Virginia T. O'Brien - Higgins Community Academy

We All Scream For Ice Cream

Virginia T. O'Brien            Higgins Community Academy
                               11710 S. Morgan
                               CHICAGO IL 60643
                               (773) 535-5625 

Objective(s): 

Demonstrate freezing point depression, one of the colligative properties.  

Materials Needed:

One large and one small zip lock baggie, one thermometer and one spoon for 
each student; granulated sugar, vanilla, a variety of milk products (skim, 2%, 
whole, chocolate, and cream), measuring cups, measuring spoons, vanilla 
extract, salt, ice cubes, chocolate syrup.  


Strategy:
  
Speculate as to what occurs when sodium chloride or calcium chloride, types of 
salt, is sprinkled on icy streets and sidewalks.  Students may think they cause 
the temperature to rise.  Give each participant a small and a large zip lock 
baggie and a thermometer.  Fill large baggie half full with ice. Place 
thermometer in baggie, seal the baggie and set it aside.  In small baggie, put 
4 oz. of a milk product, 1 tbs. granulated sugar, and 1/4 tsp. of vanilla.  
Press air out of baggie, seal it, squeeze it to dissolve the sugar and set it 
aside.  Record the temperature on the thermometer through the bag in the large 
baggie, making sure that the tip of the thermometer is in the ice.  Take the 
thermometer out of the baggie.  In large baggie add 6 tbs. of table salt, sodium 
chloride, to the ice.  Seal and shake for a short time to mix up the salt and 
ice.  Place thermometer back in the large bag, in the salt ice mixture, and 
after 3 minutes, record the temperature as before.  Remove the thermometer.  
Place small baggie inside of large baggie.  Push some of the air out of the 
large baggie and seal.  Vigorously shake the baggies for about 5 minutes.  Take 
small baggie out of big bag, observe the consistency of the mixture, and if so 
desired, eat it. 
       
It should be observed that sodium chloride lowers the freezing point of the 
ice/water mixture and helped change the physical state of the milk mixture into  
homemade ice cream.  For this reason, calcium chloride and sodium chloride are 
used to deice sidewalks and streets, but if the outdoor temperature falls below 
this new freezing point, ice will still form.  

Performance Assessment:

Participation and following directions.  Carefully reading and recording the 
two thermometer temperatures and making and recording careful and accurate 
observations.

Conclusions:

At the end students will be able to understand that salt lowers the freezing 
point of water/ice and that this principle underlies the deicing ability of 
salt as well as its use in making ice cream.