What carbohydrate(s) would have the following test results?
Produces a reddish-orange solid with Benedict’s and a red color with Seliwanoff’s reagent in 1 minute.
Gives a color change with Benedict’s test, a light solar with Seliwanoff’s reagent after 5 minutes, and produces bubbles during fermentation.
Gives no color change with Benedict’s or Seliwanoff’s test, but turns a blue-black color with iodine reagent.
E. Hydrolysis of Disaccharides and Polysaccharides
How do the results of the Benedict’s test indicate that hydrolysis of sucrose and starch occurred?
The Correct Answer and Explanation is:
To determine which carbohydrate(s) match the described test results and to understand how the Benedict’s test indicates hydrolysis, let’s break down each statement and what it reveals:
Test Result 1
“Produces a reddish-orange solid with Benedict’s and a red color with Seliwanoff’s reagent in 1 minute.”
- Benedict’s Test: A reddish-orange precipitate indicates the presence of reducing sugars, typically monosaccharides like glucose or fructose.
- Seliwanoff’s Test: A red color in 1 minute strongly indicates the presence of a ketose sugar, such as fructose.
✅ Answer: Fructose
Test Result 2
“Gives a color change with Benedict’s test, a light color with Seliwanoff’s reagent after 5 minutes, and produces bubbles during fermentation.”
- Benedict’s Test: Color change suggests reducing sugar is present.
- Seliwanoff’s Test: A slow reaction suggests an aldose sugar or a disaccharide slowly hydrolyzing to a ketose.
- Fermentation: Bubbles show it can be fermented by yeast, meaning it’s a fermentable sugar.
✅ Answer: Glucose
Test Result 3
“Gives no color change with Benedict’s or Seliwanoff’s test, but turns a blue-black color with iodine reagent.”
- No reaction with Benedict’s or Seliwanoff’s: Not a reducing sugar or ketose.
- Blue-black with iodine: Classic indicator of starch, a polysaccharide.
✅ Answer: Starch
Hydrolysis of Disaccharides and Polysaccharides (E)
How do the results of the Benedict’s test indicate that hydrolysis of sucrose and starch occurred?
✅ Correct Answer:
Benedict’s test shows a color change (from blue to green/yellow/orange/red) after hydrolysis, indicating the formation of reducing sugars.
✅ Explanation (300+ words):
The Benedict’s test detects reducing sugars, which are carbohydrates with free aldehyde or ketone groups capable of reducing copper (II) ions in the reagent to copper (I), forming a colored precipitate. Normally, sucrose (a disaccharide composed of glucose and fructose) and starch (a polysaccharide made of glucose units) do not test positive with Benedict’s reagent because their glycosidic bonds prevent free aldehyde or ketone groups from being exposed.
However, when hydrolysis occurs—either through acid or enzymatic action—these glycosidic bonds are broken, releasing the monosaccharide units. In the case of sucrose, hydrolysis yields glucose and fructose, both of which are reducing sugars. Once free, these monosaccharides can now react with Benedict’s reagent, producing the characteristic red-orange precipitate. Thus, a previously negative Benedict’s test becomes positive, signaling that hydrolysis has occurred.
For starch, hydrolysis breaks the polysaccharide into maltose and eventually into glucose. As the large, non-reducing starch molecules break down into smaller, reducing sugars, a positive Benedict’s result appears. Before hydrolysis, starch only reacts with iodine, not Benedict’s. But after hydrolysis, Benedict’s test becomes positive, indicating the formation of glucose.
In conclusion, a color change in Benedict’s test after hydrolysis clearly indicates that non-reducing sugars like sucrose and starch have been converted into their reducing sugar components. This transformation is the key indicator of hydrolysis having occurred, making Benedict’s reagent a useful tool in identifying both the presence of reducing sugars and the successful breakdown of complex carbohydrates.