Making Ice Cream Is Really Just Chemistry

Making Ice Cream Is Really Just Chemistry

MAKING ICE CREAM IS REALLY JUST CHEMISTRY

PURPOSE:

To determine the freezing point depression of water surrounding an ice

cream maker given the concentration of salt in that water solution.

BACKGROUND INFORMATION:

When a substance is dissolved into a pure solution, that solution’s boiling point is elevated, and the solution’s freezing point is depressed. These properties are known as colligative properties, because they do not depend on the nature of the solute being used, but rather, on the concentration of the solute particles. The freezing point depression for any solvent can be determined by the equation:

Tf = Kf m i

Where Tfis the decrease freezing point of the solvent in question (usually water), Kfis the molal freezing point constant, inC0/m, m is the molality of the solution, which is the concentration of the solution, and i is the number of particles that dissociate in solution. Molality, remember, is calculated by taking the moles of solute and dividing by the kilograms of solution. Kfis different for various solvents – for water it is 1.86 C0/m.

MATERIALS NEEDED:

3 quarts half and half1 large mixing spoon

3 Tb vanillaplastic spoons and bowls

2 1/4 cups sugar2 containers of table salt

2 bags of icetoppings or flavorings

Materials for molality analysis

PROCEDURE:

-MAKING THE ICE CREAM

Obtain an ice cream maker. Mix the half and half, sugar, vanilla into the main container. Place the ice and salt in alternating layers around the ice cream maker. Turn on the ice cream maker, and as it turns, periodically add new layers of ice and salt to the surrounding chamber. Be careful the chamber does not freeze. You might want to conduct this experiment in the sink, because water leakage becomes an issue. For wooden ice cream makers with small sides, use smaller ice cubes! After the ice cream has reached a good consistency, save 100 mL of the ice/salt solution for analysis. Record the temperature of your solution three times throughout the course of your experiment for comparison to your calculated freezing point depression.

- ANALYZING THE FREEZING POINT DEPRESSION

During the second day of the experiment, weigh an evaporating dish and watch glass. Fill the evaporating dish to 1/2 of its capacity with your saved salt water solution. Weigh the dish, glass, and solution. Evaporate the solvent with a Bunsen burner and ring stand. Make sure that the evaporating dish is covered with the watch glass! Weigh the COOL dish, glass, and solute.

CALCULATIONS:

Calculate the following quantities and include them in your data table. Your calculations portion of your lab report should include the following DETAILED calculations:

Grams of solute, grams of solvent, moles of solute, Kg of solvent, molality of solution, change in temperature of freezing point, the new freezing point, the observed freezing point (measured with a thermometer – the lowest temperature reached) and the predicted freezing point (calculated using formula).

DATA:

You should create a data table that includes ALL data that you have obtained during the course of the lab, and ALL data that you have calculated. This includes the mass of the evaporating dish, lid, solution, salt, and all of the calculated data listed above.

CONCLUSION:

  1. Why does the freezing point of a solvent decrease when a solute is added?
  2. A colligative property is one that depends on the amount of a substance, and not its identity. Does the freezing point depression really depend only on the amount of a substance, and not on the identity of it? Explain!
  3. Would using a different salt (ionic compound) besides NaCl have affected our results? Why or why not?
  4. Did your solution decrease in temperature and reach the depressed freezing point that you calculated? Should it reach this temperature? Some solutions got colder or were warmer than the calculated temperature. Why is this?
  5. Calculate the molal freezing point constant for a solution that has 150 g of Na2CO3 dissolved in 835 g of solvent, if the freezing point has dropped 19.83 0C. What is the solvent? (Hint – find a chart that has molal freezing point constants on it!)
Top View