10 Questions
What does a change in color from deep blue to orange or red indicate in the experiment described?
The presence of monosaccharides
In Barfoed’s Test, if a red precipitate forms within two minutes, what type of carbohydrate is likely present?
Monosaccharide
What does a bluish black color indicate in the Iodine Test for carbohydrates?
Presence of starch
Why is iodine used in the Iodine Test for carbohydrates?
To interact with coiled molecules of glucose
What is the purpose of adding Fehling A & Fehling B in the experiment procedure?
To detect the presence of carbohydrates
What is the composition of Molisch's reagent used in the qualitative test for carbohydrates?
Alpha-naphthol dissolved in ethanol
In Molisch's test, what causes the formation of a violet or purple colored complex?
Dehydration of carbohydrates to furfural compounds
What is the purpose of adding concentrated sulphuric acid in Molisch's test for carbohydrates?
To dehydrate carbohydrates to furfural compounds
What observation indicates a positive result in Molisch's test for carbohydrates?
Appearance of a violet colored ring at the junction between two layers
Which type of sugars can reduce cupric ions in alkaline medium to produce cuprous ions?
Sugars with free aldehyde groups
Study Notes
General Test for Carbohydrates (Molisch's Test)
- Molisch's reagent is composed of alpha-naphthol (C10H8OH) dissolved in ethanol (C2H5OH).
- The principle involves concentrated sulphuric acid dehydrating carbohydrates to form "furfural" compounds, which react with α-naphthol to produce a violet or purple colored complex.
- The procedure involves adding 2 ml of the test carbohydrate solution and 2 drops of α-naphthol solution, followed by careful addition of 1 ml of concentrated H2SO4.
- A "violet colored ring" appears at the junction between the two layers, indicating the presence of carbohydrates.
Reactions of Reducing Sugars
- Sugars with free aldehyde groups can reduce either cupric ions in alkaline medium or silver ions in alkaline medium.
- Fehling's reagent produces a red or orange colored precipitate of cuprous ions, while Tollen's reagent produces a silver mirror.
- Heating in a boiling water bath is necessary for these reactions.
Fehling's Test
- Fehling's reagent is composed of Fehling A (CuSO4 dissolved in dil.H2SO4) and Fehling B (sodium potassium tartarate dissolved in dil.NaOH).
- The procedure involves heating the test carbohydrate solution with Fehling A and B, and observing for a change in color from deep blue (cupric ion) to orange or red (cuprous oxide).
Barfoed's Test
- Barfoed's reagent is prepared by adding a 0.33 molar solution of neutral cupric (II) acetate to a 1% acetic acid solution.
- A red precipitate formed within two minutes indicates the presence of a monosaccharide, while a red precipitate formed after ten minutes of heating indicates the presence of a disaccharide.
Iodine Test
- The basis of the test is that starch is a coiled polymer of glucose, which reacts with iodine to produce a bluish black color.
- A bluish black color indicates a positive test for starch, while a yellow-ish brown color (no color change) indicates a negative test for starch.
- The iodine test reagent is composed of iodine dissolved in an aqueous solution of potassium iodide.
This quiz covers the composition of Molisch reagent, the principle behind Molisch's test for carbohydrates, and the procedure to conduct the test. Learn about how concentrated sulphuric acid interacts with carbohydrates to form furfural compounds, resulting in a distinctive violet or purple colored complex with alpha-naphthol.
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