Qualitative Tests of Carbohydrates
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Questions and Answers

What type of test is Molisch's test classified as?

  • General test for all carbohydrates (correct)
  • Specific test for monosaccharides
  • Specific test for reducing sugars
  • None of the tests for carbohydrates

Which sugars are classified as reducing sugars?

  • Starch and glycogen
  • Glucose and cellulose
  • Glucose and fructose (correct)
  • Fructose and sucrose

Which carbohydrate is not classified as a sugar?

  • Sucrose
  • Cellulose (correct)
  • Fructose
  • Glucose

What determines whether a monosaccharide is classified as an aldose or a ketose?

<p>The presence of a keto or aldehyde group (C)</p> Signup and view all the answers

What indicates the presence of carbohydrates when using alcoholic alpha naphthol?

<p>Formation of a reddish-violet ring (B)</p> Signup and view all the answers

Which of the following substances is known to function as a storage polysaccharide?

<p>Glycogen (D)</p> Signup and view all the answers

What color change is observed in the Anthrone test for carbohydrates?

<p>Bluish green (A)</p> Signup and view all the answers

What is the common molecular ratio of hydrogen to oxygen in carbohydrates?

<p>2:1 (D)</p> Signup and view all the answers

Which reagent is used in the Iodine test for detecting polysaccharides?

<p>Iodine solution with potassium iodide (A)</p> Signup and view all the answers

Which test is NOT used to detect the reducing properties of sugars?

<p>Dabrowski's test (D)</p> Signup and view all the answers

Which of the following statements is true about carbohydrates?

<p>Carbohydrates can act as structural components (A)</p> Signup and view all the answers

What happens if concentrated organic solutions are tested with sulfuric acid?

<p>A red color may indicate char formation (B)</p> Signup and view all the answers

Which of the following does NOT form a complex with iodine during testing?

<p>Cellulose (D)</p> Signup and view all the answers

The reaction between furfural and which agent results in a colored complex during the Anthrone test?

<p>Anthrone (A)</p> Signup and view all the answers

What is the role of potassium iodide in the iodine reagent?

<p>To enhance iodine solubility in water (B)</p> Signup and view all the answers

In the Iodine test, a blue color indicates the presence of which carbohydrate?

<p>Starch (D)</p> Signup and view all the answers

What color indicates the presence of ketoses in the Seliwanoff's test?

<p>Fiery red (A)</p> Signup and view all the answers

What is detected by Fehling's test?

<p>Reducing sugars (D)</p> Signup and view all the answers

Which of the following statements about Seliwanoff's reagent is false?

<p>It is effective at detecting non-reducing sugars. (C)</p> Signup and view all the answers

What happens to glucose when prolonged boiling occurs in Seliwanoff's test?

<p>It is converted to fructose. (C)</p> Signup and view all the answers

What role does Rochelle salt play in Fehling's test?

<p>It serves as a chelating agent. (B)</p> Signup and view all the answers

How does Benedict's test differ from Fehling's test?

<p>Benedict's reagent is more stable. (A)</p> Signup and view all the answers

What indicates a positive result in Benedict's test?

<p>Appearance of a red precipitate. (D)</p> Signup and view all the answers

What is the primary visual indicator of reducing sugars during Fehling's test?

<p>Brownish red precipitate (B)</p> Signup and view all the answers

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Flashcards

Seliwanoff's Test

A test that distinguishes between ketoses and aldoses. Ketoses, like fructose, produce a fiery red color, while aldoses, like glucose, give a weak and slow reaction.

Fehling's Test

A chemical test used to detect the presence of reducing sugars. These sugars have a free aldehyde or ketone group that can reduce copper sulfate to cuprous oxide.

Rochelle Salt

A chelating agent used in Fehling's test to bind to copper ions and prevent interference in the reaction.

Cuprous Oxide

The visible result of a positive Fehling's test. It's a yellow or brownish red precipitate that indicates the presence of reducing sugars.

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Benedict's Test

A test used to detect the presence of reducing sugars. It's more convenient than Fehling's test and uses sodium citrate as a chelating agent.

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Sodium Citrate

A chelating agent used in Benedict's test to bind to copper ions and prevent interference in the reaction.

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Red Precipitate

The visible result of a positive Benedict's test. A red precipitate that indicates the presence of reducing sugars.

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Reducing Sugars

Sugars with a free aldehyde or ketone group that can reduce copper sulfate to cuprous oxide.

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Carbohydrate Classification

Carbohydrates are classified based on the number of sugar units they have.

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Monosaccharides

Monosaccharides are the simplest sugars, like glucose and fructose.

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Disaccharides

Disaccharides are formed when two monosaccharides combine, like sucrose and lactose.

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Polysaccharides

Polysaccharides are complex carbohydrates with many sugar units, like starch and cellulose.

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Non-reducing Sugars

Sugars without a free aldehyde or ketone group are called non-reducing sugars, like sucrose.

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Molisch's Test

The Molisch test is a general test for all carbohydrates.

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Tests for Reducing Sugars

Fehling's test, Benedict's test, and Tollen's test are used to identify reducing sugars.

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How does Molisch's test work?

This test relies on the dehydration of carbohydrates by strong acid to form furfural derivatives, which then react with alpha-naphthol.

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What causes the color change in Molisch's test?

The furfural derivatives produced by the reaction with concentrated sulfuric acid react with alpha-naphthol to form a characteristic reddish-violet colored complex.

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Anthrone Test

A chemical test that utilizes anthrone to detect the presence of carbohydrates.

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How does the Anthrone test work?

Anthrone reacts with furfural derivatives, produced by the dehydration of carbohydrates in the presence of strong acid, to form a bluish-green colored complex.

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Iodine Test

A simple test that uses iodine solution to detect the presence of polysaccharides, such as starch, glycogen, and dextrins.

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What colors are produced in the iodine test?

Starch forms a blue complex with iodine, while glycogen forms a reddish-brown complex. This difference in color makes the iodine test useful for distinguishing between these two polysaccharides.

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What is the basis of the iodine test?

The iodine test is based on the ability of iodine to form colored adsorption complexes, not actual chemical bonds, with polysaccharides.

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Study Notes

Qualitative Tests of Carbohydrates

  • Carbohydrates are essential components of food, primarily obtained as starch from plants.
  • In the body, glucose is utilized or stored as glycogen.
  • Carbohydrates consist of carbon, hydrogen, and oxygen atoms, usually in a 2:1 ratio to hydrogen as in water.
  • Classified into monosaccharides (cannot be further hydrolyzed), oligosaccharides (hydrolyze into 2-10 monosaccharides), and polysaccharides (hydrolyze into numerous monosaccharides).
  • Monosaccharides are further categorized by the number of carbon atoms (e.g., hexose) and functional group (aldose or ketose).
  • Sugars are either reducing (e.g., glucose, fructose) or non-reducing (e.g., sucrose).
  • Reducing properties of sugars are tested using Fehling's, Benedict's, and Tollen's tests.
  • Molisch's test, Anthrone test, Iodine test, Seliwanoff's test, Picric acid test, Mucic acid test, Bial's test, and Osazone test are specific tests for different carbohydrate types.

Molisch's Test

  • A general test for carbohydrates.
  • Concentrated sulfuric acid reacts with carbohydrates to produce furfural derivatives
  • These derivatives react with α-naphthol to form a reddish-violet ring.
  • Polysaccharides and glycoproteins exhibit positive results.

Anthrone Test

  • Another general test for carbohydrates.
  • Furfural produced in the reaction with concentrated sulfuric acid reacts with anthrone.
  • Results in bluish-green complex formation.

Iodine Test

  • Specifically tests for starch and related substances like glycogen.
  • Starch reacts with iodine to form a dark blue/black color.
  • Glycogen exhibits reddish-brown color with iodine.

Seliwanoff's Test

  • Differentiates between aldoses and ketoses.
  • Ketoses, particularly fructose, react rapidly to yield a reddish-colored complex.
  • Aldoses, such as glucose, react more slowly to produce a weaker or negative result.

Fehling's and Benedict's Tests

  • Detects reducing sugars.
  • Reducing sugars reduce copper ions in Fehling's and Benedict's solutions.
  • This reduction is visually detected by a red or orange precipitate.
  • Benedict's test is considered more convenient and stable than Fehling's.

Picric Acid Test

  • Detects reducing sugars.
  • Reducing sugars react with picric acid in alkaline medium to form a red colored picramic acid.

Bial's Test

  • Specific for pentose sugars.
  • Reaction yields a blue-green colored complex in the presence of pentoses.

Osazone Test

  • Distinguishes different sugars by forming specific crystal structures.
  • Identifies reducing sugars.

Tollen's Test

  • Identifies reducing sugars using ammoniacal silver nitrate solution
  • Reducing sugars form a silver mirror on the test tube walls.

Reduction Tests (Including Phenylhydrazine)

  • Carbohydrates with aldehyde/ketone groups reduce metallic ions in solution.
  • Tests help confirm reducing sugar identities.

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Description

Explore the qualitative tests used to identify different types of carbohydrates, including sugars. This quiz covers classifications such as monosaccharides, oligosaccharides, and polysaccharides, as well as various testing methods like Molisch's test and Benedict's test. Test your knowledge of carbohydrate chemistry and its importance in biology.

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