.

Guzdziol, Ed., lecture summer, 1993.