Ice cream delicious chemistry

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					            Ice Cream: Delicious Chemistry                              assistant controls the stockroom so no one ever gets these
                                                                        thermometers for other uses. If this is a problem, you could
James Martino                                                           also arrange to borrow these special thermometers from the
4713 Booth Rd.                                                          home economics department.
Oxford, OH 45056
                                                                           While the ice cream is freezing. the student erouos discuss
  Chemistry labs and textbooks usually deal with theory and             the questions at the end of the ;kperiment an2 prepare one
seldom let the student see chemistry as a real-life subject.            written reoort ner erouo: dissentine reports can be filed should
Chemical bonding, molecular structure, and colligative
properties can be especially dry. The interest and excit&nent
generated bv the experiment described below help to over-               the questions in class the next day to further reinforce the
come these problem8. I assign the experiment after units on             conce-ots.
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          Many teachers pay class visits that day. The school secretaries
we have developed. I t is time for the students to realize why          also come in, as do custodians, administrators, guidance
they study these topics!                                                counselors, and other personnel. It is always nice to have them
                                                                        owe a favor. I t also helps create enthusiasm for chemistry. In
Administrative Details                                                  these days of declining enrollment, recruit!
   Ice cream freezers are not standard lab equipment, but they
can be borrowed from the students' parents. One-gallon,                 Some Chemistry
                                                            . -
motor-driven freezers work best. I divide mv class into croups            With the administrative details of the lab solved, the
of six, each group with a freezer, and appoint a leader who is          chemistry becomes easy. My chemistry course covers the
resoonsihle for all of the secretarial and administrative work          normal concepts concerning bonding. Students have learned
forthe group.                                                           about ionic. covalent. metallic. and van der Wads bonds. Thev
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   A lab oeriod of 55 min is enouah for conducting the exper-           have also spent time listing the physical and chemical prop-
imenr ii it I.; urrr~t~izcd 'l'hr inyrtdtenti ;Itt,uld he krl~t
                           well.                                        erties which are to be exoected from each bondine tvoe.
                                                                        ~   ~~
                                                                                                                               . ..
in u rt.trtrcratur until nel.dt 11. Sludenta >huuld 113e~ ~ l e n01 v
                                                                  t        Ftrr t l a . icecream lab, heat,rat111wuer wlulril:tv are
salt on thk ice to speed cooling. If the lab is done in co&ec&ve                                            ti.
                                                                        the most imutwtanr n m ~ w ~ TIwic iden; are revicwrd i n the
~ e r i o d sthe brine can be saved and used in later periods to
             .                                                          questions at'the end of the lab. Questions dealing with each
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.
                                                                                W tr, t h ~ sssbe. lne Secondao, Scnou S#:cl on \ p cased lo re1.m
   The ice cream can he frozen in 40 min, if necessary, but it              tc I* pngesa co bmn a e m e a l o c*pw mann. norh. Ner ecldrer and
is better if more time is allowed. I t is especially helpful if one         lalluridory c*permenls an0 0 recllon\ n 'cam ng cnemrslr, lhrouqn
person from each group arrives early and completes the mixing               the use of the laboratory will be provided in this feature. Experiments
step (step 2). If a spare freezer is available, it can be left un-          will be fully detailed and will be field-tested before they are pub-
assembled for students to investigate (step 1) once the freezing            lished.
operation is begun.                                                            We are equally pleased to introduce the new editor, Robert Reeves.
   Prior to the day of the lab, a list should be made of the                Reeves did his undergraduate work at Pomona College and has a masters
materials that will be needed from an outside source. Students              degree in chemistry from Wesleyan University (Connecticut)and one
                                                                            in natural sciences-from Renssalaer where he participated old
are quite helpful in procuring the necessary items. We have
                                                                            NSF-sponsored summer programs. He has taught chemistry and other
found several sources to be esoeciallv helpful. Serving spoons              physical sciences for 20 years. Most of his teaching has been in the
can be rented from the cafeteria or home economicsdepart-                   northeast, but last year, hemaved to Los Angeles and the Marlborough
ment. (Pay for these with ice cream.) Ice cream can be served               School, a girls' independent school. There he divides his time among
in expanded polystyrene cups and eaten with plastic spoons.                 his family, his chemistry students, and the Marlborough Computer
The cuws can he used later in the lab as calorimeters or con-               Center.
tainers. They can even be washed and used again. Plastic                        Contributions for this column should be sent directly to the feature
spoons also come in handy in future labs, or can be washed and              editor.
stored for the next year.                                                                                             Mickey Sarquis, Editor
   The thermometers that are placed in the ice cream during                                                             Secondary School Section
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
part of the ice cream freezer give students some insight into              Procedure
why each bonding type is useful and needed.                                 1) Observe the ice cream freezer. Make a simple sketch of the freezer
   The properties of solutions are also common topics covered                  your group is using, paying special attention to the part designed
in high school chemistry. Due t o time limitations, I do not                   to hold the ice cream. Label the various parts of the apparatus,
oresent a lab on freezing point depression or spend time                       i.e., the paddle, ice bucket, and ice cream container.
growing crystals in class. ~ " t t i n g expanded polystyene cup
                                       an                                   2) Mix all the ingredients except the milk.
of ice on the teacher's desk d u r i n this lhb allows students t o
                                         ~                                  3) Pour the mixed ingredients into the freezer.
compare their brine temperatur& t o the ice water tempera-                  4) Add the milk to the freezer, fill to mark on the side of the
tures. The only difference is the salt. Thus, students can oh-              5) Assemble the freezer, turn it on and add ice and salt to the ap-
serve t h a t impurities do affect the freezing point of water.                propriate location.
   A simple model of a n ice cream freezer can he made by                   6) Measure the temperature of the salt brine every 5 min with a -10
nuttine some ice cream mixture in a test tube. T h e test tnhe                 to 110°C thermometer.
can b e i u t in a n expanded polystyrene cup. The space around             7) Stop the freezer every 10 min. Remove enough ice so no brine will
the test tube can be filled with ice and salt. The resultine      "            enter the ice cream. Open the top of the ice cream container and
mixture freezes more slowly than a stirred mixture. There is                   measure the temperature of the ice cream with the -10 to 36O0C
some separation, showing that the ice cream is not a solution.                 thermometer. (Note. this thermometer is used only for food work
T h e water in this test tnhe forms larger ice crystals than form              and is NOT used in regular chemistrv en~eriments.i
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                 has expired.
is available in our farming community, for this experiment                 10) At the end of 40 min eat the ice cream. Note its texture, eonsist-
because the butterfat acts as an emulsifying agent to limit the                ancy, and uniformity. All students should observe the ice cream
growth of ice crystals. The whole milk also affects the taste of               as it is removed from their group's freezer. Any ice cream not
the ice cream. Although not a part of the usual chemistry                      served the fimt time should be left in the freezer until needed. The
course, emulsifying agents become a part of the post-lab dis-
cussion. Many of my teaching examples relate to the home,
and emulsifying agents are common items in cooking and
cleaning.                                                                  Questions
The Experiment                                                              1) Is ice cream a solution? What solution is formed? Identify the
                                                                                solvent and solute and describe the type of bonding involved.
   A common salid that most people enjoy is ice cream. Many solids          2) List specific physical changes that are taking place within the
have a crystalline structure. The texture of a product depends on the           freezer and the ice bucket.
size of the crystalline material that makes it up. (If the crystals are     3) Why must the ice cream be cooled below O0C before it freezes?
very large, the solids are stiff. Smallcrystals will make amore flexible    4) Why must salt be added to the ice mixture to make ice cream?
solid.) Most people want a flexible or creamy salid when making ice         5 ) Does the quantity of salt added to the ice matter and why?
cream and thus make all efforts to keep the crystal size very small.        6) Why is there undissolved salt in the brine bucket?
                                                                            7) Describe the desired heat transfer in the system.
Ingredients                                                                 8) Describe the dissolving process far making the brine.
  6 eggs                                                                    9) What is the purpose of each part sketched in procedure #I?
  2 cups sugar                                                             10) Why is each part of the system made with the material it is?
  2 Tablespoons vanilla                                                         Answer in terms of the bonds involved.
  1 can condensed milk                                                     11) What is done to insure the formation of smallcrystals during this
  'h gallon fresh milk                                                          process?
blend. freeze in the ice cream freezer                                     12) What type(s) of intermolecular bonds are found in ice cream?

                                                                                 Volume 60       Number 11       November 1983               1005

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