Acid-Base Character of Period 3 Oxides

Across Period 3 the oxides change from basic, through amphoteric, to acidic. This follows the change in bonding from ionic to covalent.Basic oxides: Na₂O and MgO. These are ionic and contain the oxide ion, O²⁻. The oxide ion is a proton acceptor (a base), so these oxides react with acids to form a salt and water, and they give alkaline solutions in water.Amphoteric oxide: Al₂O₃. Aluminium oxide is ionic with some covalent character, and it reacts with both acids and bases. It is insoluble in water, so it does not change the pH.Acidic oxides: SiO₂, P₄O₁₀, SO₂ and SO₃. These are covalent. They react with bases to form a salt and water, and the soluble ones give acidic solutions in water. Silicon dioxide is insoluble in water, so it has no effect on pH, but it is still classed as acidic because it reacts with hot, concentrated sodium hydroxide solution.So the key idea is: ionic oxide → basic, covalent oxide → acidic, with aluminium oxide (in between) being amphoteric. You judge acid-base character by what an oxide reacts with, not only by the pH it gives in water.Worked example: an oxide is a white solid with a very high melting point, insoluble in water, which dissolves in both dilute hydrochloric acid and sodium hydroxide solution. It reacts with an acid and a base, so it is amphoteric: aluminium oxide.