Solubility of an ionic compound in water is mainly dependent on:
a.Lattice enthalpy , b. Hydration enthalpy
Both these factors oppose each other and the resultant of these determines the solubility of an ionic compound in water. If lattice enthalpy has greater value, the compound is less soluble.
In case hydration enthalpy has greater value, the compound is
highly soluble in water.
Compound of alkaline earth metals are less soluble than
alkali metals, due to:

Options

A. Their high hydration enthalpy
B. Their high lattice enthalpy
C. Their increases covalent character
D. Their high ionisation enthalpy.

Explanation

To solve the question regarding the solubility of ionic compounds in water and the comparison between alkaline earth metals and alkali metals, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Solubility Factors**:
The solubility of an ionic compound in water is primarily influenced by two factors: lattice enthalpy and hydration enthalpy. Lattice enthalpy refers to the energy required to separate one mole of an ionic solid into its gaseous ions, while hydration enthalpy refers to the energy released when gaseous ions are surrounded by water molecules.

**Hint**: Remember that lattice enthalpy opposes solubility, while hydration enthalpy promotes it.

2. **Relationship Between Lattice and Hydration Enthalpy**:
These two enthalpies work against each other. If the lattice enthalpy is greater than the hydration enthalpy, the compound will be less soluble in water. Conversely, if the hydration enthalpy is greater, the compound will be more soluble.

**Hint**: Think of it as a balance: higher lattice enthalpy = lower solubility, and higher hydration enthalpy = higher solubility.

3. **Comparing Alkaline Earth Metals and Alkali Metals**:
Alkaline earth metals (Group 2) have a higher ionic charge (+2) compared to alkali metals (Group 1), which have a charge of +1. This higher charge leads to a greater lattice enthalpy for alkaline earth metals.

**Hint**: Recall that charge and size affect lattice enthalpy; higher charge usually means higher lattice enthalpy.

4. **Effect of Ionic Charge and Size**:
The alkaline earth metals also tend to have smaller ionic radii compared to alkali metals. The combination of a higher charge and smaller size results in significantly higher lattice enthalpy for alkaline earth metals.

**Hint**: Consider how ionic size and charge influence the strength of ionic bonds in a lattice.

5. **Conclusion on Solubility**:
Since alkaline earth metals have higher lattice enthalpy and lower hydration enthalpy compared to alkali metals, they are less soluble in water. The higher lattice enthalpy outweighs the hydration enthalpy, leading to lower solubility.

**Hint**: Summarize the findings: higher lattice enthalpy = lower solubility, especially in alkaline earth metals compared to alkali metals.

### Final Answer:
The compounds of alkaline earth metals are less soluble than those of alkali metals due to their higher lattice enthalpy resulting from their higher ionic charge and smaller ionic size.