Burning Calories—Literally!

Background Information for Teachers

This section contains a quick review for teachers of the science and concepts covered in this lesson.

With increased public emphasis on lifelong good nutrition habits to combat obesity and related diseases, it can be helpful for students to demystify the nutrition facts label (Figure 1) and the energy or Calorie content of food.

Our bodies need energy to survive and thrive. What we eat is broken down to release its stored energy, which we use to survive, grow and be active; whatever is leftover is stored as fat. Our food comes in many forms, but it basically consists of three main types of nutrients: sugars (also called carbohydrates), proteins, and fats (also called lipids). These complex food molecules store energy in the chemical bonds that hold them together. This type of energy is called chemical energy and can be expressed in Calories (note the capital "C" used to indicate one kilocalorie, or one thousand calories, with a lowercase "c"), the unit of energy used in the United States to quantify food energy. Inside our bodies, ingested food molecules undergo a series of changes—oxidations, which is a type of burning—that break the bonds and slowly release stored energy. In this lesson, you and your students will break down food much more rapidly by burning it in air. You will use a calorimeter to catch and measure the released energy.

A calorimeter, as shown in Figure 2, is an instrument used to measure the amount of heat created by a chemical or physical change. Heat is the type of energy that flows between two bodies due to a difference in temperature. Students will design and build their own calorimeters to measure the heat created by releasing the energy stored in food items. The basic idea is to release all the stored food energy at once and capture all the released heat with a reservoir of water. Measuring the change in temperature of the water will allow you to calculate the energy required to heat up the water. As this energy comes from breaking down food, the calculation will reveal the actual amount of energy provided, or the energy originally stored in the food. However, this is only true if all the energy released by the food is used to heat the water, and none of it gets "lost."

Calorimeters that are used by scientists are made so that practically all of the energy released during the chemical or physical change is captured by the water. The homemade version will not reach the same efficiency in catching the heat released when burning the food; in other words, only a fraction of energy stored in the food will be converted to thermal energy of the water and measured by your calorimeter. For example, some of the energy might go into heating up the surrounding air instead of the water. Capturing even half of the energy released (an efficiency of 0.5 or 50%) is acceptable for a homemade calorimeter, as it is very difficult to transfer all the energy that is released to the water reservoir. Even with low efficiencies, this lesson allows your students to rank different foods based on their Calorie content. With reasonable accuracy, it allows your students to calculate the ratio of Calorie content of different foods.

With the homemade calorimeter, your students will measure the temperature change (ΔT) of a reservoir of water due to the burning of a food item. Multiply this by the mass of the water (m_water) that has been heated, and the specific heat capacity of water c (0.001 Cal/(g °C)) to calculate the amount of energy used to heat up the water (Q_water). This is also shown in Equation 1.

Equation 1:

The following example where we burn 1.1 g of almonds will make the equation clear. We start out with 150 milliliters (mL) of water in the calorimeter. Since 1 mL of water has a mass of exactly 1 g, this water has a mass of 150 g (m_water = 150 g). If initially the temperature of the water is 20.0°C, and after burning the nuts in the calorimeter we measure a water temperature of 33.3°C, then the heat captured by the calorimeter is:

If all the energy released is used to heat up the water, this would be a measure of the energy stored in the nuts we burned. In reality, some energy will get lost so the real energy content of the burned food is likely to be higher.

To compare different foods, we need to calculate the energy released by a standard amount of food. We do this by dividing the calculated energy by the mass difference of the food item before and after burning. This specific energy, or the energy content per unit of mass (Q_{Food, 1 g}), is what allows us to identify Calorie-rich food items. In the above example, (Q_{Almond, 1 g}) equals 2.0 Cal/1.1 g, or 1.81 Cal/g.

Food as a source of energy contains two aspects: a quantitative aspect focusing on the amount of energy in food and how this might vary depending on the type of food and, a qualitative aspect focusing on whether some sources of food energy are healthier than others. This lesson emphasizes the quantitative aspect of food. The Variations section includes information on how you can explore the health and nutritional aspect of food.

Eating a balanced, nourishing diet and getting enough exercise are fundamental to good health. The energy balance between what we take in and what we use is important for health, but it is not the only component of a healthy diet. To adequately nourish our bodies, our diets need to provide items from all the food groups in sufficient amounts. If one needs to cut down on Calories, it is a good idea to start cutting down on processed foods and refined sugars, as these have little nutritional value while still adding to the energy balance.

Additional Background Links

• Food-A-Pedia, USDA
• Calories from Fat, Protein, Carbohydrates, Alcohol, NutriStrategy
• Heat (Thermal Energy), eSchoolToday
• Twenty-Six Calories Can Lift an SUV, NPR
• Calorimeters and Calorimetry, the Physics Classroom
• How to Understand and Use the Nutrition Facts Label, US Food and Drug Administration
• Learn about the Nutrition Facts Label, US Food and Drug Administration
• How Do Food Manufacturers Calculate the Calorie Count of Packaged Foods?, Scientific American

Prep Work (10 minutes)

1. For each food item students will measure:
   1. Fill one transparent container with 100 gram of the food.
   2. Take a picture of the nutrition facts label or bring the original bag/box to class.

Engage (45 minutes)

Introducing the lesson topic

In this lesson, we will examine food as a source of energy. How much energy is in food? Do all types of food have the same amount of energy?

Evaluating prior knowledge / providing background information

Divide the class into groups of 2 to 4 students, and present them with the following task.

In your groups, discuss the following scenario. You just finished some vigorous exercise. You don't feel that hungry yet, but you know you need energy. You want to get as much energy as possible from a small amount of food. Which of these foods would you choose and why? Use data provided on the food labels to support your reasoning. Note the question focuses on the energy food provides. Personal tastes or preferences are not an argument here.

Provide the nutrition facts labels of peanuts, a protein bar, raisins, bread, and apple sauce to the students.

You calculate the Calorie content per gram food by dividing the Calorie content per serving by the serving size in grams (see Table 1). For this list, peanuts deliver the most energy per gram consumed; it is the most energy dense food item.

This activity leads to the following facts:

• The Calorie is a unit of energy mainly used to indicate food energy.
• Nutrition facts labels contain information on:
  • Serving size, or the amount of food customarily eaten in one sitting; and
  • Calorie content per serving, or the energy contained in one serving.
• Serving sizes differ for different foods.
• When evaluating how much energy you consume, you need to consider both the amount of food consumed (number of servings or the mass or volume consumed) and the energy per serving, unit mass or volume.
• The energy source containing the most Calories per gram (or fixed amount) of food is the most energy dense food item; it delivers the most energy for a fixed mass of food consumed.
• Dividing the Calorie content per serving by the serving size (expressed in grams) provides the energy content (or Calories) per gram of food.

Introducing the activity

Scientists use a special instrument called a calorimeter to measure the energy stored in food. In this lesson, you will design, build, and test such a device.

Explore (150 minutes)

Designing a calorimeter

Explain to the students that, in their small groups, they will design a calorimeter or an instrument to measure the energy content of food items.

Have the following items ready for each group:

• Two nesting cans (larger can has both ends removed and holes near one end, smaller one has 2 holes opposite from each other near the top)
• Metal wire
• Pie pan
• Cork
• Needles (3)
• Dowel
• Long matches, a Bunsen burner, or a multipurpose lighter
• Thermometer to measure temperature of a liquid
• Scale
• Safety glasses
• Oven mitt
• Water
• Graduated cylinder, 250 mL
• Scissors
• Knife
• Optional: Material to prop up the can and leave an opening for air flow
• Optional: Aluminum foil

In your groups, plan how to build a calorimeter out of the provided materials.

• Brainstorm ideas about how you can burn the food to release its energy, how you will capture the released energy with a volume of water, what you will measure, and how this can give you a measure of the food energy.
• Narrow the ideas down to something that is feasible.
• Make a sketch on your worksheet of what the instrument will look like.
• Write a brief description on your worksheet of how and why this instrument will work.

A group discussion should result in the following requirements for a working calorimeter:

• It allows the food to burn as completely as possible, so all its energy is transformed into heat.
• It captures as much of the released energy as possible in a water reservoir.
• You can measure the change in temperature of the water reservoir as a measure of how much energy was put into heating up the water.
• Food items are safely accessible before and after the burning.

The following questions can further evaluate the models:

• Will most of the energy released by burning the food be captured? Where else could the energy go?
• Will the food keep burning in the calorimeter? Fire needs air in addition to fuel.

A group discussion helps explain why some models might work better than others. Table 1 lists some effective and less effective design choices.

Let each group evaluate their model. Select a model to build as class, or select the model described in the Calorimeter assembling instructions document. Let students sketch the selected model on their worksheet.

If your students are not yet familiar with the notions of dependent, independent, and control variables, it might be better to change this group work into a class discussion.

Go back to your groups to work on the following assignments:

• State the dependent, independent and control variables of the calorimeter.
• How can we make sure different groups in our class will be able to compare their measured temperature changes? (Hint: look back at the control variables.)
• The calorimeter can only function because energy changes from one form to another. Add arrows to the calorimeter sketch visualizing how energy flows through the instrument.
• Label the different types of energy on your sketch.

A temperature change of the water in the calorimeter is a measure of its total thermal energy change. This is because thermal energy is the internal energy due to temperature. It depends on the temperature, the amount and the state of the substance. As the amount and state of the water stay constant, a thermal energy change of the water induces a temperature change. The added thermal energy came from the energy released by burning the food, which equals the energy stored in the food. As a result, the temperature change is a measure of the energy stored in food.

The following example may help explain to students how the volume of water in the reservoir is a variable that needs to be controlled. Imagine you have two identical pots, one filled up to the brim and the other containing barely any water. Then, allow the same amount of heat to flow to the pots. Will the total change in thermal energy of the water in both pots be identical? Yes, as you allowed the same amount of heat to flow to the pots, so you added the same amount of energy. Is the effect of this change on the temperature of the water in both pots identical? No, the same amount of energy used to heat up less water will make its temperature rise more. The amount of water plays a role.

If groups want to compare their measured temperature changes, their calorimeters will need to have similar values for the control variables. The following measures help ensure this:

• Use the same model or design so the air flow and energy lost to the environment are as similar as possible.
• Use the same amount of water in the reservoir.
• Burn the same amount of food or normalize the findings to 1 g of food. To explain why, you can refer to the section on comparing food energy from the information contained on food labels. There, students also had to correct for different serving sizes before evaluating which food was most densely packed with energy.

Figure 3 shows an example of how students can use arrows to indicate how energy flows through the instrument. Scientists call this an energy flow diagram. Remind students that internal energy due to temperature is called thermal energy. If thermal energy has been studied in prior lessons, this is a good point to connect to that knowledge. Heat is the energy transferred due to temperature differences.

Students should understand that the energy flow in a calorimeter has three components:

• The food contains stored chemical energy.
• This energy is released in the form of heat, which spontaneously flows to colder objects (the water reservoir).
• As a result, the thermal energy content of the water increases, which is observed as an increase in temperature (or the average kinetic energy of the water particles).

Building a Calorimeter

Decide on a single calorimeter design that every group will build—either one designed by the class or use the design from the Calorimeter assembling instructions document. Make sure to distribute a clear diagram and directions explaining how to build the chosen calorimeter to all groups.

In your groups, build the calorimeter.

Note that for this design, you remove the dowel with the small can and the outer insulating can to gain access to the food items, so you can pierce them on the needles, ignite them. You place the outer insulating can and the dowel with the small can back in place as soon as the food catches on fire. You remove the dowel with the small can and the outer insulating can again to gain access to the remains after burning.

Taking Measurements

Each group receives samples of each food item. Each group will perform at least one measurement (trial) for each food item.

1. Go over the safety rules for using an open flame:
   1. Work on a non-flammable surface.
   2. Long hair needs to be tied back.
   3. All students must wear safety glasses.
   4. Always use oven mitts to handle items that may be hot.
2. Perform a trial:
   1. Select a few pieces of the same food item; using a few pieces (not just one) allows you to burn a larger mass.
   2. Weigh the food pieces to be burned and record the initial total mass in the column "Food: M_i" on the worksheet.
   3. Put the pieces of food in place. Make sure all the pieces touch. This will allow the flame to go from one piece to the next.
   4. Place your thermometer in the small can, so it can adjust its temperature to the water inside.
   5. Light the food with the long matches, Bunsen burner, or lighter. See the troubleshooting section if you have trouble igniting the food.
   6. Immediately stir the water in the small can and read the initial temperature (T_i). Record this temperature in the column "Water: T_i" on the worksheet. Remove the thermometer from the can as the metal might get hot.
   7. Allow the food to burn itself out. If smoke is coming out, the burning is still in progress.
   8. As soon as the food stops burning, carefully stir the water and measure the final temperature (T_f). Make sure the thermometer has reached a steady level before reading the temperature. Record the value on the worksheet.
   9. Weigh the remains of any food that was not completely burned (not including ashes). Record the mass in the column "Food: M_f". If all the food burned up, record a final mass "Food: M_f" of 0 g.
   10. This completes one trial for this food item.
3. While students wait for the water to cool, they can calculate value of the change in mass (M_i -M_f) and change in temperature (T_f-T_i) and write these values down on the worksheet.
4. Repeat steps 2–3 for the other food items provided.
5. Clean up the workspace:
   1. Discard all used food and water. Run matches under water before putting them in the trash.
   2. Neatly dry off all parts used in the calorimeter with a cloth or paper towels. These parts can be reused.
   3. Wipe off your bench.

Calculating Results

In your groups, calculate the energy released by the food items.

1. Provide the formulas needed and challenge students to use these and what they know about how a calorimeter works to calculate the energy released by 1 g of food. Note the formulas are also contained in the slideshow and the worksheet.

   Students fill in their results in the table provided on the worksheet. Formulas: $$Q_{water} = m_{water} \:c \left(T_{f} - T_{i} \right)$$ Where: Qwater is the heat captured by the water when heated, expressed in Cal mwater is the mass of the water that is heated, expressed in g c is the specific heat capacity of water, which is 0.001 Cal/(g °C) (Tf - Ti) is the change in temperature of the water expressed in °C QFood is the energy released when burning the food, expressed in Cal $$Q_{water} = Q_{Food}$$ Where: Qwater is the heat captured by the water when heated, expressed in Cal QFood is the energy released when burning the food, expressed in Cal $$Q_{Food,\: 1g} = \frac{Q_{Food}}{M_f - M_i}$$ Where: QFood, 1g is the energy released when burning 1 gram of food, expressed in Cal/g QFood is the energy released when burning a piece of food, expressed in Cal (Mf - Mi) is the mass of food burned, expressed in g

   Hints on how to help groups:

   • Discuss how a calorimeter can measure the energy released by the burned food and show how this is reflected in the formulas:
     • You burn food to release its energy (Q_Food)
     • The released energy heats up the water (Q_Food=Q_water)
     • The temperature change is an indication of the amount of energy used to heat up the water (Q_water = m_water c(T_f - T_i))
   • Ask the students to indicate in the formulas what they know because they measured it (m_water and (T_f - T_i)) and what is given (c, the specific heat capacity of water). Challenge them to connect these formulas so they can calculate the total energy released by the food.
   • Remind the students that the energy (Q_Food) reflects energy released by the total amount of food burned, or (M_i - M_f) grams of food burned. Refer back to how food labels contain information per serving size, and ask what was necessary before they could compare different food items.
2. Facilitate data exchange between groups. An empty data table is included in the worksheet. Let the students calculate the average Calorie content over all groups for each food item. Tell the class that scientists refer to the energy per gram as the specific energy.

   Mention that the SI (International System of Units) unit for energy is joules (J), but other units are frequently used as well, and can sometimes be more practical. For example, one small apple contains about 200,000 J and an average person consumes about 10 million J a day. Expressed in Calories (Cal; note the capital C), a small apple contains about 50 Cal and on average, people consume about 2350 Cal a day. Unfortunately, there is also the calorie (cal; note the small c), which is also an energy unit, but it is not identical to the Calorie. It is mainly used to express chemical energy and is 1000 times smaller than the Calorie: 1 Cal = 1000 cal = 1 Kilocalorie (Kcal).

Reflect (40 minutes)

Energy Content of Food

Provide each group with the nutrition fact label (or a picture of it) of one of the food items. Each food item used in the measurements should be covered by at least one group.

In your groups, calculate energy content per gram using the information listed on the food labels for one food item. Describe how this value compares to the experimentally determined value and state why they are/are not identical. Assume the values on the labels are accurate. State how you think you could improve your calorimeter or your measurement method.

Regroup the class, let the groups exchange information and add the calculated Calorie content per gram to their bar graph. An example is given in Figure 6.

Note that the Variations section provides information on how to calculate the efficiency of the calorimeter or normalize the measured values to get a better idea of what the measurements mean.

Assess

You can use this quiz to assess student learning after the activity:

• Online quiz, assignable in any LMS
• Quiz (pdf) and answer key (pdf)

You can use this worksheet to assess student learning during the activity:

• Student worksheet (pdf) and student worksheet answer key (pdf)
• Rubrics for steps 18 and 19 of the worksheet are added to the student worksheet answer key (pdf)

Lesson Plan Variations

• This lesson concentrates on the amount of energy contained in food. You might like your students to think about whether all sources of food energy are equally healthy. Consider following questions:
  • Why do you think some foods contain more energy per gram than others?
  • Do you believe that energy-rich foods are unhealthy, healthy, or sometimes healthy and sometimes unhealthy? Why?
  • What other factors would you consider when selecting healthy foods, and why do you consider these?

  Students could use the data collected in this lesson to support their answers. A discussion could provide evidence for the fact that while each Calorie provides the same energy, its health impact can vary. The nutrition facts label can help point out which factors to consider when selecting healthy foods. The following reference provided by the US Food and Drug Administration can help you understand and use nutrition facts labels .

• In a homemade calorimeter, only part of the energy contained in the food and released during burning transfers into thermal energy of the water. Because of this, the measured values only reflect a fraction of the energy contained in the food. Some of the energy will get lost to, for example, heating up the surrounding air and the aluminum cans. Students can calculate the efficiency of their homemade calorimeter by comparing the measured values to listed energy content calculated from the information found on food labels.
  $$Efficiency = \frac{Q_{Food, 1g,\: Measured}}{Q_{Food,1g,\: from\: label }}$$
  Where Q_{Food, 1 g, from label} is obtained by dividing the energy content of 1 serving by the mass of 1 serving.
• Challenge your students to improve the efficiency of the calorimeter. An example could be tenting the large can with aluminum foil, leaving a small opening at the top to allow air circulation.
• Students can normalize the Calorie content of food items to the Calorie content of one food item. To do this, choose one food item from your list (e.g. Cheerios®) and divide the energy content in 1 gram of each food item by that of 1 gram of Cheerios.
  $$\frac{Q_{Food, 1g}}{Q_{Cheerio,1g}}$$
  This list will clearly show how much more or less energy is contained in 1 gram of each food item. The normalized Calorie content calculated from the measurements will also match more closely to the normalized Calorie content calculated from the energy content values found on labels than the individual values (Q_{Food, 1 g} ) do.
• Let the students do some background research to find out the approximate proportions of the different basic food components (fats, sugars or carbohydrates, and proteins) in each of the food items you tested. Can students use the information to draw conclusions about the relative amounts of energy available in 1 gram of each of these different food components?
• Let students calculate the nutritional content of a meal they shared using an app or online program like this recipe nutrition calculator.
• Explore the health aspects of foods in more detail. How does the students' food intake break down in terms of the three components (fats, sugars or carbohydrates, and proteins)? How does this compare to the advised breakdown for their age group? What other health considerations might be overlooked when focusing too much on these three components?
• Consider the lesson Get Energized with Cellular Respiration!, which explores how our body releases the energy of food so we can use it.