Chemistry for Everyone edited by
View from My Classroom
David L. Byrum Flowing Wells High School Tuscon, AZ 85705-3099
A Spoonful of C12H22O11 Makes the Chemistry Go Down: Candy Motivations in the High School Chemistry Classroom Fanny K. Ennever Department of Physical Science, Bronx High School of Science, 75 West 205th Street, Bronx, NY 10468;
[email protected] “Your students have just come from Spanish and math, and are about to go to gym and history. Just saying ‘let’s continue where we left off yesterday’ won’t grab their attention for chemistry,” as my first principal said. Ideally, every lesson should start with an intriguing observation that needs explanation. Mixing reagents in beakers can be amazing, but so can objects from everyday life, like candy, which is inexpensive, safe, and welcomed by the students, particularly around lunchtime. Published articles have described a number of hands-on activities with candies or other food suitable for high school classes or lab sessions (1–6). This article describes additional “food” motivations that I use in a high school class, some of which may take only a few minutes to elicit an appropriate question that can start the inquiry for the remainder of the lesson. [I also use candy as a reward for doing homework: every Monday, any student who did all the homework in the previous weeks gets a piece of candy. I have not measured how much difference this reward makes, in addition to a grade penalty for not doing the homework. Interestingly, high doses of glucose have been found to improve the memory (7).] Hygienic precautions should be followed, particularly noting any allergies at the beginning of the year and ensuring no one eats food touched by anyone else unless wrapped. Food Motivations
Measurement Groups of three students are given a roll of Fruit-bythe-Foot (about three-feet long) and asked to divide it into three parts. This motivates a discussion on accuracy (how much do the three pieces differ) and precision (is any piece 12-inches long).
Chemical Names A volunteer reads the ingredients in a packaged food— the food must be carefully selected so the ingredients contain chemical names. Students ask some variation on “what is sodium bicarbonate”, which prompts a discussion of naming conventions, including that the accepted name for sodium bicarbonate is now sodium hydrogen carbonate.
Stoichiometry I tell the class that a chemist’s recipe for apple pie uses 100 grams of sugar per apple and a mole ratio of salt to sugar
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of 1:20. (The students generally recognize that even chemists do not actually cook this way.) I ask how much salt I should put in a pie made with 7 apples. We work out the answer: 700 g C12H22O11 [MW = 12(12.01) + 22(1.008) + 11(16.00) = 342.30 g mol᎑1] is 2.05 mol sugar, 1兾20 of that is 0.102 mol NaCl (MW = 58.5 g mol᎑1), which is 5.98 g of salt; since 1 teaspoon of salt weighs ∼6 g, this is about 1 teaspoon of salt.
Distillation Simply having students taste distilled water versus tap water makes them ask what the difference is (dissolved salts and chlorine in tap water).
Periodic Table Students taste potassium chloride (about half will find it disgusting) and ask why it is used as a salt substitute. This emphasizes the similarity of properties among groups. Once a student said, “Hey, I can taste the extra electrons!”
Polarity Students each get one large marshmallow and four small marshmallows on toothpicks and are instructed to stick the toothpicks into the large marshmallow so the small marshmallows are as far apart as possible. With prodding, they create a tetrahedral structure: methane (or carbon tetra-fluoride, -chloride, -bromide, or -iodide). This can be expanded by removing small marshmallows (but not the toothpicks, which represent nonbonding electron pairs) one at a time to show ammonia, water, and hydrogen fluoride and investigate their polarity. Attractions Between Nonpolar Molecules Students get a Nik-L-Nip wax bottle (other items made of wax are just as suitable) and are asked why wax is good for this packaging (soft, low melting, etc.).
Effect of Pressure on Boiling Point A student is given a cake box and asked to read the highaltitude directions. Why do cakes cook differently at high altitude? (Lower pressure means the temperature is lower at the boiling point of water, so the cake has to cook longer and more water is needed. An additional effect of the lower pressure is to make the bubbles of CO2 from the baking powder larger so additional flour is needed to add gluten to make the bubble walls stronger.)
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Chemistry for Everyone
Rate of Solution Students are given a piece of hard candy and asked how to keep it from dissolving: do not bite it (surface area); do not suck it (stirring); hold it in your teeth (lower temperature, less contact with solute). Catalysis Students are given Junior Mints and asked how they get the semi-solid center. The ingredients list catalase, an enzyme that converts sucrose into a glucose兾fructose mixture (as bees do). In the factory, the center is hard like crystallized sucrose, but the enzyme turns it into a honey-like, gooey mixture over a period of a few days. Entropy Two students come to the front of the class. Each is given a bag of colored candy (Skittles and M&Ms work well, as would many others). One is normal, the other has been divided into same-color smaller plastic bags. Each gets a jar and puts the candy in the jar, in careful layers in the case of the sorted bags. The one with the normal candy is asked to shake her jar until it looks like the layered one. This sparks a discussion of how random motion cannot create order. Someone suggests instead shaking the layered one, which of course does make the two jars the same. Acids Students are given a choice of WarHeads or hard candy. Both contain organic acids, but the WarHeads have them concentrated on the surface. Some students will not eat WarHeads, and asking why induces the students to summarize the properties of acids they have learned in previous science courses. Redox 1 Students are shown a piece of bauxite (aluminum ore) and several Hershey’s kisses in their wrappers and are asked which “form” of aluminum is more useful. This introduces the historical importance of metals, giving rise to the names of two ages (Bronze and Iron) and to the literal meaning of reduction—the mass of metal is reduced compared to the mass of ore. Redox 2 Students compare commercial apple slices to an apple sliced a few hours ago. They look at the citric acid in the
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ingredients to explain why the commercial slices are not brown (citric acid is an antioxidant that prevents oxygen from reacting with the compounds in apples that “rust”).
Organic Compounds Students are given Jelly Belly jelly beans and are prompted to wonder where all the flavors come from (they are all organic compounds, illustrating the variety of compounds carbon can form; many are esters).
Saturated Organic Compounds Students read the nutrition label on food (Fritos, Oreos, etc.) and ask “What are saturated, unsaturated, and trans fats?” (Saturated and unsaturated are terms in a typical high school curriculum; trans is not but is often of considerable interest to students.) As of now, “hydrogenated” can be found in the ingredients, which can be explained by an addition reaction, but this may change with the reformulation to remove trans fats. Safety Be careful when bringing food into the classroom. Be sure that candy is not consumed after touching hazardous materials or at the laboratory bench. Conclusion I hope that these “candy” motivations at a minimum make students encounter something sweet that, after some thought, can be explained by chemistry. It may also connect chemical concepts to their observable world and possibly give them the habit of noticing things to ask questions about. Literature Cited 1. Jacobsen, E. K.; Maynard, J. J. Chem. Educ. 2004, 81, 1544. 2. Jennings, L. D.; Keller, S. W. J. Chem. Educ. 2005, 82, 549– 550. 3. Levine, E. H. Sci. Teacher 1996, 63, 18–21. 4. Mabrouk, S. T. J. Chem. Educ. 2003, 80, 894–898. 5. Merlo, C.; Turner, K. E. J. Chem. Educ. 1993, 70, 453. 6. Ross, M. R. J. Chem. Educ. 2000, 77, 1015. 7. Messier, C. Euro. J. Pharmacol. 2004, 490, 33–57.
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