What Is Magnesium Oxide?
Magnesium Oxide (MgO), commonly known as magnesia, is an inorganic compound formed from one magnesium atom and one oxygen atom. It appears as a white, powdery solid at room temperature and is one of the most widely used metal oxides in the world — spanning industries from steelmaking to medicine.
Molecular Structure and Bonding
MgO has a simple but highly stable crystal structure. It adopts a rock salt (NaCl-type) crystal lattice, where each magnesium ion is octahedrally coordinated by six oxygen ions, and vice versa. This ionic bonding arrangement is extremely strong, which explains many of MgO's remarkable physical properties.
- Molecular formula: MgO
- Molar mass: 40.30 g/mol
- Crystal system: Cubic (face-centred)
- Bond type: Ionic
Key Physical Properties
| Property | Value |
|---|---|
| Melting Point | 2,852 °C (5,166 °F) |
| Boiling Point | 3,600 °C (6,512 °F) |
| Density | 3.58 g/cm³ |
| Appearance | White powder or solid |
| Solubility in Water | Slightly soluble (0.86 g/L at 20 °C) |
| Hardness (Mohs) | 5.5–6 |
Chemical Reactivity
Despite its stability, MgO does react under certain conditions:
- With water: MgO slowly reacts with water to form magnesium hydroxide (Mg(OH)₂), a mildly alkaline compound. This process is called hydration or slaking:
MgO + H₂O → Mg(OH)₂ - With acids: MgO is a basic oxide and readily dissolves in acids, forming magnesium salts. For example:
MgO + H₂SO₄ → MgSO₄ + H₂O - With CO₂: In the presence of moisture and carbon dioxide, MgO can slowly carbonate to form magnesium carbonate (MgCO₃).
Calcination and MgO Grades
The properties of MgO are significantly influenced by how it is manufactured. Magnesium oxide is typically produced by calcining (heating) magnesium carbonate or magnesium hydroxide. The temperature of calcination determines the reactivity and surface area of the resulting product:
- Light-Burned MgO (caustic calcined): Calcined at 700–1,000 °C. Highly reactive, large surface area. Used in chemical, agricultural, and environmental applications.
- Hard-Burned MgO: Calcined at 1,000–1,500 °C. Moderate reactivity. Common in refractory and construction uses.
- Dead-Burned MgO (sintered magnesia): Calcined above 1,500 °C. Very low reactivity, high density. Preferred for high-temperature refractory applications.
- Fused MgO (electrofused): Melted in an electric arc furnace. Maximum density and purity. Used in the most demanding refractory and electrical applications.
Purity Levels and What They Mean
Commercial MgO is sold in a range of purity grades, typically expressed as a percentage of MgO content. Higher purity is required for pharmaceutical, food-grade, and high-performance refractory applications, while lower grades suffice for agriculture and general industrial use. Always check the specification sheet for impurity levels — particularly calcium oxide (CaO) and silica (SiO₂) content — when selecting MgO for a specific application.
Why These Properties Matter
Understanding MgO's chemistry is the foundation for choosing the right grade and form for your application. Its exceptional melting point makes it indispensable in furnace linings, its basic nature makes it useful for neutralisation and soil correction, and its biocompatibility makes it suitable for health and food applications. Each use case traces directly back to its fundamental chemical and physical character.