Reactions of Sodium and Magnesium with Water

Sodium reacts vigorously with cold water. It floats, melts into a ball (the reaction is exothermic and sodium has a low melting point), fizzes as hydrogen is given off and moves around on the surface until it disappears. The solution left is strongly alkaline (pH about 12 to 14).2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)Magnesium reacts only very slowly with cold water, giving a few bubbles of hydrogen over a long time. Magnesium hydroxide is only slightly soluble, so the solution is only weakly alkaline (pH about 9 to 10).Mg + 2H₂O → Mg(OH)₂ + H₂Magnesium reacts much faster with steam. It burns with a bright white light and forms magnesium oxide instead of the hydroxide:Mg(s) + H₂O(g) → MgO(s) + H₂(g)Why is sodium more reactive? Sodium only has to lose one electron to form Na⁺, but magnesium must lose two electrons to form Mg²⁺. Much more energy is needed to remove two electrons, so magnesium reacts more slowly.Worked example: 0.46 g of sodium (A_r = 23.0) reacts with water. Moles of Na = 0.46 ÷ 23.0 = 0.020 mol. From the equation 2 mol Na gives 1 mol H₂, so 0.010 mol of hydrogen is formed.