Question

22 Pre-Lab Study Questions

1. What happens to glucose or galactose when the Cu²⁺ in Benedict’s reagent is reduced?

2. Would you expect fructose or glucose to form a red color rapidly with Seliwanoff’s reagent? Explain your answer.

3. Why don’t all the disaccharides undergo fermentation with yeast?

4. How can the iodine test be used to distinguish between amylose and glycogen?

Answer 1

1. Gluconic acid is formed when Cu2+ Benedict's reagent is reduced by glucose or galactose.Cu2+ is blue, it forms a red precipitate, Cu2O, a Cu+ compound. Copper is reduced from +2 to +1. That means the sugar was oxidized. The aldehyde oxidized to a carboxyl group
2. Fructose will form a red color more rapidly with Seliwanoff's since it is a ketose  Seliwanoff's agent is most sensitive to ketose. Glucose, which is an aldose will develop slowly giving off a pink color.
3. Fermentation is due to enzymes. Yeast does not contain all the specific enzymes to breakdown all the disaccharides.
4. The iodine test is used to detect whether a substance contains starch or not. If a substance contains.amylose is a type of starch hence it shows the iodine test positive . And glycogen is not starch hence it shows the iodine test negative, hence iodine test be used to distinguish between amylose and glycogen.