- Cream will freeze quickly if the temperature is lowered around it to well below the freezing point of water, 0 deg. C. The frozen cream will be smooth if it is in motion as it freezes. In this investigation, you will use rock salt to lower the temperature of melting ice below 0 deg. C, and make ice cream.
MATERIALS: For 2 to 3 batches.
Two coffee cans, 1 and 3 lb.
Ice cream recipe, 3 cups:
- 3 cups of half and half (half milk and half cream)
- 3/4 cup of sugar
- 2 tsp of vanilla extract
- Heat the mixture to dissolve the sugar.
- Do not boil. Let the mixture cool.
Rock salt (NaCl), 2 cups
Crushed ice, 2 qt.
Duct tape or masking tape
Mittens
Thermometer (that measures down to -20 deg. CPROCEDURE:
REMEMBER: You are going to eat this, so don't do it in the lab.
1. Pour the chilled ice cream mixture described in Materials list into the small can until the can is about three-fourths full. Put on the lid and tape around it.
2. Place the small can into the larger can.
3. Add alternate layers of ice and salt, around the small can, so that the amount of ice is about 4 times the amount of salt.
4. Put on the outer lid. Place tape around it.
5. Put on your mittens.
6. Several students can sit in a circle and roll the can back and forth, or one person can roll it back and forth on a table. A towel may be wrapped around the can to insulate it.
REMEMBER: If you don't keep it rolling, the ice crystals in your ice cream will be larger. 7. The ice will melt. When very little ice remains, you may add with a little more salt.
8. After 20-30 min of rolling, open the cans. Enjoy your ice cream!
9. Check the temperature of the water-ice-salt mixture with a thermometer.
REACTION:
- When the salt dissolves, its particles (ions) interfere with the freezing process, that is, the formation of ice from water. The salt particles become surrounded by water molecules. Even though the temperature is below 0 deg. C, the ice can melt more easily than the water can freeze because the water molecules are now associated strongly with the salt particles. Energy is needed to melt the ice, and this energy comes from the water. As a result, the water becomes colder. Thus, the salt dissolves in the water, energy goes into melting the ice, and the water-ice-salt mixture gets colder. The mixture should go to about -15 deg. C. As energy (heat) is removed from the ice cream mixture, it gets very cold and ice cream forms.
QUESTIONS:
1. Why is the inner can filled only three-fourths full with ice cream mix?
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2. How do you explain the rapid increase in the amount of water in the can?
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3. What temperature did you find the water-ice-salt mixture to be? _______ deg. C
4. How does rolling or churning ice cream make the ice cream smooth rather than full of noticeable ice crystals?
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Making Ice Cream