Magnesium is a lightweight, silvery-white metal that belongs to the alkaline earth metal group. Magnesium reacts with water, but the speed and nature of this reaction depend heavily on the water’s physical state and temperature. Under ambient conditions, the reaction is slow or negligible, while under high heat, it can become vigorous and release significant energy.
Reaction with Liquid Water (Temperature Dependence)
When magnesium metal is placed in cold liquid water, the reaction is extremely slow, often appearing not to react at all. This occurs because the metal immediately forms a thin, nearly insoluble layer of magnesium hydroxide (\(\text{Mg}(\text{OH})_2\)) on its surface. This layer acts as a protective barrier, preventing water molecules from reaching the underlying metal and stalling the reaction process.
The chemical reaction that slowly begins to take place forms magnesium hydroxide and hydrogen gas (\(\text{H}_2\)), represented by the formula \(\text{Mg}(s) + 2\text{H}_2\text{O}(l) \rightarrow \text{Mg}(\text{OH})_2(s) + \text{H}_2(g)\). The few bubbles of hydrogen gas that form on the surface of the metal are evidence of this slow process.
As the temperature of the liquid water increases, the reaction rate becomes noticeably faster. The added thermal energy provides the necessary activation energy to destabilize the magnesium hydroxide layer. The thermal energy also increases the kinetic energy of the water molecules, leading to more frequent and forceful collisions with the magnesium surface. While the product is still primarily magnesium hydroxide, the elevated temperature accelerates the process significantly compared to cold water.
The Chemistry of Steam: Forming Magnesium Oxide
The reaction changes dramatically when magnesium is exposed to water in its gaseous state, or steam, which represents a much higher temperature environment. At these elevated temperatures, generally exceeding the boiling point of water, the reaction is vigorous and often demonstrated in laboratories to produce a bright flame. The intense heat immediately overcomes any protective layer.
The key chemical difference is that the product is no longer magnesium hydroxide but rather magnesium oxide (\(\text{MgO}\)), which is a white powder. The high temperatures cause any transient magnesium hydroxide that might form to thermally decompose instantly into magnesium oxide and water vapor.
The reaction between magnesium and steam is \(\text{Mg}(s) + \text{H}_2\text{O}(g) \rightarrow \text{MgO}(s) + \text{H}_2(g)\). This reaction is highly exothermic, meaning it releases a significant amount of heat energy, which sustains the high temperature and the speed of the reaction. The rapid and energetic nature of the reaction produces a brilliant white light and a large volume of hydrogen gas.
Physical Characteristics That Affect Reaction Speed
Beyond temperature and the state of water, the physical characteristics of the magnesium itself play a large role in determining the speed of the reaction. The most influential factor is the metal’s surface area. A solid block or thick ribbon of magnesium reacts much slower than a fine magnesium powder because the powder has a vastly greater total surface area exposed to the water.
Chemical reactions between a solid and a liquid or gas can only occur at the interface where the two reactants meet. By reducing the particle size, the number of contact points available for the reaction increases substantially, leading to a much faster rate of hydrogen gas production. This principle is why magnesium powder is often used in demonstrations to achieve a rapid reaction.
The purity of the magnesium can also slightly modify the reaction rate. Impurities within the metal can sometimes inhibit the reaction or, conversely, accelerate it by creating localized electrochemical cells on the surface. However, the influence of surface area remains the dominant physical factor affecting the reaction kinetics.