TOPICS: Solutions, phase changes, freezing point depression.

LEVEL: Middle and high school

TIME:  One class period

ADVANCE PREPARATION:  Obtain class set(s) of new test tubes (15-18 mm diameter) that may be reserved for subsequent use only in this experiment.  Prepare a cream mixture for the class.  The following is a simple recipe that makes enough for 25 students:

1 cup heavy whipping cream

1 cup milk

1/3 cup sugar

1 teaspoon vanilla

Makes about 2 1/2 cups (about 25 ml/student).

Limit the use of additional flavorings or extracts; the alcohol in these reduces the freezing point of the ice cream mixture and students may not finish within one class period .

MATERIALS: Each student makes ice cream, sharing a container and thermometer with a partner.  Coffee cans or other containers may be used instead of beakers. One or two students may be sent to collect pails of snow to be stored in the sink for class use.  A 100 ml beaker or paper cup filled with salt per group is more than adequate .

SAFETY NOTE: Stress to students that these test tubes are reserved for use in this experiment only.  Under normal circumstances, nothing in the laboratory should be eaten .

STEP C: Have students flex and bend the end of the straw; the straw tends to pull out as the ice cream is removed unless the crimped end of the straw is frozen into the ice cream .

STEP D and E. The ice cream mixture will freeze much faster if the test tube is completely surrounded by snow.

QUESTIONS 3-6 require higher order thinking skills.  We recommend that you discuss these questions in class before having students write out their answers.  You may wish to omit question 6 for middle school students .

1. Answers may vary .

2. Most of the snow will have melted, leaving a slushy solution .

3.  A nonvolatile solute will lower the freezing point of the solvent.  The salt water solution was above its freezing point and the temperature would only level off when the freezing point was reached .

4. No, sand will not dissolve and so does not affect the freezing point of the snow.  It does provide traction .

5. The water and anti-freeze form a solution which lowers the freezing point of both .

6. The snow and salt formed a solution with a freezing/melting point well below the temperature of the environment in which the solution formed.  The snow, therefore, melted; absorbing 334 joules of energy per gram of snow.  This heat energy was absorbed from the snow, ice cream mixture, and environment around the beaker, resulting in a decrease in temperature.

NOTE:  How do you make antifreeze?  Hide her pajamas!