Abstract

Objective

The goal of this project is to determine which added material will make ice melt fastest.

Introduction

To make ice cream with an old-fashioned hand-crank machine, you need ice and rock salt to make the cream mixture cold enough to freeze. If you live in a cold climate, you've seen the trucks that salt and sand the streets after a snowfall to prevent ice from building up on the roads. In both of these instances, salt is acting to lower the freezing point of water.

For the ice cream maker, because the rock salt lowers the freezing point of the ice, the temperature of the ice/rock salt mixture can go below the normal freezing point of water. This makes it possible to freeze the ice cream mixture in the inner container of the ice cream machine. For the salt spread on streets in wintertime, the lowered freezing point means that snow and ice can melt even when the weather is below the normal freezing point of water. Both the ice cream maker and road salt are examples of freezing point depression.

Salt water is an example of a chemical solution. In a solution, there is a solvent (the water in this example), and a solute (the salt in this example). A molecule of the solute will dissolve (go into solution) when the force of attraction between solute molecule and the solvent molecules is greater than the force of attraction between the molecules of the solute. Water (H₂O) is a good solvent because it is partially polarized. The hydrogen ends of the water molecule have a partial positive charge, and the oxygen end of the molecule has a partial negative charge. This is because the oxygen atom holds on more tightly to the electrons it shares with the hydrogen atoms. The partial charges make it possible for water molecules to arrange themselves around charged atoms (ions) in solution, like the sodium (Na₊) and chloride (Cl⁻) ions that dissociate when table salt dissolves in water.

Other substances that dissolve in water also lower the freezing point of the solution. The amount by which the freezing point is lowered depends only on the number of molecules dissolved, not on their chemical nature. This is an example of a colligative property. In this project, you'll investigate different substances to see how they affect the rate at which ice cubes melt. You'll test substances that dissolve in water (i.e., soluble substances), like salt and sugar, as well as substances that don't dissolve in water (i.e., insoluble substances), like sand and pepper. Which substances will speed up the melting of the ice?

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Solution
• Solute
• Solvent
• Colligative properties
• Freezing point depression
• Phases of matter
  • Solid
  • Liquid
  • Gas
  • Plasma
• Phase transitions
  • Melting
  • Freezing
  • Evaporation
  • Condensation
  • Sublimation

Questions

• Which of the suggested test substances are soluble in water?
• Which of the suggested test substances are insoluble in water?

Bibliography

• For more information on colligative properties, see:
  • Eli, Todd & Keith, date unknown. "Colligative Properties," Chemworld, ThinkQuest Library, Oracle Education Foundation [accessed September 6, 2007] http://library.thinkquest.org/C006669/data/Chem/colligative/colligative.html?tqskip1=1.
  • Nave, C.R., 2006. "Colligative Properties of Solutions," HyperPhysics, Department of Physics and Astronomy, Georgia State University [accessed September 6, 2007] http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/collig.html.
• For information on Avogadro's number and molecular weight, see:
  
  • Lachish, U., 2000. "Avogadro's Number, Atomic and Molecular Weight," [accessed September 6, 2007] http://urila.tripod.com/mole.htm.
  • Furtsch, T.A., date unknown. "Avogadro's Number," Tennessee Technological University [accessed July 22, 2010] http://iweb.tntech.edu/chem281-tf/avogadro.htm.
• To try a simulated experiment on freezing point depression or boiling point elevation, see (Flash animation, requires browser plug-in):
  Greenbowe, T.J., 2005. "Boiling-Point Elevation and Freezing-Point Depression," Department of Chemistry, Iowa State University [accessed September 6, 2007] http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/propOfSoln/colligative.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• Ice cubes
• Identical plates or saucers
• Timer
• Electronic kitchen balance (accurate to 0.1 g)
• Measuring cup
• Suggested materials to test for ice-melting ability
  • Table salt
  • Sugar
  • Sand
  • Pepper

Experimental Procedure

1. Do your background research so that you are knowledgeable about the terms, concepts, and questions, above.
2. You'll need a clean plate and several ice cubes for each of the substances to be tested.
3. Use the balance to measure the initial mass of the ice cube. For more information on how to properly use a balance see Chemistry Lab Techniques.
4. Note the starting time, then carefully sprinkle one teaspoon of the substance to be tested over the ice cube.
5. After a fixed amount of time (say, 10 minutes), pour off the melted water into a measuring cup, and use the balance to measure the mass. Subtract the mass of the empty cup, and you'll have the mass of the melted water. Wait the same amount of time for each test.
6. Measure the remaining mass of the ice cube.
7. Repeat three times for each substance to be tested.
8. Use the same procedure to measure the melting rate for ice cubes with nothing added.
9. For each test, calculate the percentage of the ice cube that melted:
   
   [mass of melt water]/[initial mass of ice cube] × 100
   
   
10. For each test, calculate the percentage of the ice cube remaining:
    
    [remaining mass of ice cube]/[initial mass of ice cube] × 100
    
    
11. For each substance you tested, calculate the average amount of melted water produced (as a percentage of initial mass), and the average remaining ice cube mass (as a percentage of initial mass).
12. Did any substances speed up melting of the ice (compared to melting rate of plain ice cubes with nothing added)?

Variations

• Does the melting rate depend on the amount of solute added? Design an experiment to find out.
• Try your experiment in the refrigerator to simulate colder weather. Alternatively, if the outside temperature is wintry, take your experiment outdoors! Be sure to monitor the temperature regularly throughout your experiment.
• For a related, more advanced experiment on freezing point depression, see the Science Buddies project Chemistry of Ice-Cream Making: Lowering the Freezing Point of Water
• Do you think salt would melt ice in your freezer? Why or why not? Try it and find out.

• For more science project ideas in this area of science, see Chemistry Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Last edit date: 2010-07-22 11:00:00