The solubility in water of sulphate down the Be group is `Be gt Mg gt Ca gt Sr gt Ba.`

Options

A. Decreasing lattice energy
B. High heat-of solvation for smallier ions like `Be^(2+)`
C. Increase in melting points
D. Increasing molecular weight

Answer

To solve the question regarding the solubility of sulphates of alkaline earth metals (Be, Mg, Ca, Sr, Ba) in water, we need to analyze the factors affecting solubility in this group of elements.

### Step-by-Step Solution:

1. **Identify the Group**:
The elements mentioned (Be, Mg, Ca, Sr, Ba) belong to Group 2 of the periodic table, known as alkaline earth metals.

2. **Understand the Sulphate Compounds**:
The sulphates in question are:
- Beryllium sulphate (BeSO4)
- Magnesium sulphate (MgSO4)
- Calcium sulphate (CaSO4)
- Strontium sulphate (SrSO4)
- Barium sulphate (BaSO4)

3. **Observe the Solubility Trend**:
The solubility of these sulphates in water decreases in the order:
\[ \text{BeSO}_4 > \text{MgSO}_4 > \text{CaSO}_4 > \text{SrSO}_4 > \text{BaSO}_4 \]

4. **Explain the Trend**:
The solubility trend can be attributed to two main factors: lattice energy and hydration energy.
- **Lattice Energy**: This is the energy required to separate the ions in the solid compound. As we move down the group, the size of the metal ions increases (Be²⁺ < Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺), which leads to a decrease in lattice energy.
- **Hydration Energy**: This is the energy released when ions are solvated by water molecules. Smaller ions (like Be²⁺ and Mg²⁺) have higher hydration energies due to their smaller size, which allows them to interact more effectively with water molecules.

5. **Conclusion**:
For the alkaline earth metal sulphates, the high hydration energy of the smaller ions (Be²⁺ and Mg²⁺) outweighs the lattice energy, leading to higher solubility. As we move down the group, the increasing size of the ions results in lower hydration energy compared to lattice energy, causing the solubility to decrease.

### Final Order of Solubility:
Thus, the order of solubility in water of sulphates down the group is:
\[ \text{BeSO}_4 > \text{MgSO}_4 > \text{CaSO}_4 > \text{SrSO}_4 > \text{BaSO}_4 \]