Problem 18 - A Complete Resource Book in Chemistry for JEE Main

Question

An alkaline solution of and citrate ions is called (a) silver chloride; Tollen's reagent (b) cupric sulphate; Benedict's solution (c) silver nitrate; Fehling's solution (d) cupric sulphate; Schiff's reagents

Step-by-Step Solution

Answer

(b) cupric sulphate; Benedict's solution

Step 1: Understand Common Chemical Tests

Chemical tests often use solutions as reagents to identify or test for other substances. For example, Tollen's reagent is used to identify aldehydes and relies on the reduction of silver ions to metallic silver.

Step 2: Identify Solution Components

The problem mentions an alkaline solution of citrate ions. This typically refers to the reagent used in Benedict's solution, which contains copper(II) ions in an alkaline citrate solution to test for reducing sugars.

Step 3: Match Components to Solution Names

Benedict's solution is known for its use of cupric sulphate (copper(II) sulfate) and citrate ions to detect aldehyde functional groups that reduce copper ions.

Step 4: Pick the Correct Answer

Based on the known characteristic of Benedict’s solution, option (b) cupric sulphate, Benedict's solution is the correct answer since it contains the alkaline citrate solution.

Key Concepts

Chemical Tests

Chemical tests are essential tools in chemistry. They allow scientists to identify substances and understand their properties. Commonly, chemical tests involve the use of reagents to cause reactions that will produce detectable changes. For example, Tollen's reagent is used to test for the presence of aldehydes. When an aldehyde is present, this reagent causes the reduction of silver ions, resulting in a silver mirror on the container's surface.

Chemical tests are widely used in laboratories due to their efficiency and practicality. They are straightforward, often only requiring basic laboratory equipment and reagents. This makes them accessible for educational purposes and professional research alike.

• Help in detecting the presence of various chemical groups.
• Enable the identification of unknown compounds.
• Provide qualitative and sometimes quantitative data.

These tests play a significant role in diagnosing and studying chemical compositions, which is why understanding how to interpret the results is crucial.

Reducing Sugars Detection

Reducing sugars are sugars that can donate electrons to another chemical. This is important because their ability to reduce other molecules makes them detectable by certain chemical tests. Benedict's solution is designed to detect these sugars. It uses copper(II) ions in an alkaline citrate solution as the reacting agents.

When a reducing sugar is present, it will reduce the copper(II) ions to copper(I) oxide. This is evident by a color change in the solution, ranging from green to red, depending on the concentration of the reducing sugar. This change provides a visual cue that is easy to interpret.

• Simple and quick test.
• Provides a visual indication of reducing sugar presence.
• Can infer the concentration of reducing sugars based on color intensity.

Such tests are important in fields like biochemistry and food science, where sugar concentration can affect research outcomes and product quality.

Copper Ions in Alkaline Citrate

Copper ions in an alkaline citrate solution are a key component of Benedict's solution. The copper(II) sulfate provides the copper ions, which are critical in the detection process. In Benedict's test for reducing sugars, these ions act as the oxidizing agent.

The alkaline citrate serves to maintain the pH at a level where the copper ions are stable and reactive with the reducing sugars. As the reaction occurs, the copper(II) ions are converted to copper(I) oxide, a reddish precipitate. The presence of citrate ions helps prevent the copper ions from precipitating too early or under normal conditions.

• Copper ions indicate oxidation-reduction reactions.
• Alkaline conditions enhance the reaction's efficiency.
• Citrate stabilizes the copper ions in solution.

Understanding the role of these components helps clarify why Benedict’s test is so reliable for detecting sugars, thus profoundly illustrating the chemistry involved in these reactions.