Ice Cream Lab

Colligative Properties Lab

Colligative Properties: Physical properties determined by the concentration of dissolved particles in a mixture, and not affected by the type of dissolved particles. Osmotic pressure (tendency of water to flow from high water concentration to low water concentration as seen when applying salt to a slug), boiling point elevation and, most importantly in this activity, freezing point depression are examples of colligative properties. When 1 mole of NaCl dissolves in a liter of water it dissociates into 1 mole of sodium ions and 1 mole of chloride ions, or a total of 2 moles of particles per liter of mixture (a 2M concentration). When 1 mole of sugar dissolves in a liter of water there is just 1 mole of sugar molecules per liter (a 1M concentration). The particle concentration of the solution has a direct relationship to the strength of the colligative effect of the solution.

Ice cream, a mixture of milk, sugar and vanilla, is really an aqueous mixture with many particles in each liter. Because of this the freezing point of ice cream is lower than that of water, so to freeze ice cream, or to keep it frozen, you must keep its temperature significantly below 0 degrees Celsius. Adding rock salt to ice produces a melting ice and saltwater mixture with a depressed freezing point in which the ice cream can be frozen.

Heat of Fusion: Melting a solid to form a liquid is an endothermic process. The heat of fusion of a substance is the amount of energy needed to change 1 gram of solid to liquid. The heat of fusion of water is 334 joules/gram.

Since the room temperature and the milk mixture are both warmer than the freezing point of the ice mixture, energy will be transferred into the ice mixture on all sides. This energy will be used in melting the ice (334 joules per gram melted) and the temperature of the ice mixture will stay at its freezing point as long as there is still ice to melt. This process will effectively draw energy out of the milk mixture, lowering its temperature and, eventually, freezing it!

The Purpose of this lab is to illustrate how adding an ionic solute changes the properties of the water.

Materials needed:

¾ cup Bovine Lacteal Secretion (WHOLE MILK or Half& Half) ¼ Cup Sucrose (SUGAR) 1 tsp. 4-hydroxy-3 methoxy-bensaldehyde (VANILLA) 1 scoop Sodium Chloride 1 squirt sterculiaceae (Optional)(Chocolate/Strawberry SYRUP) 2 Cups Solid Hydrogen Hydroxide (ICE) 1 Small Ziploc Bag 1 Large Ziploc Bag Procedure:

1) In the small zip lock bag place ¾ cup Bovine Lacteal Secretion. Add 1 tsp. 4-hydroxy-3 methoxy- bensaldehyde, ¼ Cup Sucrose and 1-2 squirts sterculiaceae (Optional). Mix thoroughly till all solutes are dissolved.

2) Close and seal the bag tightly. ( A must)

3) Fill the large Zip lock bag with 2 Cups Solid Hydrogen Hydroxide and add 1 scoop Sodium Chloride. Measure and record the pre-temperature (Do not place thermometer into the small bag mixture.)

4) Place the small Zip lock bag into the large bag and seal tightly

5) Shake and knead the large bag making sure that the smaller bag inside is being shifted through the solid water. Continue to do this for 20 minutes or until the inner bag contents begin to harden. Measure and record the post-temperature (Do not place thermometer into the small bag mixture.)

6) There are spoons and bowls to eat your ice cream. Enjoy, but please clean up your lab area.

Name ______Date. ______ANALYSIS

1) Compete the data table during the lab.

2) Show Formulas and calculations below for the following: Record A Pre –temperature 0.0 C C.  B Post- temperature C C Experimental ΔT C

F.  D Estimated Mass of Ice 0.65 Kg E Estimated mass of salt 65.0 g F Moles of salt mol G.  G Molality of ice-salt soln m Van’t Hoff multipler H for the Ice-salt soln. I.  I Theoritical ΔT

Conclusion Questions

1) List two ways adding rock salt helped to freeze the ice cream. (Consult Intro.)

2) Where does the cold (well below zero) come from, how does the ice solution get so cold?

Cold does not "come from" anywhere. Cold is the absence of heat. As the salt dissolves in the meltwater, intermolecular bonds between water molecules in the ice lattice are broken. Bond breaking processes always require energy. The energy flow to enable the breaking process comes from the surroundings. If you were to be holding the glass containing the ice-salt-water mixture, heat energy would flow out of your hand into the glass. Heat flow away from you is perceived as cold.

3) Explain briefly why we “salt” the roads when it is icy? (Please do not respond that the salt melts the snow)