AIIMS 2006 Chemistry Coagulation Assertion Reason Question

To analyze the provided question, we need to evaluate both the assertion and the reason concerning the coagulation of As₂S₃ sol (arsenic trisulfide) using Fe³⁺ ions (ferric ions).

Assertion: Fe³⁺ can be used for coagulation of As₂S₃ sol.

Explanation: Coagulation refers to the process by which small particles (like colloidal particles) aggregate to form larger particles, which can then settle. In colloidal systems, charged particles can repel each other, maintaining a stable suspension. When we introduce ions with opposite charges, such as Fe³⁺, they can neutralize the negative charges on the As₂S₃ particles, allowing them to come together and precipitate out of the solution.

Fe³⁺ ions act as coagulants because they have a high positive charge that can effectively reduce the electrostatic repulsion between negatively charged As₂S₃ particles. This makes the assertion true.

Reason: Fe³⁺ reacts with As₂S₃ to give Fe₂S₃.

Explanation: The reason provided states that Fe³⁺ reacts with As₂S₃ to form Fe₂S₃. However, this statement is incorrect. While Fe³⁺ can interact with the As₂S₃ sol, it does not form a stable compound of Fe₂S₃ directly through this reaction. Instead, the role of Fe³⁺ in coagulation is primarily due to its ability to neutralize the charge on the colloidal particles, not to form a new compound.

Conclusion:

Since the assertion is true, but the reason is false, the correct answer to the question is C) Assertion is true but Reason is false.

Clarifying Incorrect Options:

• A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion: This option is incorrect because, while the assertion is true, the reason is false.

• B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion: This option is also incorrect because the reason itself is false, which disqualifies it from being true.

• D) Both Assertion and Reason are false: This option is incorrect because the assertion is indeed true.

Key Concepts:

1. Coagulation: The process of destabilizing colloidal particles, leading to their aggregation and precipitation.
2. Charge Neutralization: The mechanism by which oppositely charged ions (like Fe³⁺) neutralize the charge on colloidal particles, promoting coagulation.
3. Colloidal Chemistry: A branch of chemistry that deals with systems where one substance is dispersed evenly throughout another.

Summary:

In summary, the assertion that Fe³⁺ can be used for coagulation of As₂S₃ sol is correct due to its ability to neutralize charges, while the reason suggesting a direct reaction to form Fe₂S₃ is incorrect. Therefore, the answer is C.