CHE XXXXXXXXXXManual 3E.pdf
Copyright © 2014 Pearson Education, Inc. 337
LABORATORY GOALS
erties of some common ca
ohydrates.
nguish between monosaccharides, disaccharides, and
polysaccharides.
LAB INFORMATION
Time: 3 h
Comments: Tear out the report sheets and place them next to the matching procedures.
Related Topics: Ca
ohydrates, aldohexoses, ketohexoses, reducing sugars, fermentation
Dispose of all chemicals as directed by your laboratory instructor.
CHEMICAL CONCEPTS
All of the monosaccharides and most of the disaccharides can be oxidized to ca
oxylic acids. When the
2Cu
ion that is reduced. Therefore, all the sugars reducing
sugars. Ketoses also act as reducing sugars because the ketone group on ca
on 2 isomerizes to give an
aldehyde group on ca
on 1.
When oxidation of a sugar occurs, the 2Cu is reduced to Cu , which forms a red precipitate of
copper(I) oxide, 2Cu O( ).s The color of the precipitate varies from green to gold to red depending on the
concentration of the reducing sugar. If the concentration of the reducing sugar is low, the sample will
have a gold or green color; if the concentration is high, the sample will have a red color. Sucrose is not a
educing sugar because it cannot revert to the open-chain form that would provide the aldehyde group
needed to reduce the copper(II) ion.
Tests for Ca
ohydrates 30
338 Laboratory Manual for General, Organic, and Biological Chemistry
are aldehydes (aldoses). With ketoses, a deep red color is formed rapidly. Aldoses give a light pink color
that takes a longer time to develop. The test is most sensitive for fructose, which is a ketose.
C. Fermentation Test
Most monosaccharides and disaccharides undergo fermentation in the presence of yeast. The products of
fermentation are ethyl alcohol 3 2(CH CH OH) and ca
on dioxide 2(CO ). The formation of bu
les
of ca
on dioxide is used to confirm the fermentation process.
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Glucose Ethanol
C H O 2C H OH 2CO ( )yeast g
Although enzymes are present in yeast for the hydrolysis of most disaccharides, they are not available
for lactose or galactose. Therefore, lactose and galactose give negative results with the fermentation test.
D. Iodine Test for Polysaccharides
When iodine 2(I ) is added to amylose, the helical shape of the un
anched polysaccharide traps iodine
molecules, producing a deep blue-black complex. Amylopectin, and cellulose, react with iodine to give
ed-to-
own colors. Glycogen produces a reddish-purple color. Monosaccharides and disaccharides are
too small to trap iodine molecules and do not form dark colors with iodine.
E. Hydrolysis of Disaccharides and Polysaccharides
Disaccharides hydrolyze in the presence of an acid to give the individual monosaccharides.
H
2Sucrose H O glucose fructose
In the laboratory, we use water and acid to hydrolyze starches, which produce smaller saccharides such
as maltose. Eventually, the hydrolysis reaction converts maltose to glucose molecules. In the body,
enzymes in our saliva and from the pancreas ca
y out the hydrolysis. Complete hydrolysis produces
glucose, which provides about 50% of our nutritional calories.
+ + +H or H or H o
amylase amylase maltase
DAmylose, amylopectin dextrins maltose many -glucose units
F. Testing Foods for Ca
ohydrates
Several of the tests, such as the iodine test, can be ca
ied out with food products such as cereals,
ead,
crackers, and pasta. Some of the ca
ohydrates we have discussed can be identified.
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Tests for Ca
ohydrates 339
EXPERIMENTAL PROCEDURES GOGGLES REQUIRED!
Materials: Test tubes, 400-mL beaker, droppers, hot plate or Bunsen burner, 5- or 10-mL graduated
lutions: glucose, fructose, lactose, sucrose,
starch, and an unknown
Place 10 drops of solutions of water (reference), glucose, fructose, lactose, sucrose, starch, and an
unknown in separate test tubes. Label each test tube.
The formation of a greenish to reddish-orange color indicates the presence of a reducing sugar. If
the solution is the same color as the Benedi
1. Record your observations.
2. Classify each as a reducing or nonreducing sugar.
Materials: Test tubes, 400-mL beaker, droppers, hot plate or Bunsen burner, 5- or 10-mL graduated
starch, and an unknown
Place 10 drops of solutions of glucose, fructose, lactose, sucrose, starch, water, and unknown in
separate test tubes. Label each test tube.
eagent to each. Mix by shaking. The reagent contains concentrated
HCl. Use carefully.
Place the test tubes in a boiling hot-water bath and note the time.
1. After 1 min, observe the colors in the test tubes. A rapid formation of a deep red color indicates
the presence of a ketose.
2. Classify each as a ketose or aldose.
C. Fermentation Test
Materials:
glucose, fructose, lactose, sucrose, starch, and an unknown
Fill the fermentation tubes with solutions of glucose, fructose, lactose, sucrose, starch, water, and
the unknown. Add 0.2 g of yeast to each fermentation tube and mix well by shaking
(see Figure 30.1).
If fermentation tubes are not available, use small test tubes placed upside down in larger test tubes.
Cover the mouth of the large test tube with filter paper or cardboard. Place your hand firmly over
the paper cover and invert. When the small test tube inside has completely filled with the mixture,
eturn the larger test tube to an upright position (see Figure XXXXXXXXXXSet the tubes aside.
FIGURE 30.1 Fermentation tube filled with a ca
ohydrate solution.
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340 Laboratory Manual for General, Organic, and Biological Chemistry
At the end of the laboratory period, and again at the next laboratory period, look for gas bu
les in
the fermentation tubes or inside the small tubes.
1. Record your observations (see Figure 30.3).
2. State whether fermentation occu
ed or not.
D. Iodine Test for Polysaccharides
Materials: Spot plate or test tubes, droppers, iodine reagent, 2% ca
ohydrate solutions in dropper
ottles: glucose, fructose, lactose, sucrose, starch, and an unknown
Using a spot plate, place 5 drops of each solution of glucose, fructose, lactose, sucrose, starch,
water, and the unknown in the wells. (If you do not have a spot plate, use small test tubes.)
Add 1 drop of iodine solution to each sample. A dark blue-black color is a positive test for
amylose in starch.
1. Record your observations.
2. Indicate whether amylose is present.
E. Hydrolysis of Disaccharides and Polysaccharides
Materials: Test tubes, 10-mL graduated cylinder, 400-mL beaker (boiling water bath), hot plate or
Bunsen burner, spot plate or watch glass, 10% HCl, 10% NaOH, red litmus paper, iodine
2% starch and sucrose solutions in dropper bottles
Place 3 mL of 2% starch in 2 test tubes.
Place 3 mL of 2% sucrose in 2 more test tubes.
To one sample each of sucrose and starch, add 20 drops of 10% HCl. Mix by shaking.
To the other samples of sucrose and starch, add 20 drops of 2H O. Mix by shaking.
Label the test tubes and heat in a boiling water bath for 10 min.
Remove the test tubes from the water bath and let them cool.
To the samples containing HCl, add 10% NaOH (about 20 drops) until 1 drop of the mixture turns
litmus paper blue, indicating the HCl has been neutralized.
FIGURE 30.2 Test tubes used as fermentation tubes.
FIGURE 30.3 Fermentation tubes with 2CO bu
les.
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Tests for Ca
ohydrates 341
Test separate samples for hydrolysis using the i test as described below.
Iodine Test
Place 5 drops of each solution on a spot plate or watch glass.
Add 1 drop of iodine reagent to each.
1. Record your observations.
2. Record your observations.
3. Determine if hydrolysis has occu
ed in each.
F. Testing Foods for Ca
ohydrates
Materials: rups (corn, maple,
fruit), foods with starches: cereals, pasta,
Obtain 2 ca
ohydrate samples to test.
Perform the iodine test on each.
1. Describe the kinds of ca
ohydrates you identify in each sample.
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CHE XXXXXXXXXXManual 3E.pdf
Copyright © 2014 Pearson Education, Inc. 343
Pre-Lab Study Questions 30
1. What happens to glucose or galactose when the 2Cu
2. Would you expect fructose or glucose to form
Explain your answer.
3. undergo fermentation with yeast?
4. How can the iodine test be used to distinguish between amylose and glycogen?
Date Name
Section Team
Instructor
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Copyright © 2014 Pearson Education, Inc. 345
Date Name
Section Team
Instructo
Test Water Glucose Fructose Lactose Sucrose Starch Unknown
#_______
for Reducing Sugars
1. Observations
2. Reducing/
Nonreducing
Test for Ketoses
1. Colors after
1 min
2. Ketose/Aldose
C. Fermentation Test
1. Observations
2. Fermentation
(Yes/No)
D. Iodine Test for Polysaccharides
1. Observations
2. Amylose
(Yes/No)
Questions and Problems
Q1 From the results in Part A, list the ca
ohydrates that are reducing sugars.
REPORT SHEET
Tests for Ca
ohydrates
LAB
30
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346 Laboratory Manual for General,