Carbohydrate Identification Chemical Tests, Notes, Lecture and MCQ with Answer
Carbohydrate Identification Chemical Tests
Carbohydrates can be identified using several chemical tests based on their structural characteristics. These tests help differentiate between monosaccharides, disaccharides, and polysaccharides.
1. Molisch’s Test (General Test for Carbohydrates)
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Principle: Detects carbohydrates by dehydrating them into furfural derivatives, which react with α-naphthol to form a purple ring.
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Reagents: α-naphthol and concentrated sulfuric acid.
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Positive Result: Formation of a purple or violet ring at the junction of the liquids.
2. Benedict’s Test (For Reducing Sugars)
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Principle: Reducing sugars (with free aldehyde or ketone groups) reduce Cu²⁺ ions to Cu₂O, forming a colored precipitate.
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Reagents: Benedict’s reagent (copper sulfate, sodium carbonate, and sodium citrate).
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Positive Result: Color change from blue → green → yellow → orange → red, depending on sugar concentration.
3. Fehling’s Test (For Reducing Sugars)
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Principle: Similar to Benedict’s test, reducing sugars reduce Fehling’s solution (Cu²⁺) to form Cu₂O precipitate.
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Reagents: Fehling’s A (copper sulfate) and Fehling’s B (alkaline tartrate).
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Positive Result: Formation of a red or brick-red precipitate.
4. Barfoed’s Test (For Monosaccharides)
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Principle: Differentiates monosaccharides from disaccharides based on the rate of reduction of Cu²⁺.
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Reagent: Barfoed’s reagent (copper acetate in acetic acid).
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Positive Result: Red precipitate forms within 2–5 minutes (monosaccharides), while disaccharides react more slowly.
5. Seliwanoff’s Test (For Ketose Sugars)
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Principle: Ketoses (e.g., fructose) dehydrate faster than aldoses, producing a red condensation product.
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Reagent: Resorcinol in hydrochloric acid.
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Positive Result: Rapid red color formation (ketoses like fructose); slower reaction for aldoses.
6. Bial’s Test (For Pentoses)
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Principle: Pentoses form furfural derivatives, which react with orcinol and ferric chloride to form a green or blue color.
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Reagent: Orcinol, hydrochloric acid, ferric chloride.
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Positive Result: Green or blue color indicating pentose presence.
7. Iodine Test (For Starch)
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Principle: Starch forms a complex with iodine, producing a characteristic color.
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Reagent: Iodine solution.
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Positive Result: Blue-black color (starch), brownish color for glycogen.
Summary Chart of Carbohydrate Identification Tests
| Test Name | Target Carbohydrate | Reagents Used | Positive Result |
|---|---|---|---|
| Molisch’s Test | All carbohydrates | α-Naphthol & H₂SO₄ | Purple/Violet ring |
| Benedict’s Test | Reducing sugars | Benedict’s reagent | Green → Red ppt. |
| Fehling’s Test | Reducing sugars | Fehling A & B | Red ppt. |
| Barfoed’s Test | Monosaccharides | Barfoed’s reagent | Red ppt. (fast) |
| Seliwanoff’s Test | Ketose sugars | Resorcinol + HCl | Red color |
| Bial’s Test | Pentoses | Orcinol, FeCl₃, HCl | Green/Blue color |
| Iodine Test | Starch | Iodine solution | Blue-Black color |
1. Which test is used as a general test for all carbohydrates?
A) Benedict’s Test
B) Molisch’s Test
C) Fehling’s Test
D) Iodine Test
Answer: B) Molisch’s Test
2. Which reagent is used in Benedict’s test for reducing sugars?
A) Orcinol and FeCl₃
B) Copper sulfate, sodium carbonate, and sodium citrate
C) α-Naphthol and sulfuric acid
D) Iodine solution
Answer: B) Copper sulfate, sodium carbonate, and sodium citrate
3. A red precipitate in Fehling’s test indicates the presence of:
A) Non-reducing sugars
B) Reducing sugars
C) Starch
D) Pentoses
Answer: B) Reducing sugars
4. Which test is used to differentiate between monosaccharides and disaccharides?
A) Barfoed’s Test
B) Benedict’s Test
C) Seliwanoff’s Test
D) Bial’s Test
Answer: A) Barfoed’s Test
5. What is the positive result for the Iodine test in starch detection?
A) Green color
B) Blue-black color
C) Red precipitate
D) Purple ring
Answer: B) Blue-black color
6. Which test is specifically used to detect pentose sugars?
A) Barfoed’s Test
B) Seliwanoff’s Test
C) Bial’s Test
D) Fehling’s Test
Answer: C) Bial’s Test
7. Seliwanoff’s test differentiates between which two types of sugars?
A) Aldoses and Ketoses
B) Reducing and Non-reducing sugars
C) Monosaccharides and Polysaccharides
D) Starch and Glycogen
Answer: A) Aldoses and Ketoses
8. In Barfoed’s test, how fast do monosaccharides react compared to disaccharides?
A) More slowly
B) At the same rate
C) Faster
D) Do not react at all
Answer: C) Faster
9. Which test involves α-naphthol and sulfuric acid to confirm carbohydrate presence?
A) Molisch’s Test
B) Benedict’s Test
C) Iodine Test
D) Barfoed’s Test
Answer: A) Molisch’s Test
10. A positive result in Seliwanoff’s test gives which color change?
A) Blue
B) Red
C) Green
D) Yellow
Answer: B) Red