Carbohydrates - Experiment No. 3

Biochemistry class
University of Makati
III-BSE General Science

Made by: yourstruly

1. CARBOHYDRATES
Activity no. 3
Prepared by: Group 2

2. OBJECTIVES
To be able to identify the different types of carbohydrates using the different
specific chemical tests.

4. S
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5. 10 test tubes
graduated cylinder
test tube brush
water bath
tripod
Bunsen burner
test tube holder
Spot plate
Crucible tong A
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6. A
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aspirator
pipette
watch glass
gauze beaker
Iron stand
funnel
Evaporating dish
Mortar and pestle

7. I. TEST BASED ON THE FORMATION OF
FURFURAL AND ITS DERIVATIVES:
A.GENERAL TEST FOR CARBOHYDRATES
1.MOLISCHTEST
2.BIAL’S ORCINOLTEST
3.SELIWANOFFTEST

8. 1. MOLISCH TEST
a)Place 4 mL of 3% solution of glucose, xylose,
lactose, sucrose and starch into separate labeled
test tube. Add 2 drops of the Molisch reagent on
each test tube. And mix the contents by gently
shaking the test tube.
b)Cautiously add about 1-2 ml of concentrated
sulfuric acid, allowing the acid to run down the
side of the tube. Due to the density of sulfuric
acid, it will form a layer under the glucose
solution. Note the color of the ring formed at the
junction of the two liquids.
c)A brown color due to charring is not an indication
of carbohydrates and the test needs to be
repeated.
d)Record all your results.
Sucrose
Starch
Glucose Lactose

9. Substance
Tested
Description
Glucose In the first trial, it turned brown ring and in a
second attempt it formed clear (no purple color) It
means, it is a negative result.
Lactose Formedpurple and immediately turned to a brown
color after a minute, the second attempt is the
same, from violet slowly formed brown.
Sucrose Purple-red ring formed at theinterface of the tow
solutions. This indicates a positive result, sucrose
is a carbohydrate.
Starch Greenblack substance is formed with a touch of
purple in it.
1. MOLISCH TEST
Based on the result of the experiment, it
is the sucrose that gave a true positive
result. It formed purple ring as it is.

10. Analysis:
The formation of a purple or purplish-red ring at the point of
contact between H
2SO
4and the MolischReagent mixture
confirms the presence of carbohydrates.
The sucrose possess purplish-red ring, indicates the
presence of carbohydrates.

11. 2. BIAL'SORCINOLTEST
a)Place 5 drops of 3% solution of ribose
and glucose in separate labeled test
tubes.
b)Add 5 mL of bial'sreagent to each test
tube.
c)Carefully heat each tube over a bunsen
flame until the solution begins to boil.
d)Note the blue green color of the product
formed in the sample tube.
e)Record your results in the table below.
After: Glucose After: Ribose

12. Substance Tested Results
Ribose Green colorwas formed and the bottom
of the test tube have soot observed.
Glucose There is no change in color(yellow-
green) but it produce soot to the test
tube.
Based on the result of the experiment,
both ribose green-yellow product,
Hexose generally react to green
products. Pentose indicates the
formation of bluish product.
2. BIAL'SORCINOLTEST

13. Analysis:
Bial’stest is specific for pentoses. Hexoses generally react to
form green, red, or brown products. The formation of bluish
product is positive, therefore riboseand glucoseindicates a
negative result for pentoses.

14. 3. SELIWANOFF'STEST
a)Place 1 mL each of 3 % glucose, fructose,
maltose and sucrose in separate labeled test
tubes.
b)Add 4 mL of seliwanoff'sreagent to each test
tube.
c)Place the place tubes in a boiling water bath
and allow it to stay there for exactly 30
seconds
d)Note the changes and record which test tube
gives the test in the shortest time.
e)Continue heating and observe the color
change for every 1 minute interval until you
have achieved 5 mins.
f)Record the time required for a positive test for
each sample.

15. Fructose formed cherry red color rapidly
when added to a solution that indicated a
positive test.
3. SELIWANOFF'STEST
Substance Tested Time Results
3% of Glucose 5 minutes Dark yellow
(Original color of
reagent)
3% of Fructose5 minutes Cherry redcolor
is formed.
3% of Maltose 5 minutes Dark yellow
(Original color of
reagent)
3% of Sucrose 5 minutes Dark yellow
(Original color of
reagent)

16. Analysis:
Based on the experiments the substances that been tested by putting a
SELIWANOFF'S reagent resulting into different form. In Fructose it resulted
in having a Cherry red color in which it is a positive and belongs to "Ketose.
While Glucose, Maltose, and Sucrose there color becomes Dark yellow in
which results to negative and belongs to "Aldose". SELIWANOFF'S test is a
chemical test which distinguishes between the Aldose and ketose sugars.

17. II. TEST BASED ON THE REDUCING
PROPERTY OF SUGARS
A.GENERAL TEST FOR CARBOHYDRATES
1.BENEDICT’S TEST
2.BARFOED’STEST
3.IODINE TEST FOR POLYSACCHARIDES

18. 1. BENEDICT'S TEST
a)Place 1 mL each of 3% solutions of glucose,
xylose, fructose, lactose, sucrose and starch. In
separate labeled test tubes add 5 mL of
benedict's reagent in each test tubes.
b)Place all the tubes in boiling water bath for 2-3
mins.
c)Observe the color of the solution and note
whether a precipitate has formed (a change in
color of the solution is not indicative of a positive
reaction; a precipitate must appear).
d)Avoid prolonged heating
e)Record your results.
LactoseSucrose Glucose StarchFructose

19. Green –very low concentration
Yellow –low concentration
Orange –brown –medium concentrate
Brick red –high concentration
1. BENEDICT'S TEST
Substance Tested Result Explanation
3% of Glucose
Orange Brown
precipitate
Moderate
(mono, reducing)
3% of Fructose
Dark Brown
precipitate
Moderate
(mono, reducing)
3% of Lactose
Orange precipitateModerate
(di, reducing)
3% of Sucrose
Green precipitateTraceable/ very
low
(di-, non reducing)
3% of Starch Crystal blue Noreducing sugar

20. Analysis:
When Benedict’s reagent solution and reducing sugars are heated
together, the solution changes its color to orange-red/brick red.
This color is due to presence of simple carbohydrates. In specific,
lactose, fructose and glucose are involved.

21. 2. BARFOED'STEST
a)Place 1 mL each of 3% solutions of
glucose, maltose, sucrose in separate
labeled test tubes.
b)Add 3 mL of barfoed'sreagent in each
test tube.
c)Place all the tubes in a boiling water
bath for 10 mins. Record your
observations
glucose
sucrose
maltose

22. 2. BARFOED'STEST
Substance
Tested
Result Explanation
3% of Glucose
It formed red-brick
precipitate at the top side
of the test tube.
the combination of disaccharide and acid
hydrolyze the disaccharide which result to
formation of monosaccharide, can
classify as positive test.
3% of Maltose
There is no change in colorMaltose can distinguish as disaccharide
that leads to negative test .
3% of Sucrose
There is no change in colorSucrose can also distinguish as
disaccharide that results to negative test

23. Analysis:
Barfoedtest is specific to monosaccharides, this is use
to differentiate monosaccharides (+ive) and
dissacharides(-ive). It is observed that glucose
(monosaccharide) has red-brick precipitate at the side
of the test tube and a slight change in color, blue-violet.

24. 3. IODINE TEST FOR POLYSACCHARIDES
a)Place 1 mL each of 3% solutions
of starch, glucose, and sucrose
in separate labeled test tubes.
b)To each spot, add 2 drops of
iodine solution.
c)Take note of the color formed.
d)Record your observations.
Sucrose Glucose Starch

25. Sucrose and Glucose did not
react to iodine where as starch
reacted to iodine and produced
blue-black pigment because
iodine solution is reagent which
help determine the presence of
starch.
3. IODINE TEST FOR POLYSACCHARIDES
Substance
Tested
Result
3% Starch
Blueblack color
3% of Glucose
Reddish orange color
3% of Sucrose
Reddish orange color

26. Analysis:
Iodine test is used to test starch. Whenthe starchis
mixed withiodinesolution, an intensely blue
coloredstarch/iodinecomplex is formed.

27. QUESTIONS:
1.Which of the different carbohydrate test would give a positive result
for maltose? Describe the color change.
Carbohydrate test that has positive tests are molischtest, Benedict's
test Seliwanoff'stest, Barfoed'stest. Maltose is a reducing sugar that
reacts that is why they produce red precipitate after boiled.

28. QUESTIONS:
2. Can seliwanoff’stest be used to distinguish from fructose? Why?
Yesit is used to distinguishaldoses from ketoses. On treatment with
conc. Acid, ketoses are dehydrated more rapidly to give furfural
derivatives and on condensation with resorcinol give cherry red
complex. Thetest willbe answered byfructose,sucroseand other keto
containing carbohydrates.

29. QUESTIONS:
3.Inulin is a polysaccharide composed entirely of fructose units.
Which test would be best used to identify the presence of fructose?
The best test to identify the presence of fructose is the Benedict’s Test,
when benedict’s reagent solution and reducing sugars are heated
together, the solution changes its color to orange-red/brick red.

30. CONCLUSION
Therefore, the positive results in the test for carbohydrates are the sucrose
(molischtest) that possess purplish-red ring, indicates the presence of
carbohydrates. While in bial’stest, ribose and glucose indicates a negative
result for pentoses. In seliwanoff, fructose resulted having a cherry red color in
which it is a positive. Then in benedict’s test,Lactose, fructose and glucose are
involved that results to a moderate reducing sugar. Then in Barfoed’stest it is
observed that glucose (monosaccharide) has red-brick precipitate at the side of
the test tube and a slight change in color, blue-violet. Lastly in the iodine test,
starchis mixed withiodinesolution, an intensely blue
coloredstarch/iodinecomplex is formed.