Magnesium, an alkaline earth metal, is a reactive element, but its interaction with water often appears passive due to a thin, protective coating on its surface. Its relationship with water changes dramatically depending on the temperature.
The Reaction with Cold Water
Magnesium metal placed in cold water appears largely unreactive, but a subtle chemical process is occurring. The metal slowly reacts with liquid water to form magnesium hydroxide and hydrogen gas. This displacement reaction is represented by the chemical equation: \(\text{Mg(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(s)} + \text{H}_2\text{(g)}\).
The sluggishness of this reaction is primarily due to a passivation layer of magnesium oxide that naturally forms on the metal’s surface. Furthermore, the reaction’s product, magnesium hydroxide, is only slightly soluble in water. As the reaction proceeds, this insoluble magnesium hydroxide forms a physical barrier, coating the metal and preventing further water molecules from reaching the fresh magnesium underneath.
Accelerated Reaction with Hot Water and Steam
The chemical interaction between magnesium and water intensifies when the temperature is raised. Increasing the heat provides the necessary energy to overcome the protective layers and accelerate the reaction. The reaction with hot water is faster than with cold water, but the most vigorous reaction occurs when magnesium encounters steam.
When magnesium is heated in the presence of steam, it produces different products than the cold water reaction. At these elevated temperatures, the primary product is magnesium oxide, a white solid, instead of the hydroxide: \(\text{Mg(s)} + \text{H}_2\text{O(g)} \rightarrow \text{MgO(s)} + \text{H}_2\text{(g)}\). The high temperature causes any magnesium hydroxide that might form to immediately decompose into magnesium oxide and water. If fine magnesium powder or ribbon is used, this intense steam reaction often ignites and burns with a bright light.
Real-World Context of Magnesium Reactivity
The varying reactivity of magnesium with water impacts both technology and safety. Magnesium’s ability to react intensely with steam and burn brightly is the principle behind its use in pyrotechnics and flares. Since magnesium can steal oxygen from the water molecule, it can burn even in a water-rich environment like steam.
In industrial settings, the reaction of magnesium with water poses a safety hazard, especially with finely divided powder or shavings. Water must never be used to extinguish a magnesium fire because the reaction produces flammable hydrogen gas and generates immense heat. Specialized Class D fire extinguishers containing dry agents are required for these metal fires.
Magnesium’s reactivity is also harnessed for constructive purposes in items like water heaters. A magnesium rod is installed inside the steel tank as a sacrificial anode. Because magnesium is more electrochemically active than steel, it corrodes preferentially in the water, sacrificing itself to protect the steel tank from rust. This process extends the lifespan of the water heater.