Question

Both Glucose and Sucrose give Fehling’s test:
(A).True
(B).False

Answer 1

Hint: Glucose is a reducing sugar and it gives a positive result for Fehling’s test. As cane sugar is non-reducing sugar it does not give positive results for Fehling’s test. To differentiate ketone and carbohydrates which are water soluble, Fehling’s test is used.

Complete answer:
- One of the popular tests used for the estimation or detection of reducing sugars and non-reducing sugars is the Fehling’s test.
Solutions that are used in Fehling’s test should be prepared fresh in laboratories. Fehling’s test is conducted using two solutions; solution A and Solution B that are separate. Fehling’s solution A contains Copper(II) sulphate which has blue color. Fehling solution B is composed of Rochelle’s salt which is potassium sodium tartrate and sodium hydroxide.
 - Fehling solution A and B are prepared separately and stored. Both solutions are taken in equal volumes and mixed which results in Fehling's final solution having blue color. The deep blue ingredient is the bis(tartrate) complex of Cu2+. The tartrate tetra-anions serve as a chelating agent in the solution.
Procedure to perform Fehling’s test:
Add the sample in a dry test tube.Distilled water should be kept in another tube as control.
Fehling’s solution to be added in the tubes.The tubes must be kept in a water bath.
In case of development of red precipitate , observations to be made and recorded.
Notably, the result is positive if there is a formation of reddish brown precipitate while the result is negative if there is no indication of such change.
 -The most important monosaccharides glucose, act as reducing agents in alkaline solution. It is in open chain aldehyde or ketone form in alkaline solution. It reacts with Fehling's solution by reducing the copper(II) ions to copper(I) oxide and Thus, gives a positive result for Fehling’s test.
Sucrose does not give Fehling’s test positive and it does not have any aldehyde or ketone group free to undergo oxidation.

Hence the correct option is (B) False.

Note:
It is used to distinguish between aldehyde and ketone. Aldehydes are easily oxidized and give positive Fehling’s test. Ketones apart from alpha-hydroxy-ketones do not give positive Fehling’s test as they are not oxidized easily.Aldose monosaccharides and ketose monosaccharides both give positive Fehling’s test.Presence of glucose in urine can be determined using Fehling’s test. It helps to know whether the person is diabetic or not.