Magnesium is a lightweight metal often used in alloys for aerospace and automotive industries. As an alkaline earth metal, it possesses high chemical reactivity. The short answer to whether magnesium reacts with air is a definitive yes, though the speed and products of this reaction depend heavily on the surrounding conditions. This inherent reactivity influences how the metal is stored and used in industrial applications.
Magnesium’s Slow Reaction at Room Temperature
When pure magnesium metal is exposed to the atmosphere at room temperature, it immediately begins a slow reaction with the oxygen present in the air. This oxidation process forms magnesium oxide (\(\text{MgO}\)) on the metal surface, causing the normally bright, silvery surface to dull.
This thin layer of magnesium oxide is often referred to as tarnish, appearing as a dull gray film. The oxide layer is dense and stable, acting as a protective barrier that adheres tightly to the underlying magnesium metal. Because this barrier prevents oxygen from reaching the bulk of the metal, the slow reaction effectively stops after the initial surface layer is complete. This self-passivation allows magnesium metal to be handled and stored under normal conditions.
Rapid Combustion and Intense Heat
The behavior of magnesium changes when heat is introduced, moving from a slow tarnish to rapid combustion. Once the metal reaches its ignition point, it reacts vigorously with oxygen in an exothermic process that releases intense heat and a brilliant white light. This combustion reaction is energetic enough that it is still found in signal flares and fireworks today.
The extreme reactivity means that conventional fire extinguishing agents like water or carbon dioxide (\(\text{CO}_2\)) are ineffective and can even be dangerous against a magnesium fire. Magnesium is powerful enough to strip the oxygen from both water and carbon dioxide molecules, continuing to burn while simultaneously producing flammable hydrogen gas or powdered carbon. For this reason, specialized Class D dry powder extinguishers, which work by smothering the fire and excluding air, are the only safe option for metal fires.
The Unique Reaction with Nitrogen
While the primary product of magnesium combustion is magnesium oxide, magnesium is unique among common metals because it also reacts directly with the nitrogen (\(\text{N}_2\)) component of the air. Atmospheric air consists of about 78% nitrogen, which typically has a strong triple bond that makes it chemically inert in most reactions. However, the intense heat generated during magnesium combustion provides enough energy to break this bond.
This secondary reaction forms a compound known as magnesium nitride (\(\text{Mg}_3\text{N}_2\)), which is often visible as a yellowish-green residue mixed with the white magnesium oxide ash. When magnesium nitride comes into contact with water, it undergoes a hydrolysis reaction, converting into magnesium hydroxide and releasing ammonia gas. This process highlights magnesium’s reactivity, as very few metals are capable of reacting directly with atmospheric nitrogen.