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Questions and Answers
How is the iodine test used to differentiate between simple sugars and polysaccharides?
The iodine test involves adding a few drops of iodine to the carbohydrate solution. If there is no change in the yellow color of iodine, it indicates the presence of simple sugars (mono- and disaccharides). If a blue-black color appears, it indicates the presence of polysaccharides, such as starch.
What is the principle behind Molisch's test for carbohydrates?
In the presence of concentrated sulfuric acid (H2SO4), sugars are dehydrated to form furfural (in pentoses) or hydroxymethylfurfural (in hexoses), which then condenses with -naphthol to produce colored condensation products.
What is the purpose of Fehling and Benedict test in carbohydrate analysis?
The Fehling and Benedict tests are used to differentiate between reducing and non-reducing sugars. All monosaccharides and disaccharides are reducing sugars, except sucrose, which is non-reducing. The tests involve the use of specific reagents to detect the presence of reducing sugars based on the formation of red precipitate.
What are the components of Fehling's reagent and Benedict's reagent?
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How does the presence of reducing sugars lead to the formation of red precipitate in Fehling and Benedict test?
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Study Notes
Carbohydrate Analysis
- Iodine test is used to differentiate between simple sugars and polysaccharides by forming a blue-black complex with starch (polysaccharide) but not with simple sugars.
- Molisch's test is used to detect the presence of carbohydrates, based on the principle that carbohydrates dehydrate in the presence of HCl to produce an aldehyde, which reacts with α-naphthol to produce a purple complex.
Reducing Sugars Test
- Fehling's test and Benedict's test are used to detect the presence of reducing sugars in carbohydrates.
- The purpose of these tests is to identify the presence of reducing sugars, which can reduce Cu(II) ions to Cu(I) ions.
Components of Fehling's and Benedict's Reagents
- Fehling's reagent consists of copper(II) sulfate and sodium hydroxide.
- Benedict's reagent consists of copper(II) sulfate, sodium hydroxide, and sodium carbonate.
Formation of Red Precipitate
- In Fehling's and Benedict's test, the presence of reducing sugars leads to the formation of a red precipitate due to the reduction of Cu(II) ions to Cu(I) ions, which react with hydroxide ions to form a red cuprous oxide precipitate.
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Description
Test your knowledge of carbohydrate qualitative tests such as Molisch’s test and Iodine test. Learn about the principles behind these tests and their applications in differentiating between simple sugars.
