Question

Which of the following gives positive Fehling’s solution test?
(E) Sucrose
(F) Glucose
(G) Fats
(H) Protein

Answer 1

Hint: Fehling Test is used to distinguish between aldehydes and ketones. The output is oxidised $C{u_2}O$ which is red precipitate. Gives a positive test for a group of compounds in which there is one exception.

Complete step by step answer:
Oxidation reaction: loss of electrons is known as oxidation. A species undergoing oxidation is known as a reducing agent.
Reduction reaction: gain of electrons is known as reduction. A species undergoing reduction is known as an oxidizing agent.
The Fehling’s solution acts as oxidising agent and gives positive test for Aldehydes and negative test for ketones (except for alpha hydroxy ketones), thus gives positive test for all types of sugar, except sucrose.
The Glucose structure has an aldehyde group and due to which it gives a positive test for Fehling’s solution.

Thus, the right answer is (B) Glucose.

Additional information: Sucrose, protein and fats do not have an aldehyde group, so it gives a negative test for Fehling’s Solution. The reagent of Fehling solution which on reaction gives red brown precipitate of $C{u_2}O$ .
Fehling solution + Glucose(with aldehyde group) Red precipitate of Cuprous (I) oxide + Glucose (with acidic group)
The aldehyde is oxidised to the carboxylic group and Cupric (II) oxide is reduced and converted to Cuprous (I) oxide which is red brown precipitate.

Note:
Take care of other oxidising agents and reducing agents, along with Fehling’s solution. Sugar has an aldehyde group, thus they give positive tests and get converted to a carboxylic acid group. The solution after adding with Fehling’s reagent has to be heated a little.