Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A : With the increase in the chain length of the saturated fatty acids, their melting point increases while their solubility in water decreases.
Reason R : The intermolecular interactions between the saturated fatty acid molecules increase with the increase in their chain length.
In light of the above statements, choose the correct answer from the options given below

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

For Fatty Acids:
1. Chain Length $\uparrow$ $\rightarrow$ Melting Point $\uparrow$ $\rightarrow$ Solubility $\downarrow$.
2. Unsaturation (Double bonds) $\uparrow$ $\rightarrow$ Melting Point $\downarrow$.
This explains why butter (saturated) is solid at room temperature while oil (unsaturated) is liquid.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question tests the understanding of physical properties of fatty acids and how they relate to molecular structure (chain length).

Step 2: Detailed Explanation:

• Analyzing Assertion A: Saturated fatty acids consist of long hydrocarbon chains. As the number of carbons increases, the non-polar hydrophobic part of the molecule grows larger. This makes the molecule less soluble in water. Simultaneously, a larger molecule has more surface area for contact, which usually corresponds to a higher melting point. Thus, Assertion A is true.

• Analyzing Reason R: The primary intermolecular force in saturated fats is the Van der Waals interaction (London dispersion forces). These forces are additive. A longer carbon chain allows for more points of contact and stronger "packing" between adjacent molecules. More energy (heat) is required to overcome these increased interactions to melt the substance. Thus, Reason R is true.

• Logical Connection: Since the increase in intermolecular forces (Reason) is the direct cause of the increase in melting point (Assertion), R is the correct explanation for A.

Step 3: Final Answer:
Both statements are scientifically accurate, and the reason correctly identifies the molecular mechanism behind the trend in physical properties.