PROCEDURE
How can we measure the energy contained in foods?
BACKGROUND
Plants have evolved processes that convert light energy into the chemical bonds of complex molecules.
The chemical bonds in carbohydrates, fats, and proteins store energy until needed by the plant. The plant
can then release the energy by breaking the appropriate chemical bonds.
Every animal maintains its life processes by consuming complex molecules that store energy. The processed plants and animals we eat as foods contain varying amounts of proteins, carbohydrates, and
fats. Because each of these types of foods contains varying amounts of energy, these foods will release
varying amounts of energy when they are used by cells. Within our bodies, the energy is released slowly by
a series of chemical reactions.
By burning pieces of food, the chemical energy stored in molecular bonds is released as heat and light.
The heat can be measured in units called calories. A calorie is the amount of heat (energy) required to
increase the temperature of one gram of water by 1"C. This process is the basis of the technique of
calorimetry. The more calories a food contains, the more heat it gives off when burned. Foods high in
calories will release large amounts of energy. One gram of a protein will release far fewer calories than
one gram of fat This lab will study foods with different proportions of protein, fats, and carbohydrates to
see how much energy (calories) they release.
Once the lab is completed, answer the following questions.
1. The burning of the food sample releases energy in the form of light and heat. Explain the technique
for measuring heat in calories?
2. Does the water need to be at the same temperature at the start of each new test? Explain.
EQUIPMENT
|
ring stand with test tube clamp |
 |
PRELAB
The math involved in a calorimetry lab
is extensive. Therefore the following sample results is provided. Complete
all math calculations before beginning the lab procedure below.
FACTS TO REMEMBER:
1 calorie = heat needed to raise 1 g of H20
1oC.
1 Calorie = heat needed to raise 1000 g of H20
1oC.
1 Kilocalorie (unit of heat) = 1 Calorie (unit of food
energy)
EQUATIONS:
To
determine total amount of heat energy that escaped from burned food:
Total Kcal from burned food = Weight of water X Change
in Temperature of water
1000
To determine amount of heat energy in 1 gram of burned food:
Kcal/gram
= Total Kcal (from 1st equation
above)
Change in mass
(of burned food)
SAMPLE PROBLEM:
Food A is burned and the following data
is collected:
Beginning Weight (food) = 6.8 g Beginning
Temperature = 23oC Weight of H20
= 10 g
Ash Weight
= 6.2 g
Final Temperature = 63oC
First Calculate: Weight change =
6.8 – 6.2 = .6 g
Change in temperature = 63 – 23 = 40oC
Now Calculate: Total Kcal = 10
g X 40oC = 400 = .4 Kcal
1000 1000
Kcal/gram = .4 Kcal = .67 Kcal/g
.6 g
PROBLEM #1:
Food B is burned and the following data
is collected:
Beginning Weight (food) = 14.6 g Beginning
Temperature = 27oC Weight of H20
= 30 g
Ash Weight
= 14.4 g
Final Temperature = 84oC
Complete the following calculations:
a) Weight change = ………….
b) Change in temperature
= ……….
c) Total Kcal = ……….
d) Kcal/g = ………..
PROBLEM #2:
Food B is burned and the following data
is collected:
Beginning Weight (food) = 8.2 g Beginning
Temperature = 25oC Weight of H20
= 25 g
Ash Weight
= 7.9 g
Final Temperature = 76oC
Complete the following calculations:
a) Weight change
= ………….
b) Change in temperature
= ……….
c) Total Kcal =
……….
d) Kcal/g = ………..
PROCEDURE
CAUTION: Since there will be an open flame in the classroom, take
appropriate precautions.
A. Assemble several food samples. (NOTE – Samples must be able to
fit between pin and test tube.)
3. Predict one food that when burned, will
raise the temperature of water the most.
And the least.
B. Study the diagram above that shows the apparatus to be used. Assemble
the ring stand and
clamp so that the large test tube will
be positioned 1 cm. above the food sample.
Record all the following measurements on the data table. Then complete the calculations as asked.
C. Weigh an empty graduated cylinder. Pour approximately 10 ml of water into the cylinder.
4. Determine the weight of the water only. Record this weight as
“Water Volume (3)”. Pour this
weighed water
into the large test tube.
5. Measure the beginning temperature of the water with a digital
thermometer. Read the
temperature
to the nearest .1°C and record as “Temp. at Start (4)”.
D . Place the cork on the table on top of a piece of Aluminum foil. Determine
the weight of the cork and
foil combined using a digital pan balance.
(NOTE – This number will NOT be recorded on Data Sheet.)
Use the pin to attach the food sample to
the cork
6. Weigh the combination of cork, foil
and food sample. Determine the weight of the food sample only,
and record
as “Weight (5)”.
E. Strike a match and set the food sample on fire. Immediately move the
sample under the test tube. Let
the sample burn as completely as possible.
DO NOT relight the food while under the test tube – this
adds heat into the water from the match,
resulting in inaccurate data.
7. After the food sample is completely
burned, measure the temperature of the water again and
record as
“Temp. at End (6)”.
8. Again weigh the combination of cork,
foil and burned food sample. Determine the weight of the
burned food
sample only, and record as “Ash Weight (7)”.
9. Subtract "ash weight" from
the beginning "weight" and record as “Weight Change (8)”.
CAUTION: The test tube may be very hot after the experiment. Use test tube tongs to handle it.
F. Discard the water
and allow the tube to cool. Obtain a fresh, clean tube from the teacher
and repeat
the procedure for 5 or more food samples.
10. Calculate the temperature difference
for each sample by subtracting the Beginning water
temperature
from the Final water temperature, and record as “Temp. Difference
(9)”.
G. Complete the following calculations using the formulas in the PRELAB
above.
11. a) Calculate the kilocalories produced
by each sample. Record your calculations as
”Total Kilocalories (10)".
b) Calculate
the number of kilocalories per gram for each sample of food. Record your
calculations
as “Kcal per Gram (11)”.
Postlab Analysis
12. What are the problems with this technical design, which might be sources
of error? (List at least 3)
13. Which foods gave off the most catories/gm (list 2)? The least (list2)?