Why does magnesium react mildly with cold water but vigorously with hot water or steam producing magnesium hydroxide and hydrogen?

Magnesium, being an alkaline earth rare earth (s-block) metal, reacts with water more actively than most other transition (d-block) metals such as iron, copper, zinc, etc do. Yet the reaction is slower compared with the alkali metals such as lithium, sodium and potassium, which react violently with, even ice-cold water. 

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Magnesium reacts with water to give magnesium hydroxide and hydrogen as follows:Mg+2H{-2}O -> Mg(OH){-2}+H{-2}Normally Mg is coated with a thin layer of its oxide (MgO) when it is exposed to air. This layer induces passivity to Mg in its reaction with water.The rate of any reaction depends on the kinetic energy of the reactants (chemicals participating in the reaction). In other words, the reactants can transform chemically only when they possess a minimum kinetic energy known as `activation energy.' According to the Boltzmann distribution of energies, and the Arrhenius theory of reaction rates, the number of entities possessing activation energy and hence the reaction rate increase with increasing temperature. Therefore, we can say that the above reaction happens more speedily with hot water and steam. In cold water neither the activation energy is supplied nor the oxide layer is disturbed to promote further reaction of magnesium.The rate of any chemical reaction is also accentuated by factors that destabilise the reactants and those that remove the products from the scene of the reaction or stabilise the products.

The hydrogen atoms produced, are initially adsorbed on the surface of magnesium metal before they leave as hydrogen molecules. Thus, the metal surface is dynamically dressed by hydrogen atoms and thereby screened from further attack by water.Hence, the reaction is mild in cold water. When the magnesium metal is exposed to hot water or steam, the temperature of the metal increases and its interatomic bonds loosen (destabilise) making the magnesium atoms to give away their electrons more readily.Besides, the hydrogen atoms produced, are not effectively adsorbed on the hot surface of the magnesium metal and the metal is, thereby, continuously exposed to the attack of hot water molecules. Further, in hot water, the water molecules are more ionised (destabilised) giving more number of hydrogen ions to accept the electrons and more hydroxide ions to form the stable magnesium hydroxide. Hence, the reaction is vigorous in hot water and in steam.Another equally important reason for the vigorous reaction in hot water is that the produced hydrogen gas is more thoroughly expelled from the layer of magnesium than in cold water due to the prevailing turbulence and also as gases are generally less soluble under hot conditions in liquids.

Source: thehindu.com