Name : _______________________________ Date: __________
ICE CREAM THERMODYNAMICS LAB
LAB OBJECTIVES:
To use both the laws of thermodynamics and properties of solutions to aid in freezing an ice cream mixture.
STATE THE QUESTION: How can you lower the freezing point of water in order to freeze an ice cream mixture?
GATHER INFORMATION
It’s 35˚ C in the shade and to cool off, you are eating an ice cream cone. As you sit there you wonder just how ice cream
is made. One area of chemistry that helps to explain the making of ice cream is thermodynamics. There are three laws of
thermodynamics:
1. The total amount of energy in the universe is constant.
2. The entropy (which is a measure of disorder) of the universe is always increasing.
3. Everything with a temperature above 0˚ K has energy.
In making ice cream you remove about 1000 calories of heat from the milk/sugar (chocolate milk) mixture and transfer it
to the salt/ice mixture. Energy is conserved and the first law is satisfied. Heat is always transferred from a hot object to a
cooler one. Imagine your surprise if you had a glass of water and the water froze and the glass got hot. The 2nd law
determines the direction of heat transfer and states heat always moves from a hot object to a cooler one.
Another aspect of chemistry involved is the properties of solutions as compared to pure solvents. The presence
of solute (the thing being dissolved) particles in a solution will raise the boiling point and lower the freezing point of the
solvent (the dissolver). Therefore, because the ice cream mixture is mainly a solution of sugar and water, its freezing
point is depressed below 0˚ C.
Before refrigerators were invented, ice cream was made using ice. In this method, the “hot” ice cream mixture
has to lose energy to the “cold” ice. Since ordinary ice is only 0˚ C, this is the lowest temperature that the ice cream
could become. The ice cream mixture would still be a liquid.
To freeze the ice cream mixture, it is necessary to use “colder” ice. Again, properties of solutions provide the
answers. A salt-ice mixture has a lower freezing point than pure ice, so it acts as “colder” ice. The more salt added to the
ice, the lower the freezing point. The ice cream mixture can then lose more energy to the salt-ice mixture and freeze.
Gather Info of individual variables:
Temp of Milk ____________ Temp of ice ______________ Temp. Of ice & rock salt _______
FORM A HYPOTHESIS
If _____________________________________________________________________________________, then
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CONDUCT THE EXPERIMENT
MATERIALS:
ICE CREAM MIXTURE:
 Quart size Ziploc bag
 1 cup of chocolate milk
MAKING THE ICE CREAM MATERIALS:
 Gallon size Ziploc bag
 Ice
 Rock salt
 Thermometer
PROCEDURE:
PART 1:
1.
2.
3.
4.
5.
Add 1 cup of chocolate milk to smaller size Ziploc bag.
Seal the bag well. Try to make sure all of the air is out of the bag.
Mix the ingredients until they are blended.
Take the initial mass of the milk mixture and record in your data table.
Take the initial temperature of the milk mixture by wrapping the baggie around the thermometer. Do
not put the thermometer directly into the mixture as then it would not be safe to eat.
PART 2:
6. Fill the gallon-sized Ziploc bag about half to three-fourths full with ice.
7. Add rock salt so that it is lightly coating the ice cubes, 2 tsp. is good.
8. Take the initial temperature of the ice/ salt mixture by wrapping the baggie around the thermometer.
Record on data table.
9. Put the Ziploc sandwich bag with the ice cream mixture in it (keep it sealed!) into the gallon sized Ziploc
bag.
10. Seal the gallon sized bag with as much of the air out as possible.
11. Wrap it in newspaper (insulator) and shake for 5 minutes really fast. it will be COLD!
You can put into a white can to shake if you wish.
PART 3:
12. Once your mixture is the correct consistency, use the thermometer to find the final temperature of the
milk mixture (ice cream) and the rock salt/ice/water mixture. Record on data sheet!
13. Once you have read the temperature, pour the contents of the gallon sized bag in the sink.
14. Take the final mass of the milk/ ice cream mixture.
15. Get a plastic spoon, and enjoy your tasty thermal treat.
16. Make sure your lab station is clean and all used bags are in the trash before you leave.
COLLECT YOUR DATA
INITIAL READINGS
Initial Mass of Milk Mixture
FINAL READINGS
Final Mass of Milk Mixture
Initial Temp of Milk Mixture
Final Temp of Milk Mixture (Ice Cream)
Initial Temp of Ice/ Salt
Final Temp of Ice/ Salt
DIFFERENCE
ANALYZE YOUR DATA
DISCUSSION:
When you took the temperature of the rock salt/ice/water mixture, it should have been lower than 0 oC. How is it
possible to have ice water colder than the freezing point of water? Well, here’s an explanation…
When ice is at 0 oC, essentially water molecules are going from the solid phase to the liquid phase as fast as they are
going from the liquid phase to the solid phase. (This is called a dynamic equilibrium.) This means that at 0 oC, neither
freezing nor melting are occurring. When rock salt is added to water, the Ca+2 and Cl-1 ions slow the rate at which water
molecules go from the liquid phase to the solid phase. Thus, there is more flow of solid to liquid. In other words, we see
the ice melt.
But, how is the temperature lower than the freezing point of water? Water molecules in ice are held together by
strong intermolecular forces (like hydrogen bonding). When the ice melts, the forces holding the ice particles together
must be overcome.
Well, it takes energy to overcome these intermolecular forces. Where does this energy come from? The ice particles
use thermal energy (heat) from the surrounding liquid in order to overcome the forces holding them together. So, the
liquid water is losing heat, and the ice is using that heat to overcome the intermolecular forces that hold the ice particles
together. The temperature of the water is colder because it lost heat.
QUESTIONS:
Discussion:
1. How would you define the second law of thermodynamics?
2. How does adding salt to ice make ice “colder”?
3. Why is the salt-ice mixture needed to freeze the ice cream mixture?
4. Discuss the reason for the heat transfer that occurs as the ice melts and the ice cream mixture freezes. Draw a
diagram of the baggie-ice cream freezer. Use arrows to indicate the direction of the heat transfer.
5. How could you speed up the freezing of the ice cream mixture?
6. Graph the relationship of the temperature of the milk before shaking and after shaking it in rock salt freezer..
DRAW YOUR CONCLUSIONS
Write a DETAILED paragraph that includes the following:
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Restate your scientific question and your hypothesis.
Tell whether your hypothesis was ACCEPTED or REJECTED by the data. Use actual data (qualitative and
quantitative data) from your experiment to back it up.
Tell if you had anything go wrong/ unexpected in your experiment or if it went as you expected AND WHY. How
did that affect your results?
How could you extend this experiment? How could you make it better, what would you change next time?
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