Why does hot steam produce MgO instead of Mg(OH)2?
There are two steps in the process:

$\ce{Mg + H2O -&gt; MgO + H2}$

and

$\ce{MgO + H2O &lt;-&gt; Mg(OH)2}$

The equilibrium point in the second step is dependent on the temperature. See file Equilibrium Assemblages for more information.