Identifying Organic Compounds
Introduction
The most common organic compounds found in living organisms are lipids,
carbohydrates, proteins, and nucleic acids. Common foods, which often consist of plant
materials or substances derived from animals, are also combinations of these organic
compounds. Simple chemical tests with substances called indicators can be conducted to
determine the presence of organic compounds. A color change of an indicator is usually a
positive test for the presence of an organic compound.
In this investigation, you will use several indicators to test for the presence of lipids,
carbohydrates (both starches and sugars), and proteins in particular foods.
Materials
10 test tubes
Test tube rack
label tape
hot plate
paper towels
600 mL beaker
Biuret reagent
Benedict’s solution
20 mL samples of the following solutions:
honey
egg white
lettuce
gelatin
potato
apple juice
unknown
test tube holder
Iodine solution
Sudan III stain
Distilled water rinsing bottle
10 plastic cups for samples
corn oil
melted butter
distilled water
Procedures
1. Wash all glassware with soapy water, rinse with tap water, then quick rinse with distilled
water.
2. Mark the ten plastic cups with your initials and label with the 10 samples.
3. Mark the ten test tubes with the names of the solutions using the label tape.
4. Pour 20mL of each sample into the appropriately marked sample cup.
5. Using a 10 mL graduated cylinder, measure out 3 mL of water, pour it into one test tube, then
make a mark on all ten test tubes equal to the volume of 3 mL of water.
Testing for Lipids
1. Put 3 ml. of each solution into its respective labeled test tube. Divide a paper towel into 10
equal sections then label the sections with the names of the sample solutions.
2. Take 1-2 drops of sample from each test tube and rub the drops gently into their respectively
labeled sections on the paper towel. Set the paper towel aside to dry (about 10 – 15 minutes).
3. Now, add 5 drops of Sudan III solution to the remaining sample in each test tube. Shake each
test tube gently. Be careful! Sudan III stains. The sample should turn red throughout the
liquid if lipids are present. Record the color of the final products in the Data Table. Place a
check mark in the column “Lipids Present” for any of those substances testing positively for
lipids.
**Note: Sometimes this is an unreliable test due to the failure of dissolution of the Sudan III
solution even in a known lipid solution.
4. Hold the now dry paper towel up to the light. Some solutions have left a translucent spot on
the paper towel. This indicates a positive test for the presence of lipids. Mark the results in
the data table in the “Lipids present” column. (Does your Spot test agree with your Sudan III
test?)
5. Be sure to wash and rinse all test tubes before the next test.
Testing for Carbohydrates
1. Wash the test tubes thoroughly.
2. Sugars and starches are two common types of carbohydrates.
To test for starch,
3. Refill each cleaned test tube to the 3 mL mark with the appropriate solutions. Add 5 drops of
iodine solution to each test tube. Iodine will change color from yellow-brown to blue-black
in the presence of starch.
4. Gently shake each test tube. Record any color changes and indicate the presence of starch in
the appropriate columns on the Data Table.
5. Wash the test tubes thoroughly.
To test for sugars,
6. set up a hot water bath by filling a 600 mL beaker ½ full of water and setting it on a hot plate,
then turning the heat on High.
7. While the hot water bath is heating, fill the test tubes with 3 mL of the appropriate solutions.
Add 10 drops of the Benedict’s solution to each test tube. Benedict’s solution will change
from light blue to green, yellow, orange, brick red in the presence of a monosaccharide
(single sugar). Particular color often can be correlated with the amount of sugar present.
8. After 5 minutes, remove the test tubes from the hot water bath into the test tube rack and let
cool.
9. In the Data Table, record any color changes and make a check mark in the column next to
any solution that tests positive for sugars.
Testing for Proteins
1. Wash the test tubes thoroughly.
2. Fill the test tubes with 3 mL of the appropriate solutions. Add 5 drops of Biuret reagent to
each test tube. Be careful! Biuret reagent contains sodium hydroxide, a strong base. If
you splash any reagent on yourself, wash it off immediately with water and tell the teacher.
3. Gently shake each test tube. Biuret reagent changes color from yellow to blue-violet in the
presence of proteins. In the Data Table, record any color changes and make a check mark
beside any solution that tests positive for proteins.
Analysis and Conclusions
1. Which substances test positive for lipids? ___________________________________
_______________________________________________________________________
2. Which substances test positive for starches? _________________________________
________________________________________________________________________
3. Which substances test positive for single sugars?_____________________________
_______________________________________________________________________
4. Which substances test positive for proteins? _________________________________
________________________________________________________________________
5. Which test substances did not test positive for any of the organic compounds?______
_______________________________________________________________________
6. What is the purpose of using distilled water both as a rinse and as one of your substances?
____________________________________________________
_________________________________________________________________
_________________________________________________________________
7. What do all of the indicators you have used have in common? _____________
__________________________________________________________________
8. What do you think the unknown substance is? Why? _____________________
___________________________________________________________________
___________________________________________________________________
Critical Thinking and Application
1. Your brown lunch bag has a large, translucent spot on the bottom. What explanation could
you give for this occurrence?________________________________________
_______________________________________________________________________
2. What conclusion could you make if a positive test for any of the organic compounds
occurred in the test tube containing only distilled water?______________________
______________________________________________________________________
3. A very thin slice is removed from a peanut, then is treated with Sudan III stain and Biuret
reagent. When the slice is examined under a microscope, red and blue-violet patches are
visible on the peanut slice? What conclusions can you draw from your examination?
_______________________________________________________
_____________________________________________________________________
Extra Credit
1. During one of your free blocks, test a lunch substance for the presence of organic
compounds. Report your findings here? __________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
2. Make a three dimensional model of one of the five organic compounds. Use polystyrene
spheres, gum drops, candy, toothpicks, etc. Be sure that each different element is represented
by its own distinct color. The toothpicks can be used for the bonds among the atoms.
Data Chart : Test for Organic Compounds
Sample Name
1.
honey
2.
lettuce
3.
potato
4.
egg white
5.
gelatin
6.
apple juice
7.
corn oil
8.
butter
9.
distilled water
10. unknown
Color Before Color After
Lipids
Carbos
Sugars
Proteins