Certain substances affect the state of ice. When salt is sprinkled on ice, for example, it causes the ice to melt. This chemical reaction is usually a heat energy transfer. Water freezes at 0 degrees Celsius. A saturated salt water solution freezes at -32 degrees Celsius. What happens when you sprinkle salt on ice? As the salt combines with the melting ice, the salt water solution has a lower freezing point than the salt water. The salt lowers the freezing point of ice, causing the melting ice to be colder than the original ice.
Objectives:
To develop observation skills, understand the scientific process through active experimentation and comparison, and understand that chemicals can change the properties of water.
Materials:
- 1/2 cup milk
- 1/2 cup whipping cream (heavy cream)
- 1/4 cup sugar
- 1 tablespoon chocolate flavoring
- 3/4 cup sodium chloride (NaCl) as rock salt
- 2 cups ice
- 1-quart Ziplock bag
- 1-gallon Ziplock bag
- thermometer
- measuring cups and spoons
- cups and spoons for eating
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Procedure:
1. Add 1/4 cup sugar, 1/2 cup milk, 1/2 cup whipping cream, and 1 tablespoon chocolate flavoring to the quart Ziplock bag. Seal the bag securely.
2. Put 2 cups of ice into the gallon Ziplock bag.
3. Use a thermometer to measure and record the temperature of the ice in the gallon bag.
4. Add 1/2 to 3/4 cup salt (sodium chloride) to the bag of ice.
5. Place the sealed quart bag inside the gallon bag of ice and salt. Seal the gallon bag securely.
6. Gently rock the gallon bag from side to side. It's best to hold it by the top seal or to have gloves or a cloth between the bag and your hands because the bag will be cold enough to damage your skin.
7. Continue to rock the bag for 10-15 minutes or until the contents of the quart bag have solidified into ice cream.
8. Open the gallon bag and use the thermometer to measure and record the temperature of the ice/salt mixture.
9. Remove the quart bag, open it, serve the contents into cups with spoons and ENJOY!
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Data:
1. What kind of ice cream are you making?
I am making chocolate ice cream.
2. Document the temperature of the ice in the gallon bag before adding the salt.
0 degrees Celsius
3. Document the temperature of the ice/salt mixture (not the ice cream) in the gallon bag when the ice cream mixture begins to solidify.
-10 degrees Celsius
4. What is the temperature difference before adding the salt and after mixing for 5 minutes?
-10 degrees Celsius
5. What is the temperature difference before adding the salt and after mixing for 10 minutes?
-10 degrees Celsius
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| Adding the ice cream ingredients over the sink. |
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| Shaking the ice cream mixture, salt and ice. |
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| Serving the solid ice cream! |
Conclusion:
1. What kind of chemical reaction is the process of ice cream making, as done in our lab? Explain why.
The process of making ice cream is an endothermic reaction. In order for the ice, which cools the ice cream mixture to melt, energy must be absorbed from the outside environment to do so. Also, when salt is added to the ice, it lowers the freezing point of the ice, so even more energy has to be absorbed from the environment in order for the ice to melt.
2. Explain why salt water is harder to freeze than plain water.
A salt water mixture freezes at a much lower temperature than plain water. Salt water, which freezes at -32 degrees Celsius, would take much longer to freeze than plain water, which freezes at 0 degrees Celsius.
3. Is the process of making ice cream and physical or a chemical change? Explain your answer.
The process of making ice cream is a physical change because there is only a change in the state of matter of the ice cream during the ice cream making process. The liquid ice cream mixture is changed to a solid ice cream mixture. Also, there is no change in the chemical composition of the ice cream.
4. Why is salt used with the ice instead of sugar?
Sugar does not dissolve into multiple particles like an ionic material, such as salt. Compounds that break into two pieces upon dissolving, like NaCl breaks up into Na and Cl, are more efficient at lowering the freezing point of substances. Also, sugar does not dissolve as well as salt does in cold water.
Sources:
1. http://scienceofeverydaylife.discoveryeducation.com/families/pdfs/activities/Kitchen-Chemistry.pdf
2. http://chemistry.about.com/cs/howtos/a/aa020404a.htm
3. https://wbgu.org/wbgumultimedia/scienceatwork2/saw2_module2.pdf
4. http://www.kidspot.com.au/kids-activities-and-games/messy-activities+24/make-ice-cream-in-a-bag+12050.htm
