Instructor Information
JCE Classroom Activity: #44
An After-Dinner Trick A classic chemistry demonstration uses the phenomenon of freezing-point depression to lift an ice cube out of a glass of water with a thread (1–3). In this Activity, students investigate the effect of adding various substances to ice water and are then challenged to use their observations to lift an ice cube from a glass with thread.
The freezing point of a substance is the temperature at which the substance changes state from a liquid to a solid. For example, water freezes at 0 °C. The addition of a solute to a pure liquid lowers its freezing point, an effect known as freezing-point depression. For example, the addition of table salt to water lowers its freezing point. Freezing-point depression makes it possible to lift an ice cube with a piece of thread. The melting of ice is an endothermic process. It requires the absorption of heat from the surroundings. To lift the cube, a thread is wetted and laid on top of the cube. Salt is mounded on top of the cube and moistened thread. The freezing point of the solution formed is lower than that of pure ice. This allows some of the ice on the surface of the cube to melt. Melting absorbs heat from the immediate surroundings, which includes the moistened thread, lowering its temperature. The water that moistens the thread freezes, the thread is now attached to the ice cube, and the thread can be used to lift the ice cube out of the container.
Integrating the Activity into Your Curriculum Use of this Activity could accompany discussion on changes of states, especially the changes of freezing and melting. It could also be used during a unit on solutions chemistry and colligative properties. The Activity could be extended to a quantitative use of the freezing-point depression equation (2). Freezing-point depression also has practical applications such as the salting of icy roads and sidewalks and using ice and salt in making homemade ice cream.
About the Activity This Activity is based on a 1955 Journal of Chemical Education item, which is reproduced on the Student Side (1). An extensive discussion of the chemistry involved is available (2). Instructors may wish to have students use dark thread to lift the cube so it is more visible. Even if students know the “trick”, it may take them several tries to lift the cube. A common problem is trying to lift the cube before the thread has a chance to adhere to it strongly. Another difficulty is the tendency of the cube to tip over if a substance is sprinkled unevenly on its surface. Rectangular cubes provide a larger surface area for the addition of the substance. Cold water is used to minimize melting of the ice cubes. During our tests, the ice cube was lifted with the thread using salt, but not sugar, even though sugar lowers the freezing point. Instructors could turn the challenge into a race by seeing which student can lift the cube first, highest, longest, etc. Remind students to use a spoon to stir the solutions and not the thermometer, to avoid damage to the thermometer. Students should be sure to let the temperature of the solutions come to a constant value. This may require several minutes of stirring.
perforated
Answers to Questions 1. See Background section above. 2. The freezing-point lowering is proportional to the concentration of dissolved particles (ions or molecules). Pepper does not dissolve, so the temperature is not lowered. Salt lowers the temperature the most and sugar a smaller amount. Salt produces a higher concentration of dissolved particles than the sugar. Cream or nondairy creamer produces no measurable effect, probably because very little of the cream and nondairy creamer dissolves. Much of each substance is in suspension. Also, the mass of the creamer is smaller than that of salt and sugar. (From largest to smallest mass per two-tablespoon volume: salt, sugar, nondairy creamer, pepper.) 3. None: the freezing point and melting point of a substance are the same temperature. 4. Applying salt to icy sidewalks and roads lowers the freezing point of the ice and allows some of the ice to melt. Salt is also used with ice in ice-cream makers to lower the temperature of the solution and help freeze the ice cream mixture. 5. Sodium chloride: 2.17 g/mL; sucrose: 1.5805 g/mL. For salt: (30 mL × 2.17 g/mL)/(58.5 g/mol NaCl × 2 ions/ NaCl) = 2.23 mol ions. For sugar: (30 mL × 1.5805 g/mL)/(342 g/mol sucrose × 1 molecules/sucrose) = 0.139 mol molecules. Yes, the salt provided more particles in solution than the sugar, resulting in a lower freezing point.
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. Miller, Julius S. An After-Dinner Trick; J. Chem. Educ. 1955, 32, 205. 2. Shakhashiri, Bassam Z. Chemical Demonstration: A Handbook for Teachers of Chemistry, Vol. 3; University of Wisconsin Press: Madison, WI, 1989; pp 290–296. 3. Sarquis, A. M.; Sarquis, J. L. Fun with Chemistry, Vol. 1; Institute for Chemical Education: University of Wisconsin: Madison, WI, 1995; pp 47–48. JCE Classroom Activities are edited by Nancy S. Gettys and Erica K. Jacobsen
JChemEd.chem.wisc.edu • Vol. 79 No. 4 April 2002 • Journal of Chemical Education
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JCE Classroom Activity: #44
Student Activity
An After-Dinner Trick You are at the supper table. Dessert is about to be served. The table is cleared except for the salt and pepper and cream and sugar, and the water glasses in which reside cubes of ice. A little diversion is wanting. Supply each guest with a short length of string, say two or three inches (or with a piece a foot long if you wish). The problem is to remove a cube of ice from the water glass, using the string. Warning: No knife, spoon, or fork is to be used, and no loop tied about the ice!
Back in 1955, the Journal of Chemical Education published the challenge above. Have you seen this “trick” performed? Do you know how it works? In this Activity, you will investigate the properties of solutions that make the “trick” possible. Then, you are challenged to figure out how to lift the cube yourself!
Try This You will need: six Styrofoam cups, marker, cold water, graduated cylinders or measuring spoons and cups, ice cubes, thermometer, five spoons, salt, sugar, pepper, cream or nondairy creamer, paper towels, scissors, ruler, and thread. __1. Label six Styrofoam cups as follows: cold water, ice water, salt, pepper, cream, sugar. Place 1/2 cup (120 mL) of cold water in each cup. Add a couple of ice cubes to each cup except the cup labeled cold water. __2. Measure the temperature of the water in the cold water cup with a thermometer and record it. __3. With a spoon, stir the mixture in the ice water cup for a couple of minutes. What do you expect the temperature of the ice water will be? Measure the temperature of the water with a thermometer until it comes to a constant temperature and record the constant value. Was your prediction correct? __4. Add the substances to the cups as follows: salt cup: 2 tablespoons (30 mL) of salt cream cup: 2 tablespoons of cream or nondairy creamer pepper cup: 2 tablespoons of pepper sugar cup: 2 tablespoons of sugar __5. Stir each of the mixtures in the four cups from step 4 with a different clean spoon for a couple of minutes. __6. Measure the temperature of the mixture in each of the four cups from step 4 with a thermometer until it becomes constant and record the constant value. Rinse the thermometer with cold water after each use and dry it on a paper towel. Does adding any of these substances change the temperature of the water in the cup? How? What happens to the ice in each of the cups? __7. Remove any remaining ice from the ice water cup. Place a fresh ice cube in the cup. Cut a six-inch piece of thread. Design a procedure to remove the ice cube from the ice water cup, using the thread, without touching the ice cube with your fingers. Remember, no knife, spoon, or fork is to be used, and a loop cannot be tied around the ice. Use the information gathered in steps 1–6 to determine which, if any, of the available substances (salt, pepper, cream, and sugar) might be helpful. Your procedure may include one or more of these substances. __8. Refine and practice your procedure until you can easily remove the ice cube from the cup with the thread. Demonstrate your procedure for your instructor and classmates. Amaze your family and friends at meal time!
Questions __1. Explain how you were able to lift the ice cube. __2. Describe how the temperatures changed in the Activity. Why did each of the substances added to the ice water have the effect it did? __3. What is the difference between the melting point and the freezing point of a substance? __4. Can you think of any practical applications of the properties of solutions explored in this Activity? __5. Look up the density of sodium chloride (NaCl, table salt) and sucrose (C12H22O11, table sugar) in a reference such as the CRC Handbook of Chemistry and Physics. Calculate the approximate number of moles of particles, ions, or molecules that would be formed by 30 mL of each substance. Is the result consistent with your observations in step 6? Explain.
Information from the World Wide Web (accessed February 2002) 1. Plastic Bag Ice Cream; http://www.sme.org/memb/neweek/actice.htm 2. ChemTeam: Boiling Point Elevation and Freezing Point Depression; http://dbhs.wvusd.k12.ca.us/ColligProp/BP-Elevand-FP-Lower.html 3. CRC Handbook of Chemistry and Physics (3rd Electronic Edition); http://www.hbcpnetbase.com/hbcp/ This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 79 No. 4 April 2002 • JChemEd.chem.wisc.edu
