CBSE Class 10 Answered

 The reactive metals like potassium, sodium and calcium, produce the hydroxide when they react with water, and not the simple oxide. This is because the hydroxides of these metals are much more stable than the oxides. Indeed, potassium oxide, sodium oxide and calcium oxide will react with water (or even moist air!) to form the hydroxide of the metal. The same thing doesn't apply to metals like magnesium, zinc or iron. Because the metals lower in the series form stable oxides.

2 K + H₂O -----> 2 KOH + H₂

Mg + H₂O  -------> MgO + H₂