Name: ________________________Pd: _____ Seat #______ Color ____________
Identifying Organic Compounds
Pre-Lab Discussion
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, and proteins in particular foods.
Problem: How are indicators used to test for the presence of organic compounds?
Materials (per group)
11 test tubes
Test tube rack
Test tube holder/clamp
Masking tape
Hot plate
20 mL honey solution
20 mL egg white and water mixture
20 mL corn oil
20 mL onion and water mixture
20 mL gelatin and water solution
20 mL milk
20 mL potato and water mixture
20 mL apple juice and water mixture
20 mL distilled water
20 mL unknown substance
20 mL of purified lipid (+ control for
Sudan IV test)
20 mL of purified protein (+ control
for Biuret test)
20 mL of purified starch (+ control
for Iodine test)
20 mL of purified simple sugar (+
control for Benedict’s test)
Paper towels
600-mL beaker
Brown paper
Sudan IV stain
Benedict’s solution
Iodine solution
Biuret reagent
Safety!
Put on a laboratory apron if one is available. Put on safety goggles. Handle all glassware
carefully. Always use special caution when using any laboratory chemicals, as they may irritate
the skin or cause staining of the skin or clothing. Never touch or taste any chemical unless
instructed to do so. Use extreme care when working with heated equipment or materials to
avoid burns. Notify your teacher immediately of any accidents that occur during the lab.
Procedure
Part A. Lipids Tests
1. Obtain 11 test tubes and place them in a
test tube rack. Use masking tape to make
labels for each test tube. As shown in
Figure 1, write the name of a different
food sample (listed in the Chart – Figure
2) on each masking-tape label. Label the
tenth test tube “negative control”. Label
the 11th tube “positive control”.
Honey
Potato
Egg White
Apple Juice
Corn Oil
Unknown
Onion Juice
Gelatin
negative control
(distilled water)
Skim Milk
+ control (different
for each test! Use the
purified substance
being tested.)
Figure 2
2. Fill each test tube with 25 drops (or 1 mL) of the substance indicated on the masking
tape label. Use 25 drops of water for the negative control and 25 drops of purified
lipid (at demonstration desk) for the positive control. Add 2 drops of Sudan IV stain
(which can be found under the fume hood) to each test tube. Sudan IV stain will turn
bright red in the presence of lipids.
3. Gently shake the contents of each test tube. CAUTION: Use extreme care when
handling Sudan IV to avoid staining hands or clothing. In the Data Table record any
color changes and place a check mark next to those substances testing positively for
lipids.
4. Wash the test tubes thoroughly with a test tube brush and soapy water (brush is in
Erlenmeyer flask with soapy water).
2
Grease Spot Test for Lipids
1. For another test for lipids, divide a piece of brown paper into 11 equal sections. In each
section, write the name of one test substance.
2. In each section using the tip of the pipet, rub one drop of each substance listed in figure 2
onto the brown paper. Rub the food until a “wet” spot appears on the paper. With a paper
towel, rub off any excess pieces of food that may stick to the paper. Set the paper aside
until the spots appear dry – about 10 to 15 minutes before recording results as instructed in
the next step.
3. Hold the piece of brown paper up to a bright light or window. You will notice that some foods
leave a translucent spot on the brown paper. The translucent spot indicates the presence of
lipids.
Part C. Carbohydrates Tests
Starch Test
1. Sugars and starches are two common types of carbohydrates. To test for starch, refill each
cleaned test tube with 25 drops (or 1 mL) of the substance indicated on the masking-tape
label. Use 25 drops of water for the negative control and 25 drops of starch (at
demonstration desk) for the positive control. Add 1 drop of iodine solution to each test tube.
Iodine will change color from yellow-brown to blue-black in the presence of starch.
2. Gently shake the contents of each test tube. CAUTION: Use extreme caution when using
iodine as it is poisonous and can also stain hands and clothing.
3. In the Data Table, record the colors of the solutions in the test tubes and place a check
mark next to those substances testing positive for starch.
4. Wash the test tubes thoroughly.
Sugar Test
1. For a simple sugar test, set up a hot-water bath. Half fill the beaker with tap water. Heat the
water to a gentle boil. CAUTION: Use extreme care when working with hot water. Do not let
water splash onto your hands.
2. While the water bath is heating, fill each cleaned test tube with 25 drops of the substance
indicated on the masking-tape label. Use 25 drops of water for the negative control and 25
drops of sugar (at demonstration desk) for the positive control. Add 10 drops of Benedict’s
solution to each test tube. When heated, Benedict’s solution will change color from blue to
green, yellow, orange, or red in the presence of a simple sugar, or monosaccharide.
3. Gently shake the contents of each test tube. CAUTION: Use extreme caution when using
Benedict’s solution to avoid staining hands or clothing.
4. Place the test tubes in the hot-water bath. Heat the test tubes for 3 to 5 minutes. With the
test tube holder, remove the test tubes from the hot-water bath and place them back in the
test tube rack. CAUTION: Never touch hot test tubes with your bare hands. Always use a
test tube holder to handle hot test tubes. In the Data Table, record any color changes and
place a check mark next to any substances that test positive for a simple sugar.
5. After they have cooled, wash the test tubes thoroughly.
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Part C. Protein Test
1. Put 25 drops (1 mL) of the appropriate substance in each labeled test tube. Add 10 drops of
Biuret reagent to each test tube. Use 25 drops of water for the negative control and 25
drops of protein (at demonstration desk) for the positive control. CAUTION: Biuret reagent
contains sodium hydroxide, a strong base. If you splash any reagent on yourself, wash it off
immediately with water. Call your teacher for assistance.
2. Gently shake the contents of each test tube. Biuret reagent changes color from yellow to
violet in the presence of protein. In the Data Table, record the colors and place a check mark
next to any substances that test positively for protein.
3. Wash the test tubes thoroughly.
4.
Organic Compound Lab – Data Table
Lipids
Substances
Paper
Bag
Results
Lipid
Present?
(√)
Color
After
Sudan
Test
Carbohydrates
Lipids
Present?
(√)
Color
After
Benedict’s
Test
Simple
Sugars
Present?
(√)
Color
After
Iodine
Test
Protein
Starches
Present?
(√)
Color
After
Biuret
Test
Protein
Present?
(√)
Honey
Egg White
Corn Oil
Lettuce
Gelatin
Milk
Potato
Apple
Juice
Unknown
- control
(H2O)
+ control
**
** The + control: is the purified substance being tested: lipid for the Sudan and Grease Spot
tests, protein for the Biuret test, simple sugar for the Benedict’s test, OR starch for the
Iodide test.
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RESULTS
1. Which test substances contain lipids?
2. Which test substances contain starch?
3. Which test substances contain simple sugar?
4. Which test substances contain protein?
5. Which test substances did not test positive for any of the organic compounds?
Include these questions in your ANALYSIS:
1. List the organic molecules that were found in each food and the unknown. (For example,
cream contained lipids and sugars).
2. What were your controlled substances for each test and what was their purpose in the
experiment?
3. What do all of the indicators you used have in common?
4. Choose two of the foods and research the structure and importance of the organic
molecule(s) to the organism. For example, you may research how and why the potato
plant makes starch and how and why the honey bee makes honey and bees wax.
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Name: _______________________ Pd: ____ Seat # ______ Color __________________
Testing for Organic Compounds Pre-Lab
1. What is the purpose of this lab?
2. Name the three organic compounds we will be testing for.
3. What is the purpose of positive and negative controls? (Pre-lab class demo)
4. Complete the chart below about the four indicators we will be using.
Indicator
# of Drops
Organic Compound
Test For
Color Change in
Presence of
Organic Compound
Benedict’s Solution
Iodine
Sudan IV Stain
Biuret Reagent
5. How much of each sample is added to the nine test tubes?
_________
6. When conducting the carbohydrate test, what is unique about this test that must be done
in order to see a color change?
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