In inorganic salt analysis, for the indicatory test of \(Cl^-\), conc. \(H_2SO_4\) is used. This is because:

conc. \(H_2SO_4\) has lower volatility than \(HCl\)

The correct answer is option 4. conc. \(H_2SO_4\) has lower volatility than \(HCl\).

The test for chloride ions (\(Cl^-\)) using concentrated sulfuric acid is to detect the presence of chloride ions by observing the production of hydrochloric acid (\(HCl\)) gas.

Reasons for Using Concentrated Sulfuric Acid

Lower Volatility of Sulfuric Acid Compared to Hydrochloric Acid:

Volatility and Practical Aspects:

Concentrated Sulfuric Acid: Sulfuric acid is a dense, non-volatile liquid. It remains in the liquid phase at room temperature and does not evaporate easily.

Hydrochloric Acid: Hydrochloric acid is a gas at room temperature when released from the solution, and it is highly volatile.

Why It Matters:

During the test, when chloride salts react with concentrated sulfuric acid, hydrochloric acid (\(HCl\)) gas is produced. If a less dense acid were used, \(HCl\) gas would escape more readily into the atmosphere. The use of concentrated sulfuric acid ensures that the \(HCl\) gas produced from the reaction can be more easily detected and retained for observation, as sulfuric acid traps the gas, preventing its rapid escape.

Practical Implementation: In laboratory practice, using concentrated sulfuric acid helps to ensure that the test results are clear and that the produced gas can be captured or identified effectively.

Chemical Reaction

The typical reaction between chloride salts and concentrated sulfuric acid is:

\(\text{2NaCl + H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{2HCl}\)

Reactants: Sodium chloride (\(NaCl\)) reacts with concentrated sulfuric acid (\(H_2SO_4\)).

Products: Sodium sulfate (\(Na_2SO_4\)) and hydrochloric acid (\(HCl\)).

Observation: The production of \(HCl\) gas can be detected as it has a strong acidic odor and can turn blue litmus paper red.

Why Other Options Are Incorrect

Option 1: Concentrated \(H_2SO_4\) is a strong oxidizing agent and \(Cl_2\) is produced:

This option is not typically correct for the chloride test. Although concentrated \(H_2SO_4\) can act as an oxidizing agent, the test for chlorides primarily focuses on detecting \(HCl\) gas rather than producing \(Cl_2\).

Option 2: Concentrated \(H_2SO_4\) is a strong reducing agent and \(HCl\) is produced:

This option is incorrect because sulfuric acid is not a reducing agent; it is known for its oxidizing properties. In the chloride ion test, sulfuric acid does not reduce substances.

Option 3: Concentrated \(H_2SO_4\) is a dehydrating agent and \(Cl_2\) is produced:

While concentrated sulfuric acid does have dehydrating properties, this option is not relevant for the chloride ion test where the primary observation is the production of \(HCl\) gas rather than \(Cl_2\).

Summary

Option 4Concentrated \(H_2SO_4\) has lower volatility than \(HCl\) is correct because it explains the practical advantage of using concentrated sulfuric acid in the test. The lower volatility of sulfuric acid compared to the highly volatile \(HCl\) ensures that the produced gas can be effectively managed and detected during the analysis.

In summary, concentrated sulfuric acid is used in the test for chloride ions because its lower volatility helps to retain and observe the production of \(HCl\) gas, making the test more reliable and easier to conduct.