The Molisch Test is a qualitative test meant for assessing the presence of carbohydrates in solution
via an acidic dehydration reaction of the carbohydrate into a furfural or a furfural derivative
followed by a condensation reaction with the α-naphthol in the Molisch Reagent to produce a
compound with a purple hue. Virtually all classes of carbohydrates are expected to produce
positive results in the form of a reddish violet or purple ring at junction in the solution.
Figure 1. The dehydration reaction of D-glucose with an acid into 5-(hydroxymethyl)furfural.
As seen in Figure 1 above, the hexose sugar D-glucose was reduced to form 5(hydroxymethyl)furfural. The Molisch Reagent furthers the reaction of the furfural derivative,
producing a solution with trace purple hues.
Upon performing the Molisch Test on the kamote, rice, and potato samples, the students observed
the formation of a purple layer with all the solutions. This was the anticipated result since the
samples were all known to contain the polysaccharide starch, which is simply a chain of glucose
molecules monomers bonded at the α-(1,4) linkage. Referring to the reaction in Figure 1 above, it
is expected that glucose will undergo a dehydration reaction with the concentrated H2SO4 to form
the furfural derivative 5-(hydroxymethyl)furfural, and then condense with the α-naphthol to form
the purple dye.
The Benedict’s Test is another qualitative test for the presence of carbohydrates in solution via a
reduction reaction with simple sugars. This test is done specifically to detect the presence of
reducing sugars, which are a class of sugars that can act as reducing agents due to the presence of
aldehyde and ketone functional groups. The presence of reducing sugars in solution will produce
a brick red solution from a clear blue solution due to the oxidation of copper citrate into copper
(II) oxide as seen in the reaction in Figure 2.
Figure 2. The reaction of an aldose with Benedict’s Reagent (copper citrate) to produce copper
(II) oxide and a carboxylate.
Upon performing the Benedict’s Test on the test solutions (Fructose, Glucose, Glucose, Lactose,
Maltose, and Sucrose), all test solutions were found to produce the brick-red solution over time
except for sucrose. If one observes the structure of the above listed sugars, sucrose is the only
carbohydrate that was incapable of acting as a reducer. To reiterate, the Benedict’s Test is used to
assess the presence of reducing sugars within a solution. A lack of functional groups that can act
as reducing agents in sugars will yield know reaction with the Benedict’s Reagent. As for the
kamote, rice, and potato samples, the students observed that only the kamote sample reacted to the
Benedict’s Reagent. It was inferred that the rice and potato samples had no observable reactions
with the Benedict’s reagent because rice and potato are composed primarily of starch. Starch, while
still a sugar, does not readily function as a reducing agent, thus the lack of reaction. While kamote
contains starch, a significant amount of reducing sugars are still present in its composition (i.e.,
glucose, fructose), hence the observed reaction with the Benedict’s reagent.
The Barfoed’s Test is a qualitative test primarily used for the detection of monosaccharides via a
reduction reaction of copper (II) acetate. The aldehyde in the monosaccharide is oxidized into a
carboxylate upon reaction with copper (II) acetate. A side product of the reaction is copper (I)
oxide, which gives the solution its reddish hue.
Figure 3. The reaction of an aldose with copper ions from copper (II) acetate to produce a
carboxylate and copper (II) oxide.
Upon performing the Barfoed’s Test on the test solutions (Fructose, Glucose, Glucose, Lactose,
Maltose, and Sucrose), only Fructose, Galactose, and Glucose were observed to produce the
reddish solution. The three sugars listed above were inferred to react with the Barfoed’s Reagent
because Fructose, Galactose, and Glucose are monosaccharides that have free carbonyl groups
which can act as reducing agents for the copper (II) acetate. As for the kamote, rice, and potato
samples, the students observed that only the rice sample reacted to the Benedict’s Reagent. It was
inferred that the kamote and potato samples had no observable reactions with the Benedict’s
reagent because rice and potato are composed primarily of starch. Starch, while still a sugar, does
not readily function as a reducing agent, thus the lack of reaction. While kamote contains starch, a
significant amount of reducing sugars are still present in its composition (i.e., glucose, fructose),
hence the observed reaction with the Benedict’s reagent.
Seliwanoff’s Test is a qualitative test for distinguishing between aldose and ketose sugars. This
test relies on the principle that ketose sugars undergo dehydration reaction faster than aldose sugars
upon addition of Seliwanoff’s Reagent and exposure to heat. The presence of ketose sugars in
solution is evidenced by the color transition of the solution from clear to bright red after subjecting
the solution to heat. Aldose sugars will have a less pronounce color transition as compared to that
of ketose sugars.
Figure 4. The reaction of Seliwanoff Reagent to a sugar to produce a compound with a red hue.
As observed in Figure 4 above, the sugar is first hydrolyzed by an acid to yield a furfural
derivative, followed by a condensation reaction with the resorcinol in the Seliwanoff Reagent,
ultimately producing a compound with a bright red hue.
Upon performing Seliwanoff’s Test on the test solutions (Fructose, Glucose, Glucose, Lactose,
Maltose, and Sucrose), only Sucrose and Fructose were observed to produce the reddish solution.
Sucrose and Fructose are the only sugars listed above that contain ketone functional groups, hence
their reaction relatively more rapid color change compared to the other sugars which contained
aldehyde functional groups.
The Bial’s Test is a qualitative test meant to determine the presence of five-carbon sugars in
solution via the acid-catalyzed dehydration of the sugar into a furfural derivative, followed by a
condensation reaction to form a colored product. If pentose sugars are present in solution, the
solution will turn bluish with some precipitate formation.
Figure 5. The reaction of Bial’s Reagent to a sugar to produce a compound with a bluish hue.
Upon performing the Bial’s Test on the test solutions (Fructose, Glucose, Glucose, Lactose,
Maltose, and Sucrose), only Lactose was observed to react and change into a bluish green solution.
Observing the structures of the above listed sugars, Lactose is the only five-carbon sugar among
them. This would explain why it was the only one to show apparent color changes upon addition
of the Bial’s Reagent.
The Iodine Test is a qualitative test used to determine the presence of starch in a sample as
evidenced in the color change of a test solution due to intermolecular complexes. If starch is
present in solution, the solution transitions from brown to dark-blue.
Figure 6. The formation of a charge transfer complex for iodine and amylose which is made up
of α-D-Glucose units
As seen Figure 6 above, triiodide is generated in the presence of starch, which allows the starch
to form a charge complex with the triiodide. This complex changes the color of the solution from
brown to dark blue with some observable precipitates.
Upon performing the Iodine Test on the kamote, rice, potato, and starch samples, it was observed
that all samples produced the dark-blue solution with varying intensities in color and amounts of
precipitate. This was the inferred result, since all the above samples contained a significant amount
of starch in them. The formation of the precipitate as well as the intensity of the blue color would
give some idea how much starch a sample contained. (i.e., the starch sample was observed to have
the most intense color, the potato sample had dark blue precipitates).