LAB 4 Carbohydrates

Name: Course & Year: Score:
Instructor: Group No: Date of Activity:
Activity 4
CARBOHYDRATES
Carbohydrates include polyhydroxy aldehydes or ketones and their by-products. They

make up most of our diet and serve as the major source of energy. They play an important
part in metabolic processes by furnishing the carbon chain for compound synthesis by
living organisms. The chemical tests used to detect the presence of carbohydrates are
based on the ability to (a) form furfural and its derivatives; and (b) reduce and form
characteristic compounds with reagents.
OBJECTIVE:
To be able to identify the different types of carbohydrates using the different specific
chemical tests.
MATERIALS:
Distilled water Barfoed’s reagent

3% solutions of glucose, xylose, fructose, Phenylhydrazine
lactose, sucrose, starch Iodine solution
Molisch’s reagent Conc. H2SO4
Bial’s Orcinol reagent Conc. HCl
Seliwanoff’s reagent 10% FeCl3
Benedict’s reagent
PROCEDURES
I. Test Based on the formation of furfural and its derivatives
1. Molisch’s Test
a. Mix 4 mL of distilled water and 2 drops of the Molisch’s reagent in a test tube.
This tube will serve as the control.
b. Place 4 mL of 3% solution of glucose in the second tube. Add 2 drops of the
Molisch reagent and mix the contents by gently shaking the test tube.
c. Incline the test tube and cautiously add about 5 mL conc. H2SO4, allowing the
acid to run down the side of the tube. Sulfuric acid is denser than water and will
form a lower layer. Note the color of the ring formed at the junction of the two
liquids.
d. In the same manner of adding acid, add sulfuric acid to the control tube. What do
you observe?
e. Repeat the above test with 3% sample solutions of xylose, lactose and starch.
f. Record all results.
Substance Tested Description of visible results

Control Acid is separated from the solution
Glucose Formed a dark layer which separates
the acid and solution.
The upper substance became turbid.
Xylose The superficial layer turned black, and
the precipitate formed a light pink
substance.
Lactose Formed a dark layer which separates
the acid and solution.
The upper substance became turbid.
Starch Formed a black substance as well as a
white precipitate.
Based on the results, which carbohydrate gave positive result with Molisch
reagent?
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2. Bial’s Orcinol Test

a. Place 1 mL each of 3% solution of xylose, glucose, and starch in separately-
labeled test tubes.
b. Add 3 mL of Bial’s reagent to each test tube.
c. Carefully heat each tube over the flame until the solution starts to boil. Add 1-2
drops of 10% FeCl3 solution.
d. Note the color of the product formed.
e. Record your results.
Substance Tested Results
Glucose Clear to Light Yellow
Xylose Clear to Yellow
Starch Clear to Darker Yellow
3. Seliwanoff’s Test
a. Place 1 mL each of 3% xylose, glucose, fructose and sucrose in separately-
labeled test tubes.
b. Add 4 mL of Seliwanoff’s reagent to each test tube.
c. Place the tubes in a water bath filled with boiling water and allow them to stay
there for exactly 1 minute.
d. Note the changes and record which tube gives a positive result in the shortest
time.
e. Continue heating and observe the color changes at 1-minute intervals. (itry ng
itry)
f. Record the time required for a positive test for each sample.
Substance Tested Time Result Explanation

Xylose No Reaction Negative
Glucose No Reaction Negative
Fructose 3 minutes Positive
Sucrose 5 minutes Positive
II. Tests based on the reducing property of sugars
1. Benedict’s Test
a. Place 1 mL each of 3% solutions of glucose, xylose, fructose, lactose and
sucrose in separately-labeled test tube. Add 5 mL of Benedict’s reagent in
each tube.
b. Place all the tubes in boiling water bath for 2 to 3 minutes.
c. Observe the color of the solution and note whether a precipitate was
formed. A change in color of the solution is not considered a positive
reaction.
d. Avoid prolonged heating
e. Record your results.
Substance Tested Result Explanation
Xylose Blue to Reddish Brown
Glucose Blue to Dark Brown
Fructose Blue to Red Orange
Sucrose Blue to Light Blue
Lactose Blue to Light Brown
2. Barfoed’s Test
a. Place 1 mL each of 3% solutions of glucose, xylose, fructose, lactose and
starch in separately-labeled test tubes.
b. Add 3 mL Barfoed’s solution in each test tube
c. Prepare a control tube using distilled water instead of sugar solution.
d. Place all the tubes in a boiling water bath for 10 minutes. Record your
observations
Control No formation of
precipitate
Xylose
Glucose
Fructose
Starch
Lactose
3. Tollen’s Test
a. Place 5 drops of 3% solutions of glucose, xylose and sucrose in separate
test tubes.
b. Add 2 mL of Tollen’s reagent into each test tube
c. Boil for about 5 minutes. Note and record your observations
Xylose Turned Silver
Glucose Turned Silver
Sucrose Turned Gray
4. Phenylhydrazine
a. Place 5 mL each of glucose, lactose and maltose in separate test tubes.
Add small amount of solid phenylhydrazine and 10 drops of sodium
acetate.
b. Heat the tubes and note the formation of precipitate.
c. Let it stand for and reserve the products for the next laboratory period.
d. Observe crystals under the microscope.
e. Draw the osazone crystals as seen under the microscope.
ENRICHMENT QUESTIONS
1. Will disaccharides and polysaccharides give positive result for Molisch test?
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2. Which of the different carbohydrate tests would give a positive result for maltose?
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3. Can Seliwanoff’s test be used to distinguish sucrose from fructose? Explain.

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4. Inulin is a polysaccharide composed entirely of fructose units. Which test should be

used to best identify the presence of fructose?
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GENERALIZATION:
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Name: Course & Year: Score:

Instructor: Group No: Date of Activity:

Carbohydrates include polyhydroxy aldehydes or ketones and their by-products. They

Distilled water Barfoed’s reagent

Substance Tested Description of visible results

2. Bial’s Orcinol Test

Substance Tested Time Result Explanation

3. Can Seliwanoff’s test be used to distinguish sucrose from fructose? Explain.

4. Inulin is a polysaccharide composed entirely of fructose units. Which test should be

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