Instructor Information
JCE Classroom Activity: #45
Meltdown Showdown! Which Deicer Works Best? John W. Lyga, FMC Corporation Agricultural R&D Center, P. O. Box 8, Princeton, NJ 08543 There are several chemical deicers available to the homeowner for removing snow and ice from sidewalks. The most common contain sodium chloride (rock salt) or calcium chloride as the active ingredient. Calcium chloride products usually cost more than sodium chloride products but melt ice faster and at lower temperatures. In this Activity students decide which chemical deicer works better at melting ice and if the calcium chloride deicer is worth the extra cost. Chemical deicers work by lowering the freezing point of water, the temperature at which liquid water changes to solid ice. The number of degrees the freezing point is lowered depends on the concentration of ions or particles in the solution. Calcium chloride forms three ions (Ca2+ and 2 Cl–) when dissolved in water and sodium chloride forms only two (Na+ and Cl–). However, the molar mass of CaCl2 is almost double that of NaCl, which results in a lower concentration of CaCl2 when equal volumes of the two deicers are dissolved, since the densities of the two are nearly the same. There are two reasons why CaCl2 works better than NaCl. First, the process of dissolving calcium chloride in water is exothermic (gives off heat), which facilitates melting. See the WWW references on the Student Activity for more information. In addition, CaCl2 has a lower eutectic temperature (the lowest temperature to which the compound can depress water’s freezing point) than NaCl. At lower temperatures, CaCl2 does a better job at melting ice.
Integrating the Activity into Your Curriculum An extension to this Activity is to integrate it into a discussion about ecology. Although sodium chloride and calcium chloride are the most common deicers, they are not the most environmentally friendly. Both can cause corrosion to concrete sidewalks and vehicles, injury to plants, and water pollution from runoff. Other deicers such as CMA (calcium magnesium acetate) or urea are used as alternatives and can be included in this Activity. An experiment on this subject suitable for advanced students (1) and a related JCE Classroom Activity (2) have been published recently.
About the Activity
perforated
Students test two common chemical deicers to decide which is more effective at melting through a thin disk of ice. The ice disks are made by freezing water in a large water-resistant disposable dinner plate (not one divided into sections; divided plates do not work). Ice disks can be made in advance and stored in sealed plastic bags in the freezer. The deicers work best if they are placed about 5 cm from the edge of the disk and are well separated from each other. Calcium chloride deicer tends to stick to the disk and cannot be moved after placement. Calcium chloride deicer can be more difficult to find than rock salt. Places to try are auto parts, discount, hardware, and Photo by Erica K. Jacobsen and Jerrold J. Jacobsen grocery stores. The product purchased to test this Activity was Prestone Driveway Heat. The tests in sections I–III help students develop a hypothesis about why one deicer works better. Remind the students to use stirrers and not the thermometer to mix the deicers and water. Part I is most effective is the freezer temperature is below 20 °F. If you do not have thermometers for section II, the change in temperature is sufficient that the students can differentiate by feeling the cup. An extension can be to see if adding more deicer speeds the meltthrough process in section IV. Instructors may wish to use additional space on the ice disk to test other substances such as table salt, sugar, windshield deicers, and sand.
Answers to Questions 1. Dissolving calcium chloride in water is exothermic; the temperature increases. Dissolving sodium chloride is endothermic; the temperature decreases. The heat produced when calcium chloride dissolves melts some of the ice disk, which dissolves more of the calcium chloride. Water containing the dissolved calcium chloride freezes at a lower temperature than pure water, so it stays liquid. Sodium chloride dissolved in water also lowers the freezing temperature of water but it takes longer for the sodium chloride to dissolve. 2. Calcium chloride is hygroscopic: it absorbs moisture from the air; sodium chloride does not. The moisture absorbed helps to dissolve some of the calcium chloride and start the melting process. 3. At freezer temperatures, the calcium chloride melts a significant amount of ice while the sodium chloride melts only a small amount. At low temperatures, calcium chloride is more effective than sodium chloride at melting ice. See the graph in Web site 3 on the Student Activity. 4. Answers will vary. Rock salt costs less per pound but calcium chloride melts ice faster than rock salt and is more effective at lower temperatures. Calcium chloride can be more environmentally friendly, since less is needed.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Background
References, Additional Related Activities, and Demonstrations 1. 2.
Lantz, Juliette M.; Feindt, Jenny E.; Lewellyn Eric P. B.; Walczak, Mary M. The Bridge of Mandolin County; J. Chem. Educ. 1999, 76, 1671. JCE Editorial Staff. An After-Dinner Trick; J. Chem. Educ. 2002, 79, 480A–480B.
JCE Classroom Activities are edited by Nancy S. Gettys and Erica K. Jacobsen
JChemEd.chem.wisc.edu • Vol. 79 No. 5 May 2002 • Journal of Chemical Education
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JCE Classroom Activity: #45
Student Activity
Meltdown Showdown! Which Deicer Works Best? You may have helped your parents spread ice-melting crystals on your sidewalk in the winter to melt the ice. Have you ever looked at the ingredients on the bag to see what the crystals are made of? All ice-melting products, known as chemical deicers, are not the same, but most are made from rock salt (sodium chloride, rightmost image in the photo at the right) or calcium chloride (leftmost image). Those made from rock salt are usually less expensive. In this Activity you will test two chemical deicers to determine which melts ice better and decide if it is worth the extra cost to buy a more expensive deicer.
Photo by Erica K. Jacobsen and Jerrold J. Jacobsen
Try This You will need: rock salt or other sodium chloride deicer, calcium chloride deicer, water, large water-resistant disposable dinner plate (not a divided plate), bowl with a diameter slightly smaller than the plate, measuring cups and spoons, stirrers, marking pen, thermometer, crushed ice, six 3-oz disposable cups, access to a freezer. I. Melting ice in the freezer __1. Label two 3-oz disposable cups A and B. Place 2 tablespoons (30 mL) of crushed ice in each of the two cups. __2. Add 1 teaspoon (5 mL) of sodium chloride deicer to cup A and mix with a stirrer. Add 1 teaspoon (5 mL) of calcium chloride deicer to cup B and mix with a stirrer. Immediately place both cups in a freezer and leave for 30 min. During this 30 min period, you can continue with the rest of the Activity. After 30 min, remove the cups, observe the contents of both cups, and record your observations. II. Mixing deicers with water __1. Label two 3-oz disposable cups A and B. Add 2 tablespoons (30 mL) of water to each cup. Measure the water temperature using a thermometer and record it. __2. Add 1 teaspoon (5 mL) of sodium chloride deicer to cup A and stir with a stirrer until it dissolves. Measure and record the temperature of the solution. __3. Repeat step 2 using calcium chloride deicer. III. What happens when the deicers are exposed to air overnight? __1. Label two 3-oz disposable cups A and B. __2. Add 1 teaspoon (5 mL) of sodium chloride deicer to cup A and 1 teaspoon (5 mL) of calcium chloride deicer to cup B. Let the cups sit at room temperature exposed to air overnight. __3. Observe the contents of both cups and record your observations. IV. The Showdown __1. Add 1/2 cup of water to a water-resistant disposable dinner plate (not a divided plate). Place it in a freezer until the water freezes completely. Remove the resulting ice disk from the plate and place it on top of a bowl with a diameter slightly smaller than the plate. Set the bowl on the plate to catch any drips of water. __2. Based on your observations from parts I–III, which deicer do you think will melt through the ice disk faster? Place 1 teaspoon (5 mL) of sodium chloride deicer on the ice disk about 5 cm from the edge. Place 1 teaspoon (5 mL) of calcium chloride deicer on the other side of the ice disk 5 cm from the edge. What do you observe happening over time? Are your observations different for the two deicers? Does adding more deicer speed the process?
Questions __1. Based on your observations in section II, suggest an explanation of the fact that one of the deicers works faster. __2. For deicers to start working they need to be dissolved in water. How do your observations from section III help you explain the result from section IV? __3. Which of the deicers should work better at lower temperatures? Explain. __4. Sample prices for February 2002 are $2 for 25 pounds of sodium chloride deicer and $10 for 25 pounds of calcium chloride deicer. Would it be worth the price difference to a homeowner who uses a few pounds to clear sidewalks each winter? To a city that uses several tons to clear city streets? Consider damage the various materials would do to the surface to be cleared and to the surroundings when counting the cost. Explain your reasoning.
Information from the World Wide Web (accessed March 2002) 1. 2. 3.
Why does salt melt ice? http://antoine.fsu.umd.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml Salt Institute; http://www.saltinstitute.org Using salt and sand for winter road maintenance; http://www.usroads.com/journals/p/rmj/9712/rm971202.htm This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 79 No. 5 May 2002 • JChemEd.chem.wisc.edu
