Magnesium is an alkaline earth metal, recognized for its silvery-white luster and relatively low density. As a member of Group 2, it possesses two valence electrons, making it highly reactive. While stable at room temperature, magnesium readily reacts with oxygen once an energy barrier is overcome. The resulting chemical transformation releases a spectacular burst of energy.
The Vigorous Combustion Process
Igniting magnesium requires overcoming its activation energy barrier, typically by applying heat until it reaches its autoignition temperature of approximately 473 °C. Once this threshold is crossed, the metal undergoes rapid oxidation, reacting with oxygen in the air in a powerful, highly exothermic combustion. This process generates a large amount of energy, released visibly as intense heat and dazzling light.
The temperature of the burning magnesium can reach an estimated 3,100 °C, which is hot enough to melt many other common metals. This extreme heat drives the reaction forward, creating a brilliant, pure white light. The spectrum of this emitted light is broad, containing a significant component of high-energy ultraviolet (UV) radiation. Because the process is rapid and releases so much energy, it is often described as a flash.
The Resulting Compound
The chemical reaction that occurs during combustion is represented by the formula \(2 \text{Mg} + \text{O}_2 \rightarrow 2 \text{MgO}\). This reaction results in the formation of magnesium oxide (\(\text{MgO}\)), an ionic compound. Magnesium atoms donate two electrons to oxygen atoms to form the stable product.
The physical result of the combustion is a fine, white, powdery residue. This powder is magnesium oxide, often referred to as magnesia. The compound possesses a high melting point of about 2,852 °C. This characteristic classifies magnesium oxide as a ceramic-like material, stable even under the extreme thermal conditions that created it.
Unique Properties and Safety Considerations
The intense light and heat generated by burning magnesium have led to its use in various applications requiring bright illumination. Historically, it was used in early photography for flash powder, and today, it is a component in signal flares, fireworks, and tracer ammunition. These items leverage the metal’s ability to produce white light visible from great distances.
The high reactivity of magnesium presents safety challenges when attempting to extinguish a fire. Unlike ordinary fires, which are smothered by removing oxygen, a magnesium fire cannot be extinguished with water or carbon dioxide (\(\text{CO}_2\)). The metal is reactive enough to strip the oxygen atoms from both water and \(\text{CO}_2\) molecules to sustain combustion.
Applying water results in the production of magnesium oxide and highly flammable hydrogen gas, which can intensify the fire and potentially cause an explosion. Similarly, using a carbon dioxide extinguisher causes the magnesium to react with the gas, yielding magnesium oxide and elemental carbon (black soot). Therefore, specialized materials, such as a Class D dry chemical fire extinguisher or dry sand, must be used to smother the fire and absorb the heat without reacting chemically with the burning metal.