Alumina is an ionic compound but insoluble in water because

Options

A. its lattice energy is very high
B. its hydration energy is very high
C. both lattice energy & hydration energy are same
D. none of these

Answer

To solve the question regarding why alumina is an ionic compound but insoluble in water, we can follow these steps:

### Step 1: Understand the Nature of Alumina
Alumina, or aluminum oxide (Al₂O₃), is an ionic compound formed from aluminum ions (Al³⁺) and oxide ions (O²⁻). Ionic compounds typically dissolve in water if the energy released during the hydration of the ions is greater than the energy required to break the ionic bonds in the lattice structure.

**Hint:** Recall the definitions of ionic compounds and their typical behavior in solvents like water.

### Step 2: Consider Lattice Energy
Lattice energy is the energy required to separate one mole of a solid ionic compound into its gaseous ions. In the case of alumina, the lattice energy is very high due to the strong electrostatic forces between the Al³⁺ and O²⁻ ions.

**Hint:** High lattice energy indicates strong ionic bonds, making it difficult for water molecules to separate the ions.

### Step 3: Consider Hydration Energy
Hydration energy is the energy released when gaseous ions are surrounded by water molecules and solvated. For alumina, the hydration energy is relatively low compared to the lattice energy. This means that when water tries to dissolve alumina, the energy released from hydration is not sufficient to overcome the energy required to break the ionic bonds in the lattice.

**Hint:** Compare the magnitudes of lattice energy and hydration energy to determine solubility.

### Step 4: Conclusion
Since the lattice energy of alumina is much higher than its hydration energy, the compound does not dissolve in water. Therefore, the correct answer to the question is that alumina is insoluble in water because its lattice energy is very high compared to its hydration energy.

**Final Answer:** Alumina is insoluble in water because its lattice energy is very high.