The Question is from neet-chemistry basic-chemistry-concepts concentration-methods, and it was created by the author P SRAVAN KUMAR

Question

A mixture of two volatile liquids A and B shows a negative deviation from Raoult's law.  Which of the following statements is INCORRECT regarding this mixture?

P SRAVAN KUMAR · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
	**Primary Concept**: Raoult's Law and Non-Ideal Solutions
	**Related Topics**: Intermolecular forces, Enthalpy of mixing, Volume of mixing, Azeotropes
	**Prerequisites**: Understanding of Raoult's law for ideal solutions.
## SOLUTION PATHWAY
**Given Information**:
	The mixture shows a negative deviation from Raoult's law.
**Method & Steps**:
	1. **Negative Deviation Implications**:  A negative deviation means the total vapor pressure of the mixture is lower than predicted by Raoult's law.  This arises due to stronger intermolecular forces between the components A and B than between A-A and B-B. 
		Reasoning: Stronger A-B interactions make it harder for the molecules to escape into the vapor phase, reducing the vapor pressure.
	2. **Consequences of Negative Deviation**:
		Working:  Stronger A-B interactions lead to a negative enthalpy of mixing ($\Delta H_{mix} < 0$) because energy is released when these stronger bonds are formed. It also often leads to a negative volume of mixing ($\Delta V_{mix} < 0$) as the molecules pack more efficiently due to these interactions.
		Key Point: The stronger A-B interactions stabilize the liquid mixture, resulting in a higher boiling point than predicted by Raoult's law (as more energy is needed to overcome these interactions and vaporize the liquid).
	3. **Incorrect Statement Identification**:
		Result: The statement 'The boiling point of the mixture is lower than that predicted by Raoult's law' is incorrect.  It should be higher.
		Verification:  A negative deviation implies a higher boiling point than predicted.
 ## OPTION ANALYSIS
[A] ✓: The enthalpy of mixing is negative. This is correct as stronger A-B interactions release energy upon mixing.
[B] ✓: The volume of mixing is negative. This is often true, although not always guaranteed, as stronger interactions can lead to more efficient packing.
[C] :x: The boiling point of the mixture is lower than that predicted by Raoult's law. This is incorrect.  The boiling point is higher due to the stronger A-B interactions.
[D] ✓: The intermolecular forces between A and B are stronger than the A-A and B-B interactions. This is correct and the fundamental reason for the negative deviation.
 ## LEARNING AIDS
	**Common Mistakes**:
		1. Confusing positive and negative deviations: Remember that negative deviation means lower vapor pressure and higher boiling point.
		2. Assuming volume of mixing is always negative: While often negative, it's not a strict rule for negative deviations.
	**Quick Tips**:
		1. Negative deviation: Think 'stronger interactions, lower vapor pressure, higher boiling point'.
		2. Relate deviation to the strength of A-B interactions compared to A-A and B-B interactions.

P SRAVAN KUMAR · Answer

The enthalpy of mixing is negative.

P SRAVAN KUMAR · Answer

The volume of mixing is negative.

P SRAVAN KUMAR · Answer

The correct Option is "The boiling point of the mixture is lower than that predicted by Raoult's law."

P SRAVAN KUMAR · Answer

The intermolecular forces between A and B are stronger than the A-A and B-B interactions.

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