MOTIVATIONAL ACTIVITY

TEACHER DEMONSTRATION: ENERGY CONTENT OF FOOD

 

 

 

Energy Content of Foods

 

Energy content is an important property of food. The energy your body needs for running, talking, and thinking comes from the food you eat. Energy content is the amount of heat produced by the burning of 1 gram of a substance, and is measured in joules per gram (J/g).

You can determine energy content by burning a portion of food and capturing the heat released to a known mass of water in a calorimeter. If you measure the initial and final temperatures, the energy released can be calculated using the equation

H = D tmCp

where H = heat energy absorbed (in J), D t = change in temperature (in C), m = mass (in g), and Cp = specific heat capacity (4.18 J/gC for water). Dividing the resulting energy value by grams of food burned gives the energy content (in J/g).

 

OBJECTIVES

In this experiment, you will

· use a TI Graphing Calculator, a CBL System, and a temperature probe to measure temperature

· use a balance

· determine energy content

· compare the energy content of different foods

 

MATERIALS

CBL System

2 stirring rods

TI Graphing Calculator

balance

Vernier Temperature Probe

small can

Vernier adapter cable

slit stopper

2 food samples (nut, popcorn, or

cold water

marshmallow)

food holder

ring stand and 4" ring

matches

100-mL graduated cylinder

wooden splint

 

Figure 1

 

PROCEDURE

1. Obtain and wear goggles.

2. Plug an adapter cable into Channel 1 of the CBL System, and then plug a temperature probe into the adapter cable. Connect the CBL System to the TI Graphing Calculator with the link cable using the port on the bottom edge of each unit. Firmly press in the cable ends.

3. Turn on the CBL unit and the calculator. Press and select PHYSCI. Press , then press again to go to the MAIN MENU.

4. Set up the calculator and CBL for one temperature probe and a temperature calibration.

· Select SET UP PROBES from the MAIN MENU.

· Enter "1" as the number of probes.

· Select TEMPERATURE from the SELECT PROBE menu.

· Enter "1" as the channel number.

· Select USE STORED from the CALIBRATION menu.

5. Set up the calculator and CBL for data collection.

· Select COLLECT DATA from the MAIN MENU.

· Select TIME GRAPH from the DATA COLLECTION menu.

· Enter "5" as the time between samples, in seconds.

· Enter "96" as the number of samples (the CBL will collect data for a total of 8 minutes).

· Press , then select USE TIME SETUP to continue. Note: If you want to change the sample time or sample number you entered, select MODIFY SETUP.

· Enter "0" as the minimum temperature (Ymin).

· Enter "50" as the maximum temperature (Ymax).

· Enter "10" as the temperature increment (Yscl).

· Do not start data collection until instructed to do so in Step 8.

6. Get a sample of food and a food holder like the one shown in Figure 1. Find and record the initial mass of the food sample and food holder.

 

7. Set up the apparatus shown in Figure 1.

Determine and record the mass of an empty can.

Place about 50 mL of cold water into the can.

Determine and record the mass of the can plus water.

Use a stirring rod to suspend the can about 2.5 cm (1") above the food sample.

Use a utility clamp and slit stopper to suspend the temperature probe in the water. The probe should not touch the bottom of the can.

8. Press on the calculator to begin data collection. Remember: The temperature probe must be in the water for at least 45 seconds before you begin! Monitor the temperature on the CBL screen for a few seconds, then record the initial temperature (round to the nearest 0.1C).

9. Remove the food sample from under the can and use a wooden splint to light it. Quickly place the burning food sample directly under the center of the can. Allow the water to be heated until the food sample stops burning. CAUTION: Keep hair and clothing away from an open flame.

10. Stir the water until the temperature stops rising. Record this final temperature (round to the nearest 0.1C). After 8 minutes, data collection is completed ("DONE" appears on the CBL screen).

11. Determine the final mass of the food sample and food holder.

12. Press to display a graph of temperature vs. time. Confirm the initial and maximum values you recorded earlier by using to examine the data points along the curve. As you move the cursor right or left, the time (X) and temperature (Y) values of each data point are displayed below the graph.

13. Press , then choose YES to repeat the procedure for a second food sample. Use a new 50-mL portion of cold water.

14. When you are done, place burned food, used matches, and partly-burned wooden splints in the container supplied by the teacher.

 

DATA

Sample 1 Sample 2

Food used _______ _______

Mass of food and holder (initial) ______ g ______ g

Mass of food and holder (final) ______ g ______ g

Mass of empty can ______ g ______ g

Mass of can plus water ______ g ______ g

Initial water temperature ______ C ______ C

Final water temperature ______ C ______ C

 

 

PROCESSING THE DATA

1. Calculate change in water temperature, D t, for each sample, by subtracting the initial temperature from the final temperature (D t = tfinal tinitial).

 

2. Calculate the mass (in g) of the water heated for each sample. Subtract the mass of the empty can from the mass of the can plus water.

 

 

 

 

3. Use the results of Steps 1 and 2 to determine the heat energy gained by the water (in J). Use the equation

H = D tmCp

where H = heat absorbed (in J), D t = change in temperature (in C), m = mass of the water heated (in g), and Cp = specific heat capacity (4.18 J/gC for water).

 

 

 

4. Calculate the mass (in g) of each food sample burned. Subtract the final mass from the initial mass.

 

 

 

 

 

5. Use the results of Steps 3 and 4 to calculate the energy content (in J/g) of each food sample.

 

 

 

 

 

 

 

 

 

 

 

 

 

6. Record your results and the results of other groups below.

Class Results

Food Type Food Type Food Type Food Type

____________ ____________ ____________ ____________

_________ J/g _________ J/g _________ J/g _________ J/g

_________ J/g _________ J/g _________ J/g _________ J/g

_________ J/g _________ J/g _________ J/g _________ J/g

_________ J/g _________ J/g _________ J/g _________ J/g

_________ J/g _________ J/g _________ J/g _________ J/g

_________ J/g _________ J/g _________ J/g _________ J/g

Avg. _________ J/g _________ J/g _________ J/g _________ J/g

 

7. Which of the foods has the greatest energy content?

 

 

 

 

 

 

 

 

 

EXTENSION

1. Determine the energy content of other combustible foods.