Colligative Properties: Making Ice Cream:



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Colligative Properties: Making Ice Cream Lab

Introduction:

Colligative properties are properties of solutions that depend on the total number of solute particles in a solvent.  The number of solute particles effects both the freezing point depression and boiling point elevation of solutions.  The more particles that are in a solution, the greater the depression or elevation, respectively.

“One mole of a non-electrolyte, such as sucrose, lowers the freezing point of 1L of water by 1.86°C.    However, one mole of an electrolyte, which dissociates into ions or particles, lowers the freezing point of 1L of water even more, corresponding to the number of ions produced.  For example, one mole of sodium chloride, which dissociates into Na+ and Cl- ions, contributes two moles of particles to the solution for every mole of NaCl added.  The freezing point depression of NaCl is approximately (2)(1.86°C).  Because the ions can reassociate in the solution in a random manner, the real freezing point depression is somewhat less than the theoretical value of about 3.72°C per mole of NaCl added.” – Heidemann Fall NSC Lab Manual pg. 147

Objective: 

To determine the freezing point depression of a brine solution used to freeze an ice cream mixture.

Materials: 

Each person will need to gather the following materials.

1-1qt Ziploc 1-1gallon Ziploc 1 cup of milk

Napkin Ice ½ cup of rock salt

thermometer 3 tablespoons of sugar spoon

½ teaspoon of a flavoring (vanilla, amaretto, mint, etc.)

Procedure:

1. In the 1-quart Ziploc bag place the milk, sugar, flavoring, and seal the bag carefully.  Place this bag in the larger (1-gallon) Ziploc bag.

2. In the larger Ziploc bag put in ice (fill the bag about half full) and ½ cup of rock salt. Carefully seal the larger Ziploc bag. Place Ziploc bag into a trash bag.

3. Flip and knead the bag until the ice cream mixture becomes set. The longer you move your bag the harder your ice cream will become.

4. When your ice cream is as firm as you want it, open the larger bag and take the temperature (Celsius) of the ice/sodium chloride solution.  Record it in the chart below.  Now remove your bag of ice cream.

5. Rinse off your bag of ice cream with some cold water, and then dry your bag. Now you are ready to eat your ice cream!

6. Remember to pick up your mess when you are done!  The saltwater can be dumped down the drain.  You will need to filter out the salt chunks and place them in the trash cans next to the sinks.

Data and Calculations:

Table: Colligative Properties: Making Ice Cream

| Temperature of brine (sodium chloride) solution | |

| Mass of sodium chloride in brine solution |16.7 g |

| Mass of water in brine solution |94.68 g |

| Molality of solution (Show work) | |

| Expected freezing point (Show work) | |

| Difference between expected freezing point and observed temperature of brine solution | |

Conclusion Questions:

1. Why did we determine the molality of the solution instead of the molarity of the solution?

2. What was the theoretical (expected) freezing point of your brine solution?  Does this match what temperature you measured?  Explain why the temperature you observed my not be the expected freezing point.

Assessment

1. Cleaning up your mess after you ate your ice cream…....….…

2. Your calculations in data table…………………………..…..

3. Question #1…………………………………………………

4. Question #2………………………………………………… 

 

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