Question: What kinds of Carbohydrates are found in some everyday foods?
Background: (Highlight or underline as you read.) Most of the foods you eat contain only
complex carbohydrates. That is, they contain either complex sugars or starches. Your digestive
system must break these into simple sugars before your cells can use them.
Only a few foods contain simple sugars. The sugars are already in a form that your cells
can use, and they do not need to be broken down by your digestive system.
To test foods to see if they contain simple sugars, scientists use a color indicator - a
substance that changes color during a chemical test. Benedict’s solution is an example of a color
indicator. It begins as a blue solution, and when it is heated with a complex sugar like a
disaccharide or a polysaccharide it remains blue; but when it is heated with a simple sugar
(monosaccharide), the blue color changes to green, yellow, orange, or red.
Iodine, another chemical indicator, changes to a deep blue-black when it is mixed with a
starch or polysaccharide
This lab involves making two chemical tests. In the first test, Benedict’s solution will be
used to test known sugars. In the second test, iodine will be used to test an identical set of
samples for starch. The purpose of this is so you know what color changes to expect and become
the CONTROLS of your experiment.
Because the indicators (Benedict’s solution and iodine) must not be mixed, you will work
with a lab partners who will be responsible for conducting the second test.
Special Precautions: Follow these safety precautions while heating your samples, or
when you are around anyone else who is conducting the experiment.
-
If you have long hair, tie your hair back.
Wear safety goggles and a lab apron.
When you are heating a test tube, prevent dangerous splatters by pointing your test
tube away from other people.
Iodine is poisonous, and must not be allowed to get into your mouth or eyes. If you
suspect that any iodine has reached your mouth or eyes, tell your teacher
immediately.
Materials: Each team will obtain the following materials:
- 6 test tubes
Benedicts solution (blue)
- Test tube rack
Iodine (dropper bottle)
- Test tube holder
Paper towels
- Wax pencil/tape
Samples: 3 known and 5 unknown
- Syringe (2)
Hot bath (Beaker on a hot plated1/2 filled with
- Safety goggles and apron
with water.)
Solutions: Monosaccharide
Disaccharide
Polysaccharide
oat solution
table sugar solution
honey solution
powered sugar solution
apple juice
Procedure: READ the directions first, look over your lab area to be sure you have all the
supplies. It is important that you understand completely what you are doing and WHY!
Part A. Benedicts. Test
1. Fill a 500 ml beaker half full of water. Bring the water to a boil on a hot plate. The
boiling water is called a hot water bath.
2. Number three test tubes 1 to 3. Add 2 ml of monosaccharide solution to tube 1 with your
syringe. Rinse out afterward in a CUP of WATER. Add 2ml of the disaccharide solution
to tube 2 and 2 ml of polysaccharide solution to tube 3. Rinse syringe after each use!
3. Add 2 ml of Benedicts solution to each test tube (1,2,3) Use a new syringe.
4. Place the test tubes into the hot water bath for 3-5 minutes or just until you see at least
one tube change color.
5. Remove the test tubes from the water bath with a test tube holder.
6. Observe ANY color changes in the solutions.
7. Record those changes in the table 1 under the column “Benedicts color”.
Part B. Iodine Test
1. Number three test tubes 1 to 3.
2. Add 2 ml of monosaccharide solution to tube 1, 2ml of disaccharide solution to tube 2
and 2ml of polysaccharide solution to tube 3.
3. Using the dropper bottle with the iodine, add 2 DROPS of iodine solution to each test
tube. Mix the contents of each test tube by swirling.
4. Record the color of the tubes in the column marked “Iodine color on table 1.
Part C. Testing the Unknowns
Having tested known carbohydrates, you are now ready to test some unknown substances. By
comparing results of the Benedicts and iodine test to the table you just completed you should be
able to classify unknown substances as monosaccharide, disaccharides, or polysaccharides.
Using the knowledge you have just learned you must now identify the 5 unknowns as either
monosaccharides, disaccharides or polysacchrides.
EACH person in your lab group will be responsible for writing up and turning in a lab
report. This will be on the web site so you can down load it, save it and type on it. You should
follow the steps of the scientific method and I have provided a template for you to use for this lab
on line. You may want to check the resource section of your text book if you need further
explanation for any of the parts.
Title Page: Name, Group names, Mrs. Ziebart, Date
Purpose or Question:
Background: ( Documentation must be included) Most of this information will come from the
background section of the lab report. Here is a sample with documentation.
Carbohydrates are found in many common foods. A test called the Benedicts test can be used
to determine if a sugar is a monosaccharide or simple sugar (lab report).
Hypothesis: ( Write your own here and summarize the above hypotheses)
Materials : What did you use. Include the controls as well as the unknowns in your write up.
Procedure: Summarize in your own words what you did. Number your steps.
Observations and Results:
Table 1: Results of tests with known carbohydrates.
Tube number
Carbohydrate type
1
Monosaccharide
2
Disaccharide
3
Polysaccharide
Color change after
Heating with
Benedict’s
Color change after
Adding Iodine
Table 2: Results of tests with Unknown Carbohydrates
Carbohydrate
Honey
Oats
Table sugar
Apple juice
Powered sugar
Benedicts Color
Iodine Color
Type of
Carbohydrate
Analysis: In a paragraph include the following information. Identify the controls and the
variables in this experiment. Why are the controls needed, what did they tell you? Explain how
you got your results by writing about the tests. Did the results turn out the way you expected? If
not, then give valid reasons as to why they did not. Justify why you needed to test each
carbohydrate with BOTH tests. Determine what color changes were necessary for a positive test
for Benedicts and a positive test for Iodine. How could one use the information learned from this
lab in another situation? This is one of the most important parts of the lab and is counts as
the most points.
Conclusion: Was your hypothesis right or wrong? Summarize the outcome of the experiment.