Each reaction gives a different product

Reacts very slowly in water

  • sinks due to higher density
  • a few bubbles of hydrogen formed over several days
  • less alkaline solution of magnesium hydroxide is formed due to it being sparingly soluble in water (pH~10)
Mg(s)+ 2H2O(l) → Mg(OH)2(aq) + H2(g)

Here's a video of magnesium in water

Reacts much faster in steam

  • glows white
  • hydrogen produced which ignites and burns with a yellow flame
  • white solid magnesium oxide is formed

Mg(s)+ H2O(g) → MgO(s) + H2(g)

Here's a video of magnesium in steam

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