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 assertion and reason provided in the question, we will break down the concepts step by step.

### Step 1: Understanding the Assertion
**Assertion (A)**: The solubility of alkaline earth metal carbonates decreases down the group.

- **Explanation**: Alkaline earth metals include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). The carbonates of these metals are generally represented as MCO₃, where M is the alkaline earth metal.

### Step 2: Understanding the Reason
**Reason (R)**: Hydration enthalpy decreases down the group, whereas lattice enthalpy remains almost constant.

- **Hydration Enthalpy**: This is the energy released when ions are surrounded by water molecules. It is influenced by the charge density of the ions. As the size of the cation increases down the group, the charge density decreases, leading to a decrease in hydration enthalpy.

- **Lattice Enthalpy**: This is the energy required to separate one mole of a solid ionic compound into its gaseous ions. It depends on the charge of the ions and their sizes. In this case, the charge of the alkaline earth metal cations remains constant (+2) as we move down the group, but the size increases.

### Step 3: Analyzing the Relationship
- As we move down the group from Be to Ra, the size of the cations increases, which leads to a decrease in charge density. Consequently, the hydration enthalpy decreases because the energy released when water molecules surround the cations is less for larger cations.

- The lattice enthalpy, however, remains relatively constant because the charge of the cations does not change, even though the size increases.

### Step 4: Conclusion
- The assertion that the solubility of alkaline earth metal carbonates decreases down the group is true because the decrease in hydration enthalpy outweighs the constant lattice enthalpy, leading to lower solubility.

- The reason provided is also true, as it correctly explains the relationship between hydration enthalpy and lattice enthalpy.

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

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