Reactions of Period 3 Oxides with Water

When the Period 3 oxides are added to water, the metal oxides form hydroxides and the non-metal oxides form acids.Sodium oxide reacts exothermically and dissolves to form sodium hydroxide:Na₂O + H₂O → 2NaOHMagnesium oxide reacts to form magnesium hydroxide, which is only slightly soluble:MgO + H₂O → Mg(OH)₂Aluminium oxide and silicon dioxide are insoluble in water and do not react. The Al₂O₃ lattice is too strong to break up, and SiO₂ is a giant covalent network.Phosphorus(V) oxide reacts vigorously to form phosphoric(V) acid:P₄O₁₀ + 6H₂O → 4H₃PO₄Sulfur dioxide dissolves to form sulfurous acid (sulfuric(IV) acid), and sulfur trioxide reacts violently to form sulfuric acid:SO₂ + H₂O → H₂SO₃
SO₃ + H₂O → H₂SO₄Worked example: 2.84 g of P₄O₁₀ (M_r = 284) is added to excess water. Moles = 2.84 ÷ 284 = 0.0100 mol. Each mole forms 4 mol H₃PO₄, so 0.0400 mol H₃PO₄ forms, which is 0.0400 × 98.0 = 3.92 g.