MgO does not readily dissolve in water because

Analyze the question: We are asked why magnesium oxide (MgO) is only sparingly soluble in water.

Concept: For an ionic solid to dissolve, the energy released when its ions are hydrated (hydration energy) must be enough to overcome the energy holding the ions together in the crystal (lattice energy).

• If hydration energy ≥ lattice energy, the compound dissolves readily.
• If lattice energy > hydration energy, the compound is insoluble or sparingly soluble.

Solution:

1. MgO is made of Mg²⁺ and O²⁻ ions. These carry a 2+ and 2− charge, and both ions are small.
2. Strong attraction between such highly charged, small ions gives a very high lattice energy.
3. The hydration energy is not enough to compensate for this, so the crystal does not break up easily in water.

Checking the options:

• A. High melting point: This is a consequence of the strong lattice, not the direct reason for the low solubility. It is a related property, not the explanation.
• B. Covalent compound: Wrong. MgO is ionic (metal + non-metal, large electronegativity difference).
• C. Forms a hydroxide: The small amount that dissolves does form Mg(OH)₂, but this does not explain why it is insoluble. Mg(OH)₂ is itself only slightly soluble.
• D. Lattice energy higher than hydration energy: Correct. This is the energy-based explanation of why it does not dissolve readily.

Answer: D

Exam tip: When you see "does not dissolve" for an ionic compound, think lattice energy vs. hydration energy. Compounds with highly charged ions (2+/2−, like MgO and CaCO₃) tend to have very high lattice energies and are poorly soluble.