JCE Classroom Activity: #65
Instructor Information
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Calories—Who’s Counting? In this Activity, students determine how many calories are released per gram when marshmallows and cashews burn and then compare the quantity of energy available from carbohydrates versus fats. Students burn the food items beneath a metal soft drink can containing water and measure the resulting change in temperature of the water.
For every 14 kcal per day that an individual reduces food intake, that person’s weight is reduced by one lb; for a majority of the population, each pound lost increases life expectancy by 37 days. Thus, each kcal/day of reduced food intake extends life expectancy by about 2.5 days (1). More energy is available from fats than carbohydrates because carbon in carbohydrates is already partially oxidized. The carbon in fats is largely unoxidized. Oxidation takes place as fats and carbohydrates are converted to carbon dioxide and water. A fat also contains more carbon atoms per gram; therefore combustion of a gram of fat releases more than twice as much energy as a gram of carbohydrate. Generally fats provide about 9 Calories (9 kcal) per gram, while carbohydrates provide 4 Calories (4 kcal) per gram.
Integrating the Activity into Your Curriculum This Activity can be used to introduce calorimetry, or as an investigation of types of food molecules, and it ties in with an article published in this Journal (2). Students use the nutritional label and item price to calculate the dollar per ounce of protein, fat, and carbohydrate found in a food, as well as calories per dollar; then they make value judgments regarding nutrient and food choices. Similar experiments contain a possible extension demonstration (3) and additional hands-on experiments (4).
About the Activity
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The Activity uses a nonflammable device to hold burning food samples. Since there are multiple ways to construct a device, students are not given explicit directions for this. The design shown here is one suggestion. Begin with a base such as an inverted jar lid to collect ash as the food burns. Unbend a portion of a paperclip. Secure the portion of the paperclip that remains in its original bent conformation to the base using glue (a hot glue gun works well) or modeling clay. Push food items onto the upright portion of the clip. Since the clip point is not sharp, cashews occasionally break: push slowly and ease the nut onto the point. Cashews take longer to light, but burn steadily. Marshmallows are easier to light, but tend to need relighting to burn completely. Extra soft drink cans should be available since the tabs used to suspend the cans occasionally break. The amount of water (50 mL) used is for a half of a cashew. If students use an entire cashew, the water is likely to boil and they will have to take the heat of vaporization of water into account. Sample nutritional labels for cashews and marshmallows as well as a sample data table and calculations are available in this issue of JCE Online.W Calculations should show that the number of calories released by burning a gram of cashews is much larger than for marshmallows; marshmallows contain mainly carbohydrates, while cashews contain a large fraction of fat. Students can use a reference for information on the structure and metabolism of carbohydrates and fats. The combustion energy calculated from Activity data will be lower than the actual combustion energy of the food, mainly because the experimental setup allows for energy transfer to more than just the water.
Answers to Questions 1. A chemical reaction occurred. Food combines with oxygen to release carbon dioxide gas and water vapor. 2. The temperature of the water increases because it absorbs energy released when the food burns. Energy was transferred from the flame through the can to the water. 3. Answers will vary. A possible answer: add a shield around the burning item and the bottom of the soft drink can. 4. If the thermometer touches the can, the can temperature is being measured rather than the water temperature. If the water is not stirred periodically, the water temperature measured may not represent the overall water temperature; some regions of the water may heat faster than others. 5. The cashew releases more cal/g than the marshmallow because marshmallows contain mainly carbohydrates, while cashews contain a large amount of fat. When they burn, fats release more energy per gram than carbohydrates (see explanation in Background above).
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. Cohen, Bernard L.; Schilken, Catherine A. Calorie Content of Foods: A Laboratory Experiment Introducing Measuring by Calorimeter. J. Chem. Educ. 1994, 71, 342–345. 2. Denio, A. A.; Bennett, C. R. Nutrition and Problem Solving: Food for Thought. J. Chem. Educ. 1984, 61, 1076–1077. 3. Sullivan, Dan M. The Howling Gummy Bear. J. Chem. Educ. 1992, 69, 326. 4. Steitberger, H. Eric. How Bright Are You? Energy and Power Hands-On Activities. J. Chem. Educ. 1992, 69, 307–308. JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 81 No. 10 October 2004 •
Journal of Chemical Education
1440A
JCE Classroom Activity: #65
Student Activity
Calories—Who’s Counting? People must consume food to survive. Living cells use potential energy stored in food molecules and oxygen molecules to carry out the chemical processes that support life. Energy is sometimes measured in calories (cal). One calorie is defined as 4.184 joules, approximately the quantity of energy required to increase the temperature of 1 mL of pure water by 1 ⬚C. This calorie is not the same as the “food calorie” on nutritional labels. This unit, a Calorie, is equivalent to 1,000 calories or 1 kilocalorie (kcal). Not all types of food release the same quantity of energy in calories when they burn or are metabolized. Whether the food is fat, protein, or carbohydrate makes a difference in calorie counting. In this Activity, you will determine the quantity of energy in two different foods by measuring the increase in temperature of a volume of water heated by a burning sample of each food, and then compare the energy released.
Try This You will need: an empty, clean metal soft drink can with an intact pull tab; cold water; glass stirring rod; ring stand with ring; cashews; miniature marshmallows; wooden splint or lighter; nonflammable holder to hold the burning food; ruler; balance; small container; and thermometer. __1. Design a data table to record mass of water, initial and final water temperatures, change in water temperature, mass of holder, and initial and final mass of sample and holder combined. You will conduct two trials each for a marshmallow and cashew. __2. Using a small container, weigh ∼50 g of cold water. Pour the water into an empty, clean metal soft drink can. __3. Set up the apparatus shown in the drawings. Put a glass stirring rod through the pull tab of the can from step 2. Rest the ends of the rod on the ring of a ring stand to suspend the can by the tab from the ring. Measure and record the initial temperature of the water. The thermometer should not touch the bottom of the can. __4. Measure and record the mass of a nonflammable holder to hold the food samples. Place a miniature marshmallow or half a cashew onto the holder. Measure and record the initial mass of the sample and holder combined. __5. Place the sample with its holder under the can. Adjust the height of the ring on the ring stand so the sample is ∼2 cm from the bottom of the suspended can. __6. Remove the sample and holder from underneath the can. Light the sample with a burn- Be Safe! Work under a fume hood, in a well-ventilated ing wooden splint or lighter. Once it begins to burn, push the sample and holder underarea, or outside. Work over neath the can. (This prevents the water from being heated by the splint or lighter flame.) If the sample stops burning before it has burned completely, pull the sample and holder a nonflammable surface. out, relight the sample, and push the burning sample and holder back. Have a fire extinguisher available. __7. Use a thermometer to stir the water gently as the food burns. Record the highest temperature reached by the water. The thermometer should not touch the bottom of the can. __8. After the sample is nearly or completely burned, measure and record the final mass of the sample and holder. Empty the water from the can, and wash and dry the outside. Repeat steps 2–8 for the next sample. Conduct two trials for each type of food item. __9. Calculate the energy transferred to the water. Use q = Cpm⌬T, where q is energy (cal), Cp is the specific heat capacity of water (1 cal/g ⬚C), m is mass of water (g), ⌬T is change in water temperature (⬚C). Record q for each trial. Assume that all of the energy released by the burning food goes into heating the water. _10. Calculate the quantity of energy released in cal/g, keeping in mind the amount of sample that was actually burned. Compare the values for cashews and marshmallows. Compare the values to those on the food package.
Questions 1. 2. 3. 4.
Why did the mass of the food samples decrease? Where did this mass go? Why did the temperature of the water increase? How was energy transferred from the food to the water? Explain one way to alter the setup to assure that most of the energy from the burning food heats the water. Why is it important to keep the thermometer from touching the can while measuring the temperature? Why should the water be stirred while measuring the temperature? 5. Which food released more energy in cal/g? Why did that type of food release more energy than the other?
Information from the World Wide Web (accessed Aug 2004) Learning about Food Package Labels. http://www.askdrsears.com/html/4/T042300.asp This Classroom Activity may be reproduced for use in the subscriber’s classroom.
1440B
Journal of Chemical Education •
Vol. 81 No. 10 October 2004 •
www.JCE.DivCHED.org
