Ice Cream Lab: How cold is Ice Cream cold?

(copy to your paper all parts in blue)

 

Purpose of Lab: What is the effect of a solute on the temperature and freezing point of water? Have you ever heard the expression “cold as water”? Probably not. That’s because when something is really cold, it’s “cold as ice”.

But there is a way to make water colder than 0C without freezing it: just add salt! In this lab, you will explore the effects of a solute---something that dissolves—on the temperature and freezing point of a liquid. And you will make delicious ice cream in the process.

 

Objective: create a chemical reaction by lowering the freezing point of water to below zero allowing ice cream to form.

 

Hypothesis: If a salt solute such as NaCl is added to ice; then the freezing point of water will be lowered to allow ice to become colder than 0C.

 

Materials:

·        240 grams rock salt

·        thermometer

·        230 mL of milk/cream mixture

·        2 small Ziploc bags

·        1 gallon size freezer Ziploc bag

·        mittens/ towel

·        400 mL ice chips

 

 

 

Procedure:

Station1-add 230mL of milk/cream solution to small size Ziploc bag and seal bag tightly. Place inside second Ziploc bag and seal tightly. Make sure the solution is double bagged and sealed before moving to next station.

 

Station2- Add 240 g of Rock Salt (NaCl) using pre-marked cup to gallon size Ziploc bag. Place the double bagged milk/cream solution inside the gallon size Ziploc bag.

 

Station3- Add 400 mL of ice chips to the gallon size Ziploc bag mixing thoroughly with the rock salt. Seal the gallon size bag tightly and shake slightly to distribute salt and ice together throughout. Return to your desk.

 

1. At your desk, you will measure the initial temperature of the ice in the gallon bag. Seal the gallon bag securely.

2. Gently rock the gallon bag from side to side. It’s best to hold it by the top seal or have gloves on the bag so you hands do not get too cold from handling the bag.

3. Continue to rock the bag for the next 20 minutes and take the temperature of the ice/salt mixture every 5 minutes. Record your results in the data table.

4. Remove the gallon size Ziploc bag after 20 minutes. Empty the salt/ice mixture into the sink and discard the Ziploc bags. Open the ice cream, get a spoon and enjoy!

 

 

 

 

Results/Data:

Time (min)

 

Temperature C

cream consistency

0

 

 

 

5

 

 

 

10

 

 

 

15

 

 

 

20

 

 

 

 

Conclusion: Was your hypothesis accurate?

 

Reflection: Just like we use salt on icy roads in the winter, salt mixed with ice in this case also causes the ice to melt. When salt comes into contact with ice, the freezing point of the ice is lowered. Water will normally freeze at 0C. A 10% salt solution freezes at -7C, and a 20% solution freezes at -17C. By lowering the temperature at which ice is frozen, we are able to create an environment in which the milk mixture can freeze at a temperature below 32 degrees F into ice cream.

 

1.   How did salt affect the freezing point of water?

2.   Explain how ice on a sidewalk melts when salt is spread on it, even though the temperature of the ice remains below 0C, the freezing point of water.