Oxides of Period 3 with water: the acid–base ladder

Every oxide in Period 3 gets asked the same two questions.

1. Water test: does it react with water at all?
2. Acid–base test: does it behave as a base (reacts with acid), an acid (reacts with alkali), or both?

The answers slide smoothly from one end of the period to the other. There is no jump, no sudden switch — just a steady change in character.

Sodium and magnesium oxides are ionic. The oxide ion, $\text{O}^{2-}$, is a hungry base, so it grabs protons from water:

$\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH}$

That gives a strongly alkaline solution, pH about 13–14.

Magnesium oxide reacts too, but the product is different:

$\text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2$

$\text{Mg(OH)}_2$ is only slightly soluble, so few $\text{OH}^-$ ions reach the solution. The pH is about 9–10: alkaline, but nowhere near sodium hydroxide.