Ice Cream: Delicious Chemistry James Martino 4713 Booth Rd. Oxford, OH 45056
Chemistry labs and textbooks usually deal with theory and seldom let the student see chemistry as a real-life subject. Chemical bonding, molecular structure, and colligative properties can be especially dry. The interest and excit&nent generated bv the experiment described below help to overcome these problem8. I assign the experiment after units on solutions, atomic structure, molecular architecture, and bonding. The lab gives practical illustration of the relation of physical properties to bond type and the solution theory which we have developed. I t is time for the students to realize why they study these topics! Administrative Details Ice cream freezers are not standard lab equipment, but they can be borrowed from the students' parents. One-gallon, motor-driven freezers work best. I divide mv class into croups . of six, each group with a freezer, and appoint a leader who is resoonsihle for all of the secretarial and administrative work forthe group. A lab oeriod of 55 min is enouah for conducting the experwell. 'l'hr inyrtdtenti ;Itt,uld he krl~t imenr ii it I.; urrr~t~izcd in u rt.trtrcratur until nel.dt 11. Sludenta >huuld 113e~ ~ l e n01t v salt on thk ice to speed cooling. If the lab is done in co&ec&ve ~ e r i o d sthe . brine can be saved and used in later periods to itart c&line the ice cream mixture. It still requires lot of ice. I usually obtain the ice from the athletic department by bribing them with ice cream. The ice cream can he frozen in 40 min, if necessary, but it is better if more time is allowed. I t is especially helpful if one person from each group arrives early and completes the mixing step (step 2). If a spare freezer is available, it can be left unassembled for students to investigate (step 1) once the freezing operation is begun. Prior to the day of the lab, a list should be made of the materials that will be needed from an outside source. Students are quite helpful in procuring the necessary items. We have found several sources to be esoeciallv helpful. Serving spoons can be rented from the cafeteria or home economicsdepartment. (Pay for these with ice cream.) Ice cream can be served in expanded polystyrene cups and eaten with plastic spoons. The cuws can he used later in the lab as calorimeters or containers. They can even be washed and used again. Plastic spoons also come in handy in future labs, or can be washed and stored for the next year. The thermometers that are placed in the ice cream during this experiment are never used for non-food measurements. I keep a special set of thermometers for food labs only. My lab
1004
Journal of Chemical Education
assistant controls the stockroom so no one ever gets these thermometers for other uses. If this is a problem, you could also arrange to borrow these special thermometers from the home economics department. While the ice cream is freezing. the student erouos discuss the questions at the end of the ;kperiment an2 prepare one written reoort ner erouo: dissentine reports can be filed should the questions in class the next day to further reinforce the conce-ots. Many teachers pay class visits that day. The school secretaries also come in, as do custodians, administrators, guidance counselors, and other personnel. It is always nice to have them owe a favor. I t also helps create enthusiasm for chemistry. In these days of declining enrollment, recruit! Some Chemistry With the administrative details of the lab solved, the chemistry becomes easy. My chemistry course covers the normal concepts concerning bonding. Students have learned about ionic. covalent. metallic. and van der Wads bonds. Thev ~, have also spent time listing the physical and chemical properties which are to be exoected from each bondine. tvoe. .. Ftrr t l a . icecream lab, heat ~ran.tt,rat111wuer wlulril:tv are the most imutwtanr n m ~ w ~ TIwic t i . iden; are revicwrd i n the questions at'the end of the lab. Questions dealing with each ~~~~~~
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W tr, t h ~ sssbe. lne Secondao, Scnou S#:cl on \ p cased lo re1.m tc I* pngesa co bmn a e m e a l o c*pw mann. norh. Ner ecldrer and lalluridory c*permenls an0 0 recllon\ n 'cam ng cnemrslr, lhrouqn the use of the laboratory will be provided in this feature. Experiments will be fully detailed and will be field-tested before they are published. We are equally pleased to introduce the new editor, Robert Reeves. Reeves did his undergraduate work at Pomona College and has a masters degree in chemistry from Wesleyan University (Connecticut)and one in natural sciences-from Renssalaer where he participated old NSF-sponsored summer programs. He has taught chemistry and other physical sciences for 20 years. Most of his teaching has been in the northeast, but last year, hemaved to Los Angeles and the Marlborough School, a girls' independent school. There he divides his time among his family, his chemistry students, and the Marlborough Computer Center. Contributions for this column should be sent directly to the feature editor.
Mickey Sarquis, Editor Secondary School Section
part of the ice cream freezer give students some insight into why each bonding type is useful and needed. The properties of solutions are also common topics covered in high school chemistry. Due t o time limitations, I do not oresent a lab on freezing point depression or spend time growing crystals in class. ~ " t t i n gan expanded polystyene cup of ice on the teacher's desk d u r i n this ~ lhb allows students t o compare their brine temperatur& t o the ice water temperatures. The only difference is the salt. Thus, students can ohserve t h a t impurities do affect the freezing point of water. A simple model of a n ice cream freezer can he made by nuttine some ice cream mixture in a test tube. T h e test tnhe can b e i u t in a n expanded polystyrene cup. The space around the test tube can be filled with ice and salt. The resultine" mixture freezes more slowly than a stirred mixture. There is some separation, showing that the ice cream is not a solution. T h e water in this test tnhe forms larger ice crystals than form in a stirred ice cream mixture. Some butterfat is removed from whole milk when it is processed for sale in stores. However, I use whole milk, which is available in our farming community, for this experiment because the butterfat acts as an emulsifying agent to limit the growth of ice crystals. The whole milk also affects the taste of the ice cream. Although not a part of the usual chemistry course, emulsifying agents become a part of the post-lab discussion. Many of my teaching examples relate to the home, and emulsifying agents are common items in cooking and cleaning. The Experiment
A common salid that most people enjoy is ice cream. Many solids have a crystalline structure. The texture of a product depends on the size of the crystalline material that makes it up. (If the crystals are very large, the solids are stiff. Smallcrystals will make amore flexible solid.) Most people want a flexible or creamy salid when making ice cream and thus make all efforts to keep the crystal size very small. Ingredients 6 eggs
2 cups sugar 2 Tablespoons vanilla 1 can condensed milk 'h gallon fresh milk blend. freeze in the ice cream freezer
Procedure 1) Observe the ice cream freezer. Make a simple sketch of the freezer your group is using, paying special attention to the part designed to hold the ice cream. Label the various parts of the apparatus, i.e., the paddle, ice bucket, and ice cream container. 2) Mix all the ingredients except the milk. 3) Pour the mixed ingredients into the freezer. 4) Add the milk to the freezer, fill to mark on the side of the freezer. 5) Assemble the freezer, turn it on and add ice and salt to the appropriate location. 6) Measure the temperature of the salt brine every 5 min with a -10 to 110°C thermometer. 7) Stop the freezer every 10 min. Remove enough ice so no brine will enter the ice cream. Open the top of the ice cream container and measure the temperature of the ice cream with the -10 to 36O0C thermometer. (Note. this thermometer is used only for food work and is NOT used in regular chemistrv en~eriments.i
has expired. 10) At the end of 40 min eat the ice cream. Note its texture, eonsistancy, and uniformity. All students should observe the ice cream as it is removed from their group's freezer. Any ice cream not served the fimt time should be left in the freezer until needed. The
Questions 1) Is ice cream a solution? What solution is formed? Identify the solvent and solute and describe the type of bonding involved. 2) List specific physical changes that are taking place within the freezer and the ice bucket. 3) Why must the ice cream be cooled below O0C before it freezes? 4) Why must salt be added to the ice mixture to make ice cream? 5 ) Does the quantity of salt added to the ice matter and why? 6) Why is there undissolved salt in the brine bucket? 7) Describe the desired heat transfer in the system. 8) Describe the dissolving process far making the brine. 9) What is the purpose of each part sketched in procedure #I? 10) Why is each part of the system made with the material it is? Answer in terms of the bonds involved. 11) What is done to insure the formation of smallcrystals during this process? 12) What type(s) of intermolecular bonds are found in ice cream?
Volume 60
Number 11
November 1983
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