During this investigation, students will determine the calories—or heat content—of 3 different foods. From the experiment setup and data collected, students will have the evidence necessary to construct a model of heat transferred through the reaction of food with oxygen. Students will then apply their model to cellular respiration.
As a teacher demonstration, place a marshmallow on a paper clip and burn it until only ash remains.
Ask students what has changed and why.
PE HS-LS1-7. Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed resulting in a net transfer of energy.
Use safety glasses or goggles and be cautious with the matches and burning food samples. Check for food allergies before using food samples. Sensitive individuals should not participate in any activities that may result in exposure. Never eat or drink in lab.
Remind students to dump the water out of the can before recycling it. All food, ash, and scraps can be rolled up in the aluminum foil and disposed of in the trash.
To reduce student setup time, put 2 rings on each support stand. Place a smaller ring (to suspend the thermometer) above a larger ring from which to suspend the soda can.
Note: Students may not develop identical models. The task is to use common characteristics among the fruit to develop a classification model. At the close of the activity, discuss the different student models and compare them to the included partial key. Emphasize the differences between a classification model and a dichotomous key.
Student answers will vary in mass, and the final temperature of the water will vary with the type of food burned.
Note: This is an incomplete key. Not all classifications of fruit are represented with this sample.
1. Determine the mass of food that actually burned. (Initial Mass of Food Sample and Paper Clip – Final Mass of Food Sample and Paper Clip After Burning)
2. Determine the change in temperature of water, ∆T.
3. Calculate the energy (in calories) released by the burning food sample and absorbed by the water.
Q = mCpΔT
Q = heat absorbed by water, m = mass of water in grams, Cp = 1 cal/g °C, ∆T = change in temperature
Q = 50 g × 1 cal/g °C × 33 °C = 1650 cal
Compare your calculated calories to the food nutrition label. Describe any differences.
Student answer should be much higher because calories, NOT kilocalories, are calculated.
4. Food Calories, as read off a nutrition label, are actually kilocalories (often denoted as “Calories” with a capital C). There are 1,000 calories in a kilocalorie, or food Calorie. Determine the number of kilocalories (food Calories) released by the burning food sample (1 kilocalorie, or Calorie = 1,000 calories).
1650 cal × 1 kilocal/1000 cal = 1.65 kcal
5. Calculate the energy content of the food in kilocalories/gram.
1.65 kcal/1.5 g = 1.1 kcal/g
6. Using information on the nutrition label of the food sample, calculate the food manufacturer’s kilocalories/gram. (Divide calories per serving by the number of grams in a serving.)
90 cal/38 g = 2.37 kilocal/gram
7. Compare your experimentally determined energy content (in kilocalories/gram) to the calculated value from the nutrition label. Calculate the percent error for your experiment.
(2.37 kcal/ gram – 1.1 kcal/gram) / 2.37 kcal/gram = 0.54 × 100 = 54% Sources of error may include heat lost to the can and to the air. Some of the heat was transferred to the can to warm it up, and some may have been transferred to the air between the food and can.
8. Draw and label a model of energy transfers that take place during this activity. Be as detailed as possible.
Student models can vary but should include these energy transfers: photosynthesis stored chemical energy in plant sugars → plant sugars burned/oxidized (chemical energy is changed to thermal energy as bonds are broken and then reformed in products) → thermal energy transferred to can and water in the can through convection.
9. Explain how the calorimetry model compares to what happens in a cell.
Cells “burn” or oxidize food on a smaller level during respiration. Food is broken down through digestion and sugar molecules are broken down into usable chemical energy and thermal energy. As bonds are broken in the sugar molecules and reformed in products, energy is released in the form of heat.
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