(MIDTERM) Biochemistry Lab - SEPTEMBER 12, 2025

A

Which term best describes the sugars (like glucose and fructose) that are detected by Benedict's test due to their free aldehyde or ketone group?

A. Reducing sugars

B. Structural polysaccharides

C. Non-reducing sugars

D. Complex lipids

A

The positive result for Benedict's test is the formation of a colored precipitate. What is the chemical composition of this precipitate?

A. Copper(I) oxide (Cu2​O)

B. Copper(II) sulfate (CuSO4​)

C. Copper(II) oxide (CuO)

D. Copper(I) sulfate (Cu2​SO4​)

A

Which of the following common disaccharides is specifically mentioned as a non-reducing sugar that would typically yield a negative result in Benedict's test unless first hydrolyzed?

A. Sucrose

B. Maltose

C. Galactose

D. Lactose

A

Barfoed's test is primarily used to differentiate which two classes of carbohydrates based on their rate of reduction?

A. Monosaccharides from disaccharides

B. Polysaccharides from disaccharides

C. Aldoses from ketoses

D. Reducing from non-reducing sugars

A

Monosaccharides react faster than disaccharides in Barfoed's test because the reagent is weakly acidic and has a lower concentration of which ion compared to Benedict's reagent?

A. Copper(II) ion (Cu^2+)

B. Sodium ion (Na+)

C. Chloride ion (Cl−)

D. Hydrogen ion (H+)

C

The Iodine Test detects which specific polysaccharide?

A. Cellulose

B. Glycogen

C. Starch

D. Chitin

C

In a positive Iodine test, the iodine molecules (I2​) fit inside the helical structure of which component of starch to produce the characteristic deep blue-black color?

A. Cellulose

B. Amylopectin

C. Amylose

D. Dextrin

D

What color indicates a positive result for the Iodine Test?

A. Brick-red precipitate

B. Cherry-red color

C. Yellowish-brown color

D. Deep blue-black or purple color

B

The Seliwanoff's Test differentiates sugars based on the presence of which two functional groups?

A. Carboxyl group vs. hydroxyl group

B. Ketone group (ketoses) vs. aldehyde group (aldoses)

C. Ester group vs. ether group

D. Amino group vs. sulfide group

D

The rapid positive result in Seliwanoff's test is a "cherry-red color" and indicates the presence of which type of sugar?

A. Aldose (e.g., glucose)

B. Disaccharide (e.g., maltose)

C. Polysaccharide (e.g., starch)

D. Ketose (e.g., fructose)

C

The principle of Seliwanoff's test involves the strong acid dehydrating the sugar to form which type of derivatives?

A. Alcohols

B. Aldehyde-ketone complexes

C. Furfural derivatives

D. Monosaccharide polymers

D

Which chemical compound is the most common positive control used for the Seliwanoff's test?

A. Glucose

B. Sucrose

C. Starch

D. Fructose

B

Molisch's Test is considered a general test for virtually all carbohydrates, including which three categories?

A. Aldoses, ketoses, and polysaccharides

B. Mono-, di-, and polysaccharides

C. Reducing, non-reducing, and complex sugars

D. Glucose, maltose, and starch

D

In the Molisch's Test, concentrated sulfuric acid (H2​SO4​) is used to perform what chemical reaction on the carbohydrate?

A. Oxidation

B. Hydrolysis

C. Reduction

D. Dehydration

C

What specific observable result indicates a positive outcome for the Molisch's Test?

A. Rapid formation of a brick-red precipitate

B. Color change to deep blue-black

C. A purple or violet ring forming at the interface of the two layers

D. Formation of a yellow solution

C

The Molisch's Test requires the careful addition of H2​SO4​ down the side of the test tube to achieve what specific experimental condition?

A. Neutralize the reagent

B. Induce boiling

C. Create distinct layers and prevent mixing

D. Speed up the dehydration process

B

Which key component is not one of the main reagents listed for Seliwanoff's Test?

A. Strong acid (HCl)

B. Cu^2+

C. Resorcinol

D. H2​O (implied solvent)

C

The overall purpose of the qualitative tests for carbohydrates is to identify unknown carbohydrates based on what two key observable changes?

A. Boiling point and solubility

B. Melting point and density

C. Color changes and precipitate formation

D. pH and viscosity

D

If an unknown sample is positive for Benedict's, negative for Barfoed's, and positive for Seliwanoff's, which sugar is it likely to be, according to the example provided?

A. Sucrose

B. Glucose

C. Starch

D. Fructose

C

Which reagent listed requires the most caution due to its safety hazard, specifically mentioned in the exam reminders?

A. Iodine solution

B. Benedict's reagent

C. Concentrated sulfuric acid (H2​SO4​)

D. Resorcinol