The Basics: What Is Magnesium Oxide?
Magnesium oxide (chemical formula: MgO) is an inorganic compound consisting of one magnesium atom and one oxygen atom joined by a strong ionic bond. It is a white, odorless solid at room temperature and belongs to the class of compounds known as alkaline earth metal oxides. MgO is sometimes called magnesia, a name derived from Magnesia, a region of ancient Greece where magnesium-bearing minerals were found.
Physical and Chemical Properties
| Property | Value / Description |
|---|---|
| Molecular weight | 40.30 g/mol |
| Appearance | White powder or granules |
| Melting point | ~2,852°C (5,166°F) |
| Boiling point | ~3,600°C (6,512°F) |
| Density | ~3.58 g/cm³ |
| Solubility in water | Slightly soluble (forms Mg(OH)₂) |
| Crystal structure | Rock salt (face-centered cubic) |
| Nature | Basic (alkaline) |
The exceptionally high melting point of MgO — one of the highest of any known oxide — is a direct consequence of the strong ionic bonding between Mg²⁺ and O²⁻ ions and the high lattice energy of the rock salt crystal structure.
Key Chemical Reactions
1. Formation from Magnesium Metal
When magnesium metal burns in air or oxygen, it produces a brilliant white flame and forms magnesium oxide:
2 Mg(s) + O₂(g) → 2 MgO(s)
This highly exothermic reaction releases a large amount of energy and light, which is why burning magnesium ribbon is a classic chemistry demonstration.
2. Reaction with Water (Hydration)
MgO reacts with water to form magnesium hydroxide, though this reaction is relatively slow compared to calcium oxide:
MgO(s) + H₂O(l) → Mg(OH)₂(s)
The rate of this reaction depends on the surface area and reactivity of the MgO — which varies considerably based on how it was produced (calcination temperature).
3. Reaction with Acids
As a basic oxide, MgO reacts readily with acids in neutralization reactions. For example, with hydrochloric acid:
MgO(s) + 2 HCl(aq) → MgCl₂(aq) + H₂O(l)
This property underpins MgO's use as an antacid (neutralizing stomach acid) and in industrial pH control.
4. Reaction with Carbon Dioxide
MgO slowly reacts with atmospheric CO₂ to form magnesium carbonate:
MgO(s) + CO₂(g) → MgCO₃(s)
This reaction is relevant in long-term storage of MgO powders, as surface carbonation can reduce reactivity over time.
How Magnesium Oxide Is Produced
Industrial MgO production falls into three main routes:
Calcination of Magnesite
The most common production method involves heating magnesite (magnesium carbonate, MgCO₃) in a kiln:
MgCO₃(s) → MgO(s) + CO₂(g)
The calcination temperature used determines the reactivity of the resulting MgO. Lower temperatures (around 700–1,000°C) produce light-burned or caustic-calcined magnesia, which is highly reactive and used in industrial processes and supplements. Higher temperatures (1,500–2,000°C) produce dead-burned magnesia, which is dense and stable — ideal for refractory applications.
From Seawater and Brines
Seawater contains significant dissolved magnesium. It can be processed by adding lime or dolomite to precipitate magnesium hydroxide, which is then filtered and calcined to produce MgO. This is an important source of high-purity magnesia.
Fused Magnesia
Fused magnesia is produced by melting MgO in an electric arc furnace at temperatures exceeding 2,800°C. The resulting product has very high density, very low porosity, and is used in electrical insulation applications.
Reactivity Classes of MgO
Not all MgO is the same. The calcination temperature and raw material source strongly influence surface area, porosity, and chemical reactivity. Selecting the correct grade of MgO for a specific application — whether agricultural, industrial, pharmaceutical, or construction — is essential for achieving the desired performance.