Salt: The Essence of Life
Multidisciplinary Curriculum for Students
Unit 3: Chemistry
Chemical Reactions of NaCl
1 Class Period
Overview
Students will investigate the properties of salt in different chemical reactions. The relationship between salt and water/ice will be explored through several interesting experiments.
Background
Because of its effectiveness, availability and low cost, salt is the highway deicer in common use today. By removing ice and snow from a slick roadway surface, salt promotes winter driving safety and continued mobility, even under adverse snow and ice conditions. Rock salt and solar salt are used on highways. The lowest temperature at which sodium chloride will melt ice (the eutectic point) is -21.12° C. (-6.02° F.), at a concentration of 23.33% NaCl. Salt works best at temperatures just below 0° C. (32° F.). As the temperature falls, it takes more salt and more time to melt ice because a higher concentration of brine (salt water) is required. Fortunately, most snowstorms occur when the temperature is near freezing, where salt is very effective.
Salt is used as a deicer because it lowers the freezing point of water. The melting action of salt forms a brine, or a strong saline solution, that can penetrate below the surface of ice and packed snow. Brine prevents water from freezing and bonding to the pavement. When ice melts due to the presence of salt, heat is absorbed. This is called an endothermic reaction. The solution of salt (NaCl) in water has a lower vapor pressure than the ice so the ice changes its phase to liquid water. That is how salt reduces the freezing point.
When salt is scattered on ice, it is so soluble it immediately begins to dissolve in the moisture present on the surface of the ice, causing the ice to melt. The resulting brine dissolves more salt which in turn melts more ice and so on.
Getting Ready
Activity A
Time: 1 class period (est. 45 min.)
Materials
Per Student Group - 2 Styrofoam cups, 2 calibrated beakers, 1 cup crushed ice, 2 teaspoons salt, and graph paper
Instructions
Distribute materials to each student group. Direct students on how to assemble their experiment.
Activity B
Time: 1 class period (est. 45 min.)
Materials
Per Student Group - 2 beakers or similar containers, 2 cups crushed ice, 2 unfrozen freezer pops, measuring cup
Instructions
Distribute the materials to each student group.
Activity C
Time: 1 class period (est. 45 min. plus eating time)
Materials
Per Student Group - 1/2 package 4-ounce instant vanilla or chocolate pudding, 1/4 cup sugar, 1 cup 2% or skim milk, 1/2 cup evaporated milk, beaker, empty coffee can, 6 cups crushed ice, and 1 cup rock salt
Instructions
Distribute materials to each student group.
Activities
Activity A - Salt: The Melting Agent
1. Ask students to quickly melt a given amount of ice. Ask the class for suggestions. Heating the ice is not an acceptable answer.
2. Poke five holes in the bottom of two Styrofoam cups, and label them Cup A and Cup B.
3. Fill each Cup with the same weight of crushed ice. The weight of the ice should be measured to make sure equal amounts are placed in each Cup.
4. Mix two teaspoons of salt with the ice in Cup A. Do nothing with Cup B. Suspend both cups into two calibrated beakers and allow the melting ice to drip into the bottom of the beaker. Have students create a table to record their data.
5. Begin timing the melting process with a stop watch.
6. Every sixty seconds record the amount of water in Beaker A and Beaker B. Record for a total of 15 minutes.
7. Ask students which cup produced the most water. Students can create a graph of the data to display the results. Ask students to develop hypotheses to explain why salt had this effect on ice.
Activity B - Super Cool
1. Obtain two large beakers (1000 or 500 ml.) and fill them with equal weights of ice, about 2/3 full. Label the beakers A and B.
2. Place an unfrozen freeze pop in each of the beakers so that it is surrounded by ice. Freeze pops are popcicles with a plastic wrap, and can be purchased in most grocery stores.
3. Place a thermometer in each beaker and record the temperature of the ice.
4. Mix one cup of rock salt with the ice in Beaker A.
5. Record the temperatures in each of the beakers every five minutes.
6. Pour another 1/2 cup of salt into beaker A at 15 and 25 minutes into the experiment. Continue taking temperature readings for 30 minutes.
7. Examine the freeze pops at the end of the experiment, and determine which one has frozen and its degree of hardness. Record the results. Eat them quickly before they get soft! Discuss why the salt-ice mixture produced a lower temperature than the ice with no salt added. Explain that salt has, in a sense, lowered the temperature of the mixture. When ice melts due to the presence of salt, heat is absorbed. This is called an endothermic reaction. Ask what implications this has for different types of food processing? Salt is used to lower the temperature and increase the rate of ice melting. For example, freight and refrigerator cars use ice and salt to cool foods. The next activity will provide one example.
Activity C - Ice Cream Chemistry
1. Each student group should create an ice cream mixture made from the following ingredients:
- 1/2 package 4-ounce instant vanilla or chocolate pudding
- 1/4 cup sugar
- 1 cup 2% or skim milk
- 1/2 cup evaporated milk
2.Have a student pour the ice cream mixture into a sterilized beaker, glass or metal container.
3. Immerse the container with ice cream inside a larger container filled with 3 cups of crushed ice and 1/2 cup rock salt. Create a depression so that all but the top is surrounded by ice.
4. Stir the ice cream every few minutes. Also stir the ice-salt mixture every few minutes. Make sure that the ice-salt mixture does not spill into the ice cream.
5. As the ice melts add more ice and rock salt with the ratio of 3 tablespoons salt to 1 cup ice.
6. Continue to stir and add ice for 20 to 30 minutes, or until the ice cream is of desired consistency. Take occasional temperature readings of the ice-salt mixture.
7. Enjoy eating the ice cream!
8. Ask students the following questions:
a.What physical change took place?
b.Was the temperature of the ice mixture higher or lower than that of the ice cream?
c.What does the phrase "freezing point depression" mean, and how does it apply to making ice cream?
Assessment
Have students choose and conduct one of the three experiments. When completed, write up a brief report describing the activity steps, results, changes and the role salt played in the chemical reaction.
Conclusion
Ask students to think of other applications of salt-ice interactions for industrial uses or uses around their home or neighborhood. Students may have used salt to deice sidewalks and driveways. Have students describe any observations they have seen of salt and ice interacting, or how they may use these principles in their lives. It also may be another interesting topic to discuss why oceans tend to freeze at a much lower temperature than freshwater bodies of water.