PURPOSE:  To observe the effect of a salt on the freezing point of a solvent.

MATERIALS (for two students):  1000 ml beaker or other container, 2 new glass test tubes (15-18 mm diameter), thermometer, 2 flexible plastic drinking straws, salt (NaCl or CaCl₂), ice cream mixture, pails to collect snow.

INTRODUCTION:  It is 5 AM on a snowy, winter morning and the temperature is -5^oC.  Suddenly a snow plow rumbles down the road and sprays salt over the road.  The ice and snow slowly begin to melt making it safer for motorists.  How did the application of salt cause the snow to turn into a liquid at -5^oC, well below the freezing point of water?

Salt is not magical, but it is soluble in water.  Any nonvolatile solute (one which will not readily evaporate) like salt will lower the freezing point of a solvent like water.

People made use of this property of salt in making ice cream long before automobiles existed.  Because a cream mixture has a lower freezing point than pure water, it is not possible to make ice cream by packing it in plain ice.  Therefore, ice and salt were packed around a canister containing the cream mixture.  Many layers of snow and salt were needed as family members took turns cranking the ice cream machine until the mixture finally froze!

In this activity, you will observe the freezing point depression caused by adding a nonvolatile solute to a solvent as you make your own ice cream.

PROCEDURE:

A.  Working with a partner, pack 2-3 cm of snow in the bottom of the large beaker or other container.  Record the temperature of the snow.

B.  Sprinkle about a quarter of the salt over the snow.  Continue to add layers of snow and salt until the beaker is nearly full.

C. Fill two new test tubes about two-thirds full of the ice cream mixture. Insert a flexible straw in each tube, crimped end down.

D. Place the test tubes into the beaker so that the liquid inside the test tube is completely surrounded by snow.  Do not allow the snow-salt mixture to overflow into the cream mixture or the taste of your ice cream will be spoiled.

E. Place the thermometer into the snow-salt mixture and begin recording the temperature every minute until the ice cream has frozen completely.  Use the straw to stir the mixture to speed solidification.  Add more snow and salt as needed so that the test tube mixture is always surrounded by snow.

F.  On your data sheet, circle the temperature at which you first notice the ice cream mixture beginning to freeze.

G.  When the ice cream is frozen, warm the test tube slightly in your hand and pull gently on the straw to remove the ice cream.  Enjoy!

DATA CHART:

CONCLUSIONS:

1. What was the freezing point of the ice cream mixture?

2. Was the snow-salt mixture a solid or liquid at the end of the activity?

DISCUSSION:

3. Pure water freezes at 0^oC.  Explain how it was possible to reach the final temperature you recorded.

4. Does the application of sand have the same effect on icy roads as salt?  Explain your answer.

5. To prevent freezing, a 50:50 mixture of ethylene glycol and water (antifreeze) should be used in car radiators.  Pure ethylene glycol will freeze at a higher temperature than the antifreeze solution.  Explain this observation.

6. This activity was conducted using snow at, or near, its freezing point and a cream mixture a few degrees above 0^oC.  Both the freezing of the ice cream and the decrease in temperature of the snow-salt mixtures represent a loss of heat.  Explain how this decrease in heat energy could occur in a warm environment like your classroom.