A : Solubility of alkaline earth metal carbonates decreases down the group.
R : Hydration enthalpy decreases down the group whereas lattice enthalpy remain almost constant

Options

A. If both Assertion & Reason are true and the reason is the correct explanantion of the assertion, then mark (1).
B. If both Assertion & Reason are true and the reason is not the correct explanantion of the assertion, then mark (2).
C. If Assertion is true statement but Reason is false. Them mark (3).
D. If both Assertion and reason are false statement, then mark (4).

Answer

To analyze the given assertion and reason, we will break down the concepts step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the solubility of alkaline earth metal carbonates decreases down the group.
- This means that as we move from beryllium (Be) to barium (Ba), the ability of these carbonates to dissolve in water diminishes.

2. **Understanding the Reason**:
- The reason provided states that the hydration enthalpy decreases down the group while the lattice enthalpy remains almost constant.
- Hydration enthalpy is the energy released when ions are surrounded by water molecules, while lattice enthalpy is the energy required to separate the ions in a solid ionic compound.

3. **Analyzing Hydration Enthalpy**:
- As we move down the alkaline earth metal group, the size of the metal ions increases (from Be²⁺ to Ba²⁺).
- Larger ions have a lower charge density (charge/volume), which means they are less effective at attracting water molecules.
- Consequently, the hydration enthalpy decreases because the energy released when these larger ions are hydrated is less than that of smaller ions.

4. **Analyzing Lattice Enthalpy**:
- Lattice enthalpy is influenced by the size and charge of the ions. While the size of the ions increases down the group, the charge remains the same (2+ for alkaline earth metals).
- The increase in ionic size does not significantly affect the lattice enthalpy, which remains relatively constant.

5. **Connecting Assertion and Reason**:
- The decrease in hydration enthalpy means that the solubility of the carbonates decreases because lower hydration energy means that the ions are less able to interact with water and dissolve.
- Since the lattice enthalpy remains almost constant, the decrease in hydration enthalpy is the primary factor leading to decreased solubility.

6. **Conclusion**:
- Both the assertion and the reason are true.
- The reason correctly explains the assertion, as the decrease in hydration enthalpy leads to decreased solubility of alkaline earth metal carbonates down the group.

### Final Answer:
Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.