Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Lithium salts are hydrated.
Reason (R) : Lithium has higher polarising power than other alkali metal group members.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

The degree of hydration of alkali metal ions decreases in the order: \( \text{Li}^+>\text{Na}^+>\text{K}^+>\text{Rb}^+>\text{Cs}^+ \). Small ions attract water more strongly!

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
This question relates to the anomalous behavior of Lithium in the alkali metal group, specifically focusing on its hydration and charge density.
Step 2: Detailed Explanation:
Assertion (A): Lithium salts like \( \text{LiCl}\cdot 2\text{H}_2\text{O} \) are commonly found in hydrated forms, whereas salts of larger alkali metals (like \( \text{NaCl} \), \( \text{KCl} \)) are usually anhydrous. This is correct.
Reason (R): Lithium (\( \text{Li}^+ \)) has the smallest ionic size in the group. This leads to a very high charge-to-radius ratio, which results in high polarizing power and a high hydration enthalpy. Because of this high hydration enthalpy, water molecules are strongly attracted to the \( \text{Li}^+ \) ion, leading to the formation of hydrated salts.
The high polarizing power (or high charge density) is the fundamental reason why it interacts so strongly with polar water molecules. Thus, (R) correctly explains (A).
Step 3: Final Answer:
Both statements are true, and (R) is the correct explanation for (A).