Fehling solution A and Fehling solution B are:

Aqueous \(CuSO_4\) solution and alkaline solution of sodium potassium tartarate.

The correct answer is option 2. Aqueous \(CuSO_4\) solution and alkaline solution of sodium potassium tartarate.

Fehling's solution is used to test for reducing sugars, such as glucose, and consists of two separate solutions that are mixed just before use. Here is a detailed breakdown:

Fehling’s Solution Components

Fehling Solution A:

Composition: Aqueous copper(II) sulfate (\(CuSO_4\)).

Role: Provides \(Cu^{2+}\) ions, which are blue in color. These ions are part of the test reagent that reacts with reducing sugars.

Fehling Solution B:

Composition: An alkaline solution of sodium potassium tartrate (also known as Rochelle salt), often combined with sodium hydroxide (\(NaOH\)).

Role:

Tartrate Ions: The tartrate ions form a complex with copper(II) ions, preventing them from precipitating out as copper(II) hydroxide (\(Cu(OH)_2\)) in the alkaline medium.

Alkaline Medium: The sodium hydroxide creates an alkaline environment necessary for the reaction.

Reaction Mechanism

When Fehling's solutions A and B are mixed, the following occurs:

Formation of Complex:

In the mixture, the copper(II) ions from Fehling Solution A are complexed with the tartrate ions from Fehling Solution B. This complex prevents the copper(II) ions from forming insoluble copper(II) hydroxide (\(Cu(OH)_2\)) in the alkaline environment.

Reaction with Reducing Sugars:

When a reducing sugar (like glucose) is present, it reduces the copper(II) ions to copper(I) ions (\(Cu^+\)). The reaction is:

\(\text{R-OH (reducing sugar)} + 2\text{Cu}^{2+} + 2\text{OH}^- \rightarrow \text{R-CHO (oxidized sugar)} + \text{Cu}_2\text{O (red precipitate)} + \text{H}_2\text{O}\)

The copper(I) oxide (\(Cu_2O\)) formed is red or brick-red, which is a clear indication of a positive test for reducing sugars.

Summary

Fehling Solution A is the aqueous copper(II) sulfate solution, and Fehling Solution B is the alkaline solution of sodium potassium tartrate. When these two solutions are mixed and a reducing sugar is tested, the color change to red or brick-red indicates the presence of reducing sugars.

Thus, the correct composition of Fehling’s solutions is: 2. Aqueous \(CuSO_4\) solution and alkaline solution of sodium potassium tartarate.