Predict oxide–water reactions and describe phosphoric, sulfurous and sulfuric acids and their anions.
Separate solubility, reaction and acidity
An oxide can be acidic without dissolving or reacting appreciably in water. Test the solution after mixing, and distinguish an unchanged suspension from a true dissolved product. These reactions use sufficient water; concentrated acid mixtures and different temperatures can behave differently.
| Oxide | Outcome | Solution behaviour |
|---|---|---|
| Na₂O | Reacts to form dissolved NaOH | Strongly alkaline; pH above 7 |
| MgO | Slow reaction to form sparingly soluble Mg(OH)₂ | Weakly alkaline suspension/solution, commonly around pH 9–10 |
| Al₂O₃ | No appreciable reaction with water | Water approximately unchanged |
| SiO₂ | No appreciable reaction with water | Water approximately unchanged |
| P₄O₁₀ | Reacts strongly with sufficient water to form H₃PO₄ | Acidic; phosphoric acid is weak |
| SO₂ | Dissolves; establishes acid–base equilibria in water | Acidic; conventionally described using H₂SO₃ |
| SO₃ | Reacts vigorously to form H₂SO₄ | Acidic; strongly acidic when appreciably concentrated |
Write the species made by water
NaOH dissociates into Na⁺ and OH⁻. Magnesium hydroxide produces only a small dissolved concentration of OH⁻ because it is sparingly soluble. For sulfur dioxide, the H₂SO₃ equation is the conventional A-level representation of aqueous sulfurous acid; much dissolved sulfur dioxide remains as hydrated/dissolved SO₂. It is not converted to H₂SO₄ simply by adding water.
Why pH is not a label permanently attached to an oxide
A numerical pH depends on concentration, temperature, extent of reaction and solubility. Do not memorise “P₄O₁₀ always gives pH 0” or compare two acids at unspecified concentrations as though strength alone fixed pH. A strong acid ionises extensively; a concentrated acid contains many moles per unit volume. These are different descriptions.
Original worked example: 0.310 g Na₂O reacts completely and the solution is made up to 250.0 cm³. With Mᵣ(Na₂O) = 62.0, n(Na₂O) = 0.00500 mol, so n(OH⁻) = 0.0100 mol. [OH⁻] = 0.0400 mol dm⁻³. At 298 K, using Kw = 1.00 × 10⁻¹⁴, [H⁺] = 2.50 × 10⁻¹³ mol dm⁻³ and pH = 12.60. The volume and temperature are essential to the answer.
All the acidic hydrogens are bonded to oxygen
In the usual displayed structures, H₃PO₄ is O=P(OH)₃; H₂SO₃ is O=S(OH)₂ with a lone pair on sulfur; H₂SO₄ is O=S(=O)(OH)₂. Removing H⁺ from an O–H group leaves a negatively charged oxygen in a localised drawing. Never remove a whole OH group when forming the conjugate base. The O atoms around phosphorus in H₃PO₄ and around sulfur in H₂SO₄ are approximately tetrahedral; H₂SO₃ has a trigonal-pyramidal arrangement around sulfur because of its lone pair.
Phosphoric acid loses protons successively to H₂PO₄⁻, HPO₄²⁻ and PO₄³⁻. Sulfurous acid gives HSO₃⁻ then SO₃²⁻; sulfuric acid gives HSO₄⁻ then SO₄²⁻. Successive dissociations are not equally extensive. Resonance means the full anion is not best described by a unique, permanently localised double bond.
| Anion | Charge | Arrangement around central atom |
|---|---|---|
| Phosphate, PO₄³⁻ | 3− | Four O atoms: tetrahedral, about 109.5° |
| Sulfite, SO₃²⁻ | 2− | Three O atoms and a sulfur lone pair: trigonal pyramidal |
| Sulfate, SO₄²⁻ | 2− | Four O atoms: tetrahedral, about 109.5° |
Diagram placeholder
Acid and oxyanion displayed structures to add
Labels to include:
- H₃PO₄: one P=O and three P–O–H groups
- H₂SO₃: one S=O, two S–O–H groups and sulfur lone pair
- H₂SO₄: two S=O and two S–O–H groups
- PO₄³⁻: one P=O and three P–O⁻ in one resonance contributor
- SO₃²⁻: one S=O, two S–O⁻ and sulfur lone pair
- SO₄²⁻: two S=O and two S–O⁻ in one resonance contributor
- Brackets and total anion charges
Pair each acid with its fully deprotonated anion; put H on O, label the bonds and localised oxygen charges, and show the central geometry with appropriate wedges. Indicate equivalent resonance contributors instead of implying one oxygen is permanently special.
Use charge conservation for partial dissociation
Each stage loses one H⁺ and lowers the remaining species charge by one unit. The first stage of phosphoric acid is a weak-acid equilibrium; a later neutralisation question may instead drive several stages to completion with a base.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Give the equation for P₄O₁₀ reacting with sufficient water.Show answer
P₄O₁₀ + 6H₂O → 4H₃PO₄. This makes phosphoric(V) acid, not an oxide containing four independent phosphorus atoms in the product.
Q2. Why can SiO₂ leave the water near pH 7 and still be an acidic oxide?Show answer
It does not react appreciably with water. Its acidic classification follows from reactions with bases, such as hot concentrated NaOH, rather than requiring an acidic aqueous solution.
Q3. Draw the localised sulfite structure in words and identify its shape.Show answer
Place sulfur centrally with one S=O, two S–O⁻ bonds and one lone pair on sulfur. Enclose the ion in brackets with overall 2− charge. The three oxygen positions are trigonal pyramidal; equivalent resonance forms can put the double bond on another oxygen.
Q4. A solution contains 0.00200 mol Na₂O fully reacted in a final volume of 0.500 dm³. Calculate its pH at 298 K; Kw = 1.00 × 10⁻¹⁴.Show answer
n(OH⁻) = 2 × 0.00200 = 0.00400 mol. [OH⁻] = 0.00800 mol dm⁻³. [H⁺] = 1.25 × 10⁻¹² mol dm⁻³, so pH = 11.90.
Q5. What changes when HSO₄⁻ loses its remaining acidic proton?Show answer
An O–H bond loses H⁺ and the ion becomes SO₄²⁻. The oxygen is retained. HSO₄⁻ ⇌ H⁺ + SO₄²⁻ has −1 total charge on both sides.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- AQA 7405 inorganic chemistry specification — 3.2.4 coverage and required skills.
- Chemrevise: Properties of Period 3 elements and their oxides — Coverage checklist; explanations, data exercises and quick checks on this page are original Finesse material.
- AQA June 2023 Paper 1 mark scheme — Q03.1–03.5, pp14–15: combustion, sulfur dioxide in water, oxyanion structure, steam and acid–oxide stoichiometry; paired with report Q03 p3.
- AQA June 2023 Paper 1 examiner report — Read alongside the question-specific marking guidance; not a universal wording checklist.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.