Question

Each of these four test tubes was prepared as follows and heated:

A) Albumin + Benedict’s Reagent

B) Water + Benedict’s Reagent

C) Starch + Benedict’s Reagent

D) Glucose + Benedict’s Reagent

Which tube would give a positive result?

Which tube is the negative control?

Answer 1

1. Which tube gives a positive result

Answer - Glucose + Benedict's Reagent

2. Which tube is a negative control

Answer - Water + Benedict's Reagent

Explanation -

Benedict's test is used to detect the presence of simple carbohydrates or reducing sugars. Simple carbohydrates includes monosaccharides like glucose and disaccharides like sucrose. These simple sugars have free ketone or aldehyde groups. Presence of complex carbohydrates like starch cannot be detected by this method because it is non reducing sugars.

Simple sugars like glucose are reducing sugars and it transfer hydogen atoms to other compounds by reduction. When these reducing sugars are mixed and heated with Benedict's reagent, reduction occurs and causes colour change.

Benedict's reagent has aqua-blue color. when it is heated with a reducing sugar, the color changes yellow to brick red. Yellow to bright yellow is moderate positive result, brick red is strong positive result.

Interpretation -

A. Albumin + Benedict's reagent = Albumin is a protein and it gives a negative result. The Benedict reagent only react with reducing sugars.

B. Water + Benedict's reagent = It is the negative control. It gives a negative test result because of the absence of sugar.

C. Starch + Benedict's reagent = Starch is a complex carbohydrate and a non- reducing sugar. So it do not react with Benedict's reagent and gives a negative result.

D. Glucose + Benedict's reagent = It gives a positive result because glucose is a reducing sugar. It react with Benedict's reagent and colour changes from yellow to brick red according to the amount of glucose. So it is a positive control for sugar test.