Freezing Point Depression Lab

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Purpose

To determine how the addition of a solute alters the freezing point of a solvent and to use this principle in the production of a frozen dairy product.

Introduction

The freezing point is the temperature at which the solid and liquid phases of a substance can exist together without any net change in the amount of substance in either phase. Equilibrium exists between the solid and liquid phases of a substance at its freezing (melting) point. The freezing point of a substance is the only temperature at which the solid and liquid phases of a substance have the same vapor pressure. Vapor pressure of a solution is lower than the vapor pressure of the pure solvent. The freezing point depression is the lowering of the freezing point that occurs when substances are dissolved in the liquid. While it is common to speak of "the freezing point of the solution," it is the solvent in the solution that is actually freezing. The solute particles remain in the liquid phase of the solvent.

The amount the freezing point is lowered by solutes that are nonelectrolytes depends upon the same two factors that affect vapor pressure lowering. (1) One factor is the molal concentration of the solution. (2) The other factor is the nature of the solvent. For solutes that are nonelectrolytes, one solute will lower the freezing point of a solution the same amount as another, provided the solutions are made from the same solvent and their molal concentrations are the same. The freezing point of different solvents is lowered by different amounts. The amount that the freezing point is lowered is characteristic of the solvent. For example, 1-molal water solutions of all nonelectrolytes freeze at 1.86°C below the freezing point of pure water. That is, they all freeze at -1.86°C rather than at 0°C. These solutes forming 1-molal solutions with a different solvent will lower the freezing point by a different amount.

1.86 is called the molal freezing point constant for water. The molal freezing point constant of a solvent is numerically equal to the number of degrees the freezing point of the solvent is depressed by a nonelectrolyte solute.

In the processing of ice cream, stirring is often utilized to help maintain even temperatures and fast crystallization. Rapid crystallization tends to develop small ice crystals. The fat incorporated into ice cream is very important to its texture. Ice cream products with high fat content will be creamier and smoother. The fat interferes with the development of large crystals. It also lends a certain "mouth feel" to the product. Dairy products such as ice milk, which have low fat contents, tend to be coarser textured.

Procedure

1. Bring your own ice cream recipe and ingredients. Mix ice cream well. All ingredients should be chilled. Do not add "chunky" ingredients (chocolate chips, fruit, etc.) until freezing is almost complete.
2. Pour a small amount of unfrozen mixture into a small beaker and record its temperature. Pour your mixture into the can of the freezer. Assemble. Add ice and salt in layers.
3. Plug in or churn freezer until the motor sounds "labored" or until ice cream is of the proper consistency. You may need to drain H₂O and add ice/salt periodically. Take a final temperature of the ice cream mixture when you are finished.
4. You may work in groups of 2-4 students.
5. You must bring everything that you need to prepare and eat your ice cream.
6. You are graded on your organization, cleanup, quality and flavor of your ice cream.
7. Please bring ice cream recipes the day before we perform the experiment. Try to find recipes that do not require cooking.

Materials needed

Please be sure to label everything!

• ice chest (to hold ice and ingredients)
• ice (2 bags)
• mixing and serving spoon
• plastic cups or bowls to serve ice cream
• plastic spoons to eat with
• all ingredients
• rock salt (1/2 of a small box)
• ice cream maker (make sure that it is clean and works)

If you are adding "chunky" ingredients, do not add them until the ice cream has started to thicken. Otherwise, they will freeze to the bottom and the ice cream maker will stop turning too early.

"Whipping cream" or heavy cream is not Cool Whip or the spray stuff in a can. It comes in a carton in the dairy section of the store.

Get the telephone numbers of group members and call each other the night before or morning of the lab to remind everyone to bring their stuff. When you are deciding who will bring what, think about cost. Cream can be expensive.

Name: __________________

Period: _______

Freezing Point Depression Lab

Data Sheet

Evaluation: (rate on a scale of 1-5, with 5 being perfect)

Initial temperature of mixture: ____________

Final temperature of ice cream: ____________

Temperatures of ice salt mixture:

• 0 minutes: __________
• 5 minutes: __________
• 10 minutes: __________
• 15 minutes: __________
• 20 minutes: __________
• 25 minutes: __________
• 30 minutes: __________
• 35 minutes: __________
• 40 minutes: __________
• 45 minutes: __________

Questions

Why did you add salt to the ice when making ice cream?

Which would lower the freezing point more, 100 moles of sugar or 100 moles of NaCl? Why?

Why must the ice cream be cooled below 0° C before it freezes?

How does stirring of the ice cream while freezing affect its texture?

What effect does fat from cream or other substances have on the texture of ice cream? Why?