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Chillin' Out with Freezing Point Depression
The Science of Making Ice Cream
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Hook
The oceans surrounding Antarctica are an extremely cold and inhospitable environment. If the water here were fresh water, it would freeze solid. Fish only survive in these frigid waters because of special glycoproteins in their blood which act as antifreeze.
Because of the ocean's salinity, the freezing point of the salt water is depressed to -1.4 degrees Celsius. In this lab you will learn more about the freezing point depression of salt water. In fact, you will use the freezing point depression of saltwater to make a tasty frozen food. Just follow instructions closely and cool the "mystery solution" to below -14 deg C, and you can taste this yummy concoction.
Materials
Bottom half of plastic one gallon milk jug or suitable large bowl
9 oz plastic cup
popsicle stick or plastic knife
crushed ice to fill plastic bowl
rock salt
thermometer
2/3 cup mystery solution to be added later
Procedure
1. Place a layer of ice in bowl then sprinkle salt over the ice.
2. Place the empty plastic cup in the middle of the bowl.
3. Continue layering ice and salt around the cup, taking care not to get salt in the cup.
4. Add 100 ml of water to the ice to get melting started.
5. Take temperature of the ice / salt mixture periodically, recording the lowest temperature achieved.
6. Fill the plastic cup 1/2 full with the mystery solution.
7. Spin the cup with one hand while holding the knife against the inside edge of the cup to scrape off any frozen mixture that forms on the side. Since the edge of the cup is in contact with the ice water, the mixture will freeze to the inside edge. Continue to scrape this frozen portion until all of the mixture is frozen. This may take some time.
8. The temperature must get below -14 deg C for best results.
9. Sample product. Be sure to have teacher evaluate results.
Part 2
10. Remove 10.0 ml of the salt water solution with a graduated cylinder and place in a pre-weighed evaporating dish.
11. Record the weight of the dish plus saltwater to the nearest .01 g.
12. Allow the saltwater to evaporate overnight or gently heat with a lid in microwave. Be careful not to splatter and lose salt.
13. Weigh the remaining salt when dry.
Discussions Questions/Extensions ......
1. Calculate the molality of the salt solution using the mass of the water evaporated and the moles of the remaining salt. Molality is measured in Moles/Kg.This will be the Experimental Molality.
Experimental Molality_____________________
2. Now calculate the Theoretical Molality using the formula given in the background.
Theoretical Molality______________________
3. Calculate the percent error between the theoretical and experimental values. How close were you ?
Percent Error___________________________
4. What are ice stalactites ? Explain how they are formed. To learn about these, see data section of lab.
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