CHEMISTRY

In an ice cream maker, a mixture of cream, milk and sugar is placed in a container that is surrounded by a bed of ice. The ice, however, is not cold enough to turn the mixture into a tasty solid form. Adding table salt (sodium chloride) to the ice lowers the freezing point and causes some of the ice to melt. This makes the temperature fall sufficiently to freeze the ice cream. In a similar manner, salt is used in many other freezing and refrigeration processes.

There are many types of salt. Calcium chloride (CaCl₂) is one type of salt that is used as a road deicer. When CaCl₂ is spread on ice, it lowers the freezing point of water below 0°C, so the ice turns into its liquid form, which is much safer to drive on.

• tablespoon (tbsp) salt
• stirrer
• 2 small cups
• 2 larger cups
• 2 thermometers
• graduated cylinder
• crushed ice

1. Fill each small cup two-thirds full of crushed ice.
2. Add 1 tbsp of salt to cup B, then stir to mix.
3. Place each small cup on the mouth of a larger disposable cup so that the water dripping from the holes of the small cup will collect in the large one.
4. Insert a thermometer into each cup of ice.
5. Every 2 minutes (min), for the next 20 min, check the temperature in each cup. Record those temperatures on the data table.
6. At the end of 20 min, pour the water that has collected in cup A into a graduated cylinder to determine its volume. Record that volume in the data table. Do the same for the water in cup B.
7. Create a graph showing time on the X axis and temperature of ice on the Y axis. Graph the information on the data table. Draw the line for cup A in red and the line for cup B in blue.
8. Click on the respective cup buttons to start drawing lines on the graph.

1. In which cup did the temperature decrease the most?
2. Which cup produced the most ice water?
3. What general statement can you make about the effect of salt on ice?
4. In the winter when roads are icy, the Department of Transportation prefers to spread calcium chloride instead of sodium chloride on slick surfaces. Propose an experiment in which you could compare the effectiveness of calcium chloride to sodium chloride in lowering the melting point of water.

1. cup B.
2. cup B.
3. Salt lowers the freezing point of water. Even as temperatures drop below 0°C, the ice melts.
4. Student answers will vary; they might suggest a test similar to the one they just completed, putting calcium chloride in one of the cups and sodium chloride in the other.