A great activity that is equal parts science experiment, kitchen fun and tasty treat, Leslie Foster explains how to make your own yummy ice cream in a coffee can.
Ice on its own is not cold enough to freeze the cream mixture. The secret to lowering the freezing point of ice in order to freeze the cream mixture is salt.
Materials
• Large 1 gallon (4 litre) coffee can or other large container
• 2 – 1 quart size (1 litre) Ziploc bags or 1 pound (453g) coffee can
• Lots of crushed ice – 5-10 cups (1.25-2.5 litres)
• 6-12 tbsp. (90-170 ml) of rock salt (table salt will also work)
• Thermometer
• ½ cup (125 ml) milk
• ½ cup (125 ml) whipping cream
• ¼ cup (60 ml) sugar
• ¼ tsp. (1 ml) pure vanilla extract
• Measuring cups
• Spoons for eating!
• Towel or gloves
Experiment
1.Fill large coffee can about half full with crushed ice.
2.Measure the temperature of the ice with your thermometer.
3.Add about 6 tablespoons (90 ml) of rock salt to the ice. Seal the can and roll or shake the ice and salt for about five minutes. Wrap the can in a towel or wear gloves.
4.Remove the lid and measure the temperature of the ice mixture with a thermometer. The rock salt and ice mixture should be about -10 degrees C, colder than before you added the salt, right?
5.Use one quart-size (1 litre) Ziploc bag or your smaller coffee can to mix the following ingredients:
• ½ cup (125 ml) milk
• ½ cup (125 ml) whipping cream
• ¼ cup (60 ml) sugar
• ¼ tsp (1 ml) pure vanilla extract
6.Seal the coffee can tightly with duct tape or double bag the Ziploc bag. If using bags, remove as much air as possible from both bags. Too much air in either bag may cause them to open during shaking.
7.Place the container with cream mixture inside the large can with the ice and salt and seal the lid. Wrap the can in a towel or wear gloves. Shake or roll the can for 15-20 minutes.
8.Open the large can and remove the small coffee can or bags. Rinse well with cold water. You don’t want salt water getting into your ice cream.
9.If the ice cream mixture is still too soft, repeat steps 2, 3, 7, and 8.
10.Grab a spoon and enjoy!
The science behind the experiment
Why do we use salt?
Heat always moves from a warmer environment to a cooler environment until both are the same temperature (the second law of thermodynamics).
In order for the ice cream mixture to freeze, it has to lose energy (heat) to the “cold” ice. Ice is usually only at 0 degrees C, which would mean that the lowest temperature the ice cream mixture could reach is 0 degrees C, turning it into a milkshake consistency.
To actually freeze the cream, we have to lower the temperature of the ice even further.
Adding salt to the ice causes it to have a lower freezing point. The ice cream mixture will lose more energy to the salt-ice mixture and freeze before they reach the same temperature (thermal equilibrium). Salt mixed with ice causes the ice to melt, just like when we put salt on the roads in the winter. The more salt you add, the lower the temperature will be before the salt-water solution freezes.
When salt is added to the ice, some of the ice melts because the freezing point is lowered. The heat that causes the ice to melt comes from the warmer cream mixture next to it.
By lowering the temperature at which ice is frozen, you create an environment in which the cream mixture can freeze at a temperature below 0 degrees C. The result? Delicious ICE CREAM!