Homemade Ice Cream
Introduction
It is 30oC 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 explain the making of ice
cream is thermodynamics.
1. The total amount of energy in the universe is constant.
2. The entropy of the universe is always increasing.
3. Everything with a temperature above zero Kelvin has energy.
You may recognize the first law of thermodynamics as the law of conservation of energy. The
second law may be more familiar to you when it is expressed in the everyday language: heat
always flows from a warmer object to a cooler object. In making ice cream, it is this second law
that is of interest.
Another aspect of chemistry involved in producing ice cream deals with the physical properties
of solutions, which differ from those of pure solvents. The presence of solute particles in a
solution will raise the boiling point or lower the freezing point of the solvent, depending on the
number of particles dissolved in a given mass of solvent. The latter characteristic applies to ice
cream because the ice cream mixture is mainly a solution of sugar in water, and its freezing point
is depressed below 0oC.
Before refrigerators wee invented, ice cream was made using ice. In order to solidify by this
method, the “hot” ice cream mixture has to lose energy to the “cold” ice. Since ordinary ice in
only at 0oC, however, the lowest temperature that the ice cream mixture can reach is 0oC. With
the system at thermal equilibrium, t he ice cream mixture would still be a liquid.
To freeze the ice cream mixture, it is necessary to use “colder” ice. How do you do that? Again,
what you know about Colligative properties provides that answer: you make a solution. A saltice mixture has a lower freezing point that 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 before thermal equilibrium is reached.
Read the entire laboratory investigation and the relevant pages of your
textbook. Then answer the questions that follow.
Pre-Lab Discussion
1. How does adding salt to ice make ice “colder”?
2. Why is the salt-ice mixture needed to freeze the ice cream mixture?
3. What would be the product if pure ice were used instead of the salt-ice mixture?
4. How many grams of NaCl need to be added to 500. g of H2O so that the freezing point of
the solution is lowered to –8.3o C? Assume that the sodium chloride completely
dissociates. (Kf = 1.86o C/m)
Problem
How can you predict the lowered freezing point of water in order to freeze an ice cream mixture?
Materials
Apron
300 g Rock salt
Zip lock bag
Temperature probe
Ice cream mixture
2 spoons
4-kg Ice
Safety
Wear an apron during this investigation. Do this investigation only in a non-chemistry
laboratory setting. Make sure the zip lock bag is completely sealed so the salt-ice
mixture cannot contaminate it.
Procedure
1. Work in the room. Wash your hands with soap and water. Describe the ice cream
mixture before you begin, and record your observations in the data table.
2. Fill the zip lock bag three-fourths full of the ice cream mixture. Seal the bag
tightly and make sure it is completely sealed.
3. Measure 300 grams of NaCl in the red plastic bowl. Do not forget to compensate
for the mass of the bowl.
4. Measure the mass of ice placed into the tub. This will need about 4 kg total.
5. Measure the first half of ice and half of the rock salt added to the ice. This should
be about 150 grams of salt and 2 kg of ice. Measure and record the temperature
of the mixture.
6. Put the zip lock bag into the mixture and cover with the mixture. Do not squeeze
the bag as it might rupture. Place the rest of the ice and the other half of salt over
and around the bag.
7. Move the bag back and forth to get as much of the ice cream mixture in contact
with the salt-ice mixture.
8. Look at the mixture through the entire process.
9. Record the changes until the ice cream is firm.
10. Record the temperature of the water /salt mixture just before you remove the bag
of ice cream for the final time.
11. It should be the same as a Softie ice cream. Test the product.
12. Capture the ice water/salt mixture. Use the strainer to catch the ice so you have
just water in the number ten can. Obtain a medium sized beaker, determine the
empty weight and mass out the total of the water.
13. You should use the amount of salt added to the solution and determine the total of
the water alone.
Observations
Material
Observations
Total mass
Initial Ice
cream mixture
Final Ice cream
mixture
Salt-ice mixture
temperature
Salt
Ice
Total water/salt
mass
Calculations
1. Calculate the theoretical freezing point depression of the ice-salt mixture used in
this investigation.
2. Compare your actual freezing point to the one calculated. Determine the percent
error. Explain any differences.
Critical thinking
1. Discuss the reason for the heat transfer that occurs as the ice melts and the ice
cream mixture freezes.
2. Why are ionic compounds such as salt put on sidewalks in winter?
3. Reflect, in a paragraph, on why ionic compounds are ideal for icy surfaces. Use
the experiences from this lab to aid you in your explanation.