Assertion: Lithium salts are mostly hydrated.
Reason: The hydration enthalpies of alkalie metal ions decrease with increase in ionic sizes.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Answer

To analyze the assertion and reason provided in the question, we will break it down step by step:

### Step 1: Understanding the Assertion
The assertion states that "Lithium salts are mostly hydrated." This means that when lithium salts are dissolved in water, they tend to attract and hold onto water molecules, forming hydrated compounds.

### Step 2: Understanding the Reason
The reason given is that "The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes." Hydration enthalpy refers to the amount of energy released when ions are surrounded by water molecules. As the size of the alkali metal ions increases (from lithium to cesium), their ability to attract water molecules decreases, leading to lower hydration enthalpy.

### Step 3: Analyzing Lithium Ion Size
Lithium ion (Li⁺) is smaller than other alkali metal ions (like Na⁺, K⁺, Rb⁺, and Cs⁺). Due to its smaller size, Li⁺ has a higher charge density, which allows it to attract more water molecules effectively. This results in a higher degree of hydration for lithium salts compared to those of larger alkali metals.

### Step 4: Conclusion about the Assertion and Reason
Since lithium ions have a higher degree of hydration due to their smaller size, it supports the assertion that lithium salts are mostly hydrated. Moreover, the reason provided correctly explains why hydration enthalpy decreases with increasing ionic size, which is true for alkali metals.

### Final Answer
Both the assertion and the reason are correct, and the reason correctly explains the assertion.