3.1.5—Metal/non-metal continuum

Use balanced equations as evidence for the classification: Na₂O + H₂O → 2NaOH and SO₃ + H₂O → H₂SO₄ are representative basic and acidic cases. The bonding/electronegativity trend explains why the character changes across the period, but it does not guarantee that every oxide reacts readily with water.

Al₂O₃ is the useful boundary case: it is amphoteric, so it can react with an acid such as HCl and with a strong base such as NaOH. Do not label an oxide from the element's position alone—check the stated reaction and distinguish a water reaction from acid–base behaviour in another medium.

Environmental link: sulfur oxides dissolve and can be oxidized to acids that increase $\ce{H+}$ in rainwater, causing acid rain. Atmospheric $\ce{CO2}$ dissolves in seawater and participates in $\ce{CO2 + H2O <=> H2CO3 <=> H+ + HCO3-}$, increasing $\ce{H+}$ and lowering ocean pH. These are acidification mechanisms; do not treat every non-metal oxide as reacting with water in exactly the same way.