Problem 35 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

This question contains Statement-1 (Assertion) and Statement-2 (Reason) and has 4 choices (a), (b), (c) and (d) out of which ONLY ONE is correct. Statement-1 : Glucose gives a reddish-brown precipitate with Fehling's solution. because Statement-2 : Reaction of glucose with Fehling's solution give \(\mathrm{CuO}\) and gluconic acid. [2007] (a) Statement-1 is True, Statement- 2 is True; Statement-2 is a correct explanation for Statement-1 (b) Statement-1 is True, Statement- 2 is True; Statement-2 is not a correct explanation for Statement-1 (c) Statement-1 is True, Statement- 2 is False (d) Statement- 1 is False, Statement- 2 is True.

Step-by-Step Solution

Answer

Option (c) is correct.

Step 1: Understanding Statement-1

Statement-1 states that glucose gives a reddish-brown precipitate when treated with Fehling's solution. Fehling's solution is used to test for reducing sugars, and glucose is indeed a reducing sugar. The reaction involves reducing the copper(II) ions in Fehling's solution to copper(I) oxide (Cu2O), which is a reddish-brown precipitate. Therefore, Statement-1 is true.

Step 2: Analyzing Statement-2

Statement-2 claims that the reaction of glucose with Fehling's solution yields CuO and gluconic acid. CuO refers to copper(II) oxide, which is black, not copper(I) oxide (Cu2O), which is formed in the actual reaction and is reddish-brown. The claim that CuO is formed is incorrect. The correct reaction involves the formation of Cu2O, not CuO. Therefore, Statement-2 is false.

Step 3: Evaluating Choices

Given that Statement-1 is true and Statement-2 is false, we need to find the option that matches this assessment. Option (c) states: "Statement-1 is True, Statement- 2 is False." This correctly aligns with our analysis of the statements.

Key Concepts

Assertion and Reason Questions

Assertion and Reason Questions are a common format used in exams like JEE to assess a student’s ability to understand and connect scientific concepts.
These questions provide two statements: an Assertion (Statement-1) and a Reason (Statement-2). The student must evaluate both statements and their relationship.
Typically, this involves determining:

• Whether both statements are true.
• If the reason correctly explains the assertion.
• Whether one or both statements are false.
• If deemed true, whether the reason logically explains the assertion.

Such questions test a student’s critical thinking and ability to link cause and effect. To answer effectively, understanding the fundamental concepts behind each statement is key. In this exercise, for instance, knowing the characteristics of Fehling’s solution and its reaction with glucose helps deduce the correctness of each statement.

Fehling's Solution

Fehling's Solution is a chemical reagent used to distinguish between reducing and non-reducing sugars. Named after Hermann Fehling, it comprises two separate solutions, usually referred to as solution A and B.

• Solution A contains copper(II) sulfate, providing the distinct blue color.
• Solution B consists of a mixture of sodium potassium tartrate (Rochelle salt) and sodium hydroxide.

When these solutions are mixed for a test, the copper(II) ions ( ext{Cu}^{2+}) in the solution react with reducing sugars to produce a color change. If a reducing sugar is present, it reduces the copper(II) ions to copper(I) oxide ( ext{Cu}_{2} ext{O}), forming a red or reddish-brown precipitate.

This precipitate is crucial for visually confirming the sugar’s presence. Fehling’s solution is quite sensitive, meaning it can detect even small amounts of reducing sugars effectively.

Reducing Sugars Test

The Reducing Sugars Test, often done using Fehling's solution, is essential for identifying sugars that can act as reducing agents. Reducing sugars contain free aldehyde or ketone groups.

In the context of organic chemistry, sugars like glucose, fructose, and lactose can reduce metal ions when they oxidize themselves. During the test, a change typically from a blue solution to a brick-red or reddish-brown precipitate indicates a positive result because the sugar reduces copper(II) ions to copper(I) oxide.
This process involves:

• The sugar acting as a reducing agent.
• The blue copper(II) sulfate being reduced to red copper(I) oxide.
• The reaction being specific mostly to monosaccharides and some disaccharides.

Understanding this reaction is pivotal as it underpins several analytical techniques used in laboratories and industries to test sugar content in various substances.

Copper Oxide Precipitate

In the Reducing Sugars Test, the formation of a Copper Oxide Precipitate is a key indicator. The precipitate formed is copper(I) oxide ( ext{Cu}_{2} ext{O}), not copper(II) oxide (CuO), which distinguishes the product of this specific reaction. This reddish-brown precipitate arises because the reducing sugar donates electrons to the copper(II) ions in the solution, reducing them to copper(I) ions, which then form copper(I) oxide. Copper(II) oxide, on the other hand, is black and not typically involved in this kind of reaction. Thus, understanding the difference between these two copper oxides is essential for correctly interpreting experimental results. Students should remember these distinct characteristics, as they determine the validity of a reaction and whether or not the test indicates the presence of reducing sugars correctly.