Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

Brown ring is made for (a) \(\mathrm{I}^{-}\) (b) \(\mathrm{Cl}^{-}\) (c) \(\mathrm{NO}_{3}^{-}\) (d) \(\mathrm{Br}^{-}\)

Step-by-Step Solution

Answer

The brown ring is formed for \(\mathrm{NO}_3^-\) ions.

Step 1: Understanding the problem

The question is asking us to identify which ion forms a brown ring upon reaction. This is a common qualitative analysis test in chemistry.

Step 2: Recall the Brown Ring Test

The brown ring test is a qualitative analytical chemistry technique used to detect the presence of nitrate ions (\(\mathrm{NO}_3^-\)) in a solution.

Step 3: Identify the correct ion

Based on the brown ring test, the brown ring is specifically formed when \(\mathrm{NO}_3^-\) ions are present.

Key Concepts

Qualitative Analysis

Qualitative analysis in chemistry is focused on identifying the different substances in a mixture. Unlike quantitative analysis, which measures how much of each substance is present, qualitative analysis only confirms their presence or absence. This approach is pivotal in analytical chemistry, as it helps determine the composition of unknown samples.
There are various techniques within qualitative analysis. These techniques range from simple visual tests to more sophisticated instrumentation. Based on the nature of the sample being tested, an analytical chemist might choose one technique over another.
In educational contexts, qualitative tests like the Brown Ring Test are often favored. They provide students with an intuitive understanding of reaction principles through observable changes. As students witness a color change or a precipitate formation, they gain real-world insights into how chemical reactions manifest subtly yet visibly.

Nitrate Ions Detection

Detecting nitrate ions ( NO_3^- ) is essential in both academic settings and practical applications. The Brown Ring Test is a classic experiment for this purpose. It involves adding a solution of iron(II) sulfate ( FeSO_4 ) to the sample, followed by gently pouring concentrated sulfuric acid ( H_2SO_4 ). The acid forms a layer beneath the aqueous mixture.
The test is based on the reduction of nitrate ions by iron(II) to form nitrogen dioxide. This nitrogen dioxide reacts further to form a complex with iron, producing the distinctive brown ring at the interface between the layers. This reaction occurs as:

• NO_3^- + 3 Fe^{2+} + 4 H^+ → 3 Fe^{3+} + NO + 2 H_2O

This methodology is sensitive and provides a clear and quick way of detecting nitrate ions, enhancing students' understanding of chemical reactivity.

Analytical Chemistry Test

Analytical chemistry tests, like the Brown Ring Test, play a crucial role in the identification of ions in solutions. These tests help justify the presence of specific ions by forming visible products based on unique reactions.
Through these procedures, students learn fundamental concepts of reactivity, equilibrium, and complex formation. For example, the complexity of reactions between sulfates, acids, and nitrates showcases key topics in chemistry.
It's also important to highlight the safety aspect of these tests, as they often involve concentrated acids or other hazardous chemicals. Following proper safety protocols is a crucial part of the learning process.
Educators find analytical chemistry tests incredibly valuable for demonstrating chemical principles practically. These tests are not only pivotal for laboratory education but also provide groundwork for understanding more advanced analytical techniques used in real-world applications.