If you've ever made homemade ice cream the old-fashioned way with a hand-crank machine, you probably know that ice and rock salt are necessary to cool the cream mixture enough for it to freeze. Similarly, in cold climates, you might have seen trucks spreading salt and sand on roads after a snowfall to prevent ice buildup. In both cases, salt works by lowering the freezing point of water, causing it to change phases from solid ice to liquid water.
For the ice cream maker, the rock salt lowers the freezing point of the ice, allowing the temperature of the ice/rock salt mixture to drop below the normal freezing point of water. This allows the ice cream mixture in the machine’s inner container to freeze. For road salt during winter, the lowered freezing point allows snow and ice to melt even when the temperature is below water’s normal freezing point. Both ice cream making and salting roads are examples of freezing point depression.
Table salt (sodium chloride, NaCl) mixed with water is a good example of a chemical solution. In any solution, there are two components: the solvent (water in this case) and the solute (salt here). The solute molecules dissolve in the solvent because the attraction between the solute and solvent molecules is stronger than the attraction between the solute molecules themselves. Water (H2O) is an excellent solvent because it is partially polarized. This means that the electrons in a water molecule are unevenly distributed, with a partial positive charge on the hydrogen side and a partial negative charge on the oxygen side.
Because of this polarization, water molecules can surround charged particles, such as the sodium (Na+) and chloride (Cl-) ions that make up salt. The attraction between the water molecules and these ions is stronger than the attraction between the salt molecules, allowing the salt to dissolve and form a salty solution.
When substances dissolve in water, they can also lower the freezing point of the water. The degree to which the freezing point drops depends on the number of molecules dissolved, not on their chemical properties. This effect is an example of a colligative property.
In this science project, you will explore how different substances affect the rate at which ice cubes melt. You'll test soluble substances like salt and sugar, as well as an insoluble substance, sand. The goal is to determine which substances cause the ice to melt the fastest.
Tip for Science Fair Participants:
As you conduct your experiment, be sure to take a few photos of yourself in action and your experimental setup. These images can help make your science fair display board more engaging and informative.
1. Prepare Your Materials:
Gather the salt, sugar, sand, and measuring teaspoon so they are ready to use. Once you’ve set up the ice cubes in their bowls, you’ll need to quickly add the substances to prevent the ice from melting before you begin.
2. Set Up the Ice Cubes:
Place three ice cubes in each of the four bowls. Arrange them so that only their corners are touching, forming a triangular shape, as shown in Figure 1.
Tip: If you’re using ice cubes from a tray, let the tray sit at room temperature for about five minutes. This will make it easier to remove the ice cubes without them breaking into pieces.
3. Add the Substances:
Gently sprinkle ½ teaspoon (tsp.) of salt over the ice cubes in one bowl, as shown in Figure 2. Next, sprinkle ½ teaspoon of sugar over the ice cubes in a second bowl, and ½ teaspoon of sand over the ice cubes in the third bowl. Leave the ice cubes in the fourth bowl without adding any substances — this will serve as your control.
4. Place the Bowls in the Refrigerator:
Carefully move each bowl to an empty shelf in the refrigerator. If any of the ice cubes have shifted and no longer form a triangular shape, gently adjust them back into place.
You are conducting this experiment in the refrigerator because the effects of colligative properties are easier to observe at colder temperatures. To understand why, imagine melting an ice cube on a hot, paved road compared to melting it in a refrigerator. The road’s heat would cause the ice to melt quickly, making it difficult to notice the subtle effects of colligative properties on the melting rate.
5. Record the Starting Time:
Make a note of the starting time in your lab notebook. Inform others who may use the refrigerator that you are working on a science project and remind them not to leave the door open too long, as this could affect the temperature.
6. Monitor the Ice Cubes:
Check the ice cubes every hour. When the ice cubes in one of the bowls are at least half melted, remove all four bowls from the refrigerator and proceed to step 7. (Be sure to remove the bowls before the ice cubes in two or more bowls completely melt.)
Depending on your refrigerator's temperature, it may take about four hours for the ice cubes to melt by at least half. While waiting, create a data table similar to Table 1 in your lab notebook.
7. Pour the Liquid Water:
Carefully pour the liquid water from one of the bowls into a cup with a spout (such as a large measuring cup). Be sure the ice cubes stay in the bowl, but try to transfer as much liquid as possible into the cup. Then, carefully pour the liquid from the cup into the graduated cylinder. Record the amount of liquid in the bowl (the amount of ice that melted) in the data table in your lab notebook. After recording your results, clean and dry both the cup and the graduated cylinder.
Alternatively, you can use a funnel to transfer the liquid directly from the bowl into the graduated cylinder.
8. Repeat Step 7 for the Other Bowls:
Perform step 7 for the remaining three bowls. When pouring the liquid from the bowl with sand, try to leave as much sand behind in the bowl as possible.
9. Let the Ice Melt Completely:
Allow the ice cubes to fully melt in their bowls (you can leave them at room temperature). Once all the ice cubes are melted, repeat steps 7–8. This time, you won’t need to worry about keeping the ice cubes in the bowls. Record the amount of liquid remaining in each bowl in your data table.
10. Calculate the Total Amount of Water:
Calculate the total amount of water (originally in ice cube form) that was in each bowl by adding the “amount melted” and the “amount remaining” for each bowl. Record the total for each bowl in your data table.
For example, if the amount melted was 65 milliliters (mL) and the amount remaining was 25 mL, the total amount would be 90 mL.
11. Calculate the Percentage of Ice Melted:
For each bowl, calculate the percentage of ice that melted when you first removed the bowls from the refrigerator. To do this, divide the amount melted by the total amount.
For example, if 65 mL melted and the total amount was 90 mL, dividing 65 mL by 90 mL gives you 0.72, or 72%. This means that 72% of the ice melted.
12. Clean and Repeat:
Clean and dry the bowls. Then, repeat steps 1–11 at least two more times to complete a minimum of three trials.
13. Analyze Your Results:
Did any of the substances you tested consistently speed up the melting of the ice compared to the ice cubes with nothing added? If so, can you explain your findings?
Variations
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Effect of Solute Amount on Melting Rate:
Does the rate at which the ice melts depend on the amount of solute added? Design an experiment to investigate this by varying the quantity of solute (such as salt or sugar) you use.
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Temperature's Effect on Colligative Properties:
Explore how temperature influences the effect of colligative properties on melting ice. Try conducting the experiment at different temperatures, such as room temperature or outside on a hot day. Be sure to monitor the temperature consistently throughout the experiment.
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Testing Other Substances:
Do other substances melt ice more quickly or slowly? Choose different substances to test and repeat the experiment to observe how each affects the ice melting process.
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Advanced Freezing Point Depression Experiment:
For a more advanced experiment on freezing point depression, consider the Science Buddies project Chemistry of Ice-Cream Making: Lowering the Freezing Point of Water. This experiment explores a similar concept in a more detailed way.
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Salt in the Freezer:
Do you think salt would melt ice in your freezer? Why or why not? Try the experiment in your freezer and observe the results.
