Sweet Tooth Chemistry: Ice-Cream Making Activity
Rhea SB. Samino
Chemistry Teacher, Camarines Sur National High School
Naga City, Camarines Sur
My Chemistry class became more interesting when I tried incorporating a practical activity into the discussion. My secret ingredient: ice cream.
The class has been discussing freezing point depression, a colligative property of solutions. To illustrate the concept further, we used our one-hour period making ice cream. Not only was the concept demonstrated, the students also had fun making the ice cream mixture with their groupmates. They learned how salt lowers the temperature of ice down to -10.00C.
Aside from the freezing point depression, the concept of energy transfer was also tackled. The students experienced first-hand how heat is exchanged during phase changes like freezing and melting. Excitement rushed when ice formed in the container and when water vapor condensed and froze over the untouched part of the canister. My students called it “water molecules in action.”
Using air-tight canisters and ziplocks, my students actually saw the ice cream mixture harden while the ice melted. Ziplock plastic bags are recommended for they allow heat to pass through quickly without permitting the salt to permeate through the ice cream mixture. A thermometer is also needed to monitor the heat changes of the ice before and after adding salt to emphasize the energy changes during the reaction. Mittens are also indispensable for protecting the hands from hypothermia while the students rolled the canisters back and forth. And of course, lots of ice and table salt (about 1 kg) are needed. The ice-cream mixture is made of 1 cup evaporated milk and 1 tablespoon sugar (1:1). The crushed ice and salt were packed alternately with the ziplock bags containing the mixture at the center. After the container was sealed tightly, it was then rolled back and forth to allow air to incorporate through the ice cream mixture and to distribute evenly the ice-salt mixture surrounding the zip-lock bags.
Did my students learn the science behind ice cream making? The write-ups and the journals said it all. Not only did they capture the concepts but also the wonders of salt, water and solutions. Best of all, they had fun. It was really a wonderful Chemistry treat for the sweet tooth.
(Published 10 November 2008, Smart Communications, Inc.)
