AQA A-Level Chemistry: Properties of Period 3 elements and their oxides (7405) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.2.4 Properties of Period 3 elements and their oxides of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards. The whole section is A-level only. Inorganic chemistry is on A-level Paper 1, and A-level Paper 3 can test any content.

Learn the ideas first in the Period 3 oxides study guide and the Period 3 oxides revision notes. The course hub is AQA A-Level Chemistry, and the printable checklist lists every specification point. Melting points of the elements themselves are tested in the periodicity practice questions.

Questions

1. Which oxide reacts with water to form a solution with a pH of about 9?

• A Na₂O
• B MgO
• C SiO₂
• D SO₂

[1]

2. A small piece of sodium is added to a trough of cold water.

(a) Write an equation, with state symbols, for the reaction. [2] (b) Give two observations you would make. [2]

3. This question is about magnesium and water.

(a) Write an equation for the reaction of magnesium with cold water. [1] (b) Write an equation for the reaction of magnesium with steam and give one observation. [2] (c) Explain why sodium reacts more vigorously with cold water than magnesium does. [2]

4. This question is about the reactions of Period 3 elements with oxygen.

(a) Write equations for the burning of aluminium and of phosphorus in excess oxygen. [2] (b) Give the oxidation state of sulfur in SO₂ and in SO₃, and state what is needed to convert SO₂ into SO₃. [2] (c) Suggest why a sheet of aluminium foil does not appear to react with oxygen at room temperature. [1]

5. This question is about the melting points of the highest oxides of Period 3.

(a) Explain why the melting point of SiO₂ is much higher than that of SO₃. [3] (b) Explain why the melting point of MgO is higher than that of Na₂O. [3] (c) Al³⁺ has a higher charge than Mg²⁺, but Al₂O₃ has a lower melting point than MgO. Suggest why. [2]

6. For each oxide, write an equation for its reaction with water and give the approximate pH of the solution formed.

(a) P₄O₁₀ [2] (b) SO₂ [2] (c) Na₂O [2]

7. Explain, in terms of bonding, why Na₂O forms an alkaline solution in water, SO₃ forms an acidic solution, and SiO₂ has no effect on the pH of water. [5]

8. This question is about the products of P₄O₁₀ and SO₂ with water.

(a) Draw the structure of phosphoric(V) acid, H₃PO₄. [1] (b) Draw the structure of the sulfate(IV) ion, SO₃²⁻, showing any lone pairs on sulfur. [2] (c) State the shape of the sulfate(VI) ion, SO₄²⁻. [1]

9. Aluminium oxide is amphoteric.

(a) State the meaning of the term amphoteric. [1] (b) Write an equation for the reaction of Al₂O₃ with sulfuric acid, and an ionic equation for its reaction with hydroxide ions. [2] (c) Calculate the volume, in cm³, of 2.00 mol dm⁻³ sodium hydroxide solution that reacts exactly with 1.53 g of Al₂O₃ to form NaAl(OH)₄. (Ar: Al = 27.0, O = 16.0) [3]

10. Phosphorus(V) oxide reacts with sodium hydroxide solution to form sodium phosphate, Na₃PO₄, and water.

(a) Write an equation for the reaction. [1] (b) Calculate the mass of NaOH needed to react exactly with 1.42 g of P₄O₁₀. (Ar: H = 1.0, O = 16.0, Na = 23.0, P = 31.0) [3]

11. Three white solid oxides of Period 3 elements, X, Y and Z, are tested.

• X: very high melting point, insoluble in water, reacts with both hydrochloric acid and sodium hydroxide.
• Y: high melting point, insoluble in water, no reaction with hydrochloric acid, reacts with hot, concentrated sodium hydroxide.
• Z: low melting point, formed directly when its element burns in excess oxygen, reacts with water to give a solution of pH about 0.

(a) Identify X, Y and Z. [3] (b) State the type of structure in Y and in Z. [2] (c) Write an equation for the reaction of Y with sodium hydroxide. [1] (d) Write an equation for the reaction of Z with magnesium oxide, with no water present. [1] (e) X is classed as amphoteric but Y as acidic. Use the test results to explain why. [1]

12. A 0.240 g sample of an oxide of sulfur, either SO₂ or SO₃, is dissolved in water. The solution needs 60.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide to react completely. Each mole of the acid formed reacts with two moles of NaOH. (Ar: O = 16.0, S = 32.1)

(a) Calculate the amount, in moles, of NaOH used. [1] (b) Calculate the amount, in moles, of the oxide in the sample. [1] (c) Calculate the Mr of the oxide and identify it. [2] (d) Write an equation for the reaction of the acid formed with sodium hydroxide. [1] (e) Solutions of the two possible acids are made at the same concentration. State which has the higher pH and explain why. [2]

Answers

1. B (MgO) [1] Examiner insight: Give one letter only; two letters score nothing.

2. (a) 2Na + 2H₂O → 2NaOH + H₂ balanced [1]; state symbols (s), (l), (aq), (g) correct [1] (b) Any two from: floats; melts into a ball; fizzes; moves around; disappears [1] [1] Examiner insight: “Hydrogen is produced” is a conclusion, not an observation; give what you would see, such as fizzing.

3. (a) Mg + 2H₂O → Mg(OH)₂ + H₂ [1] (b) Mg + H₂O → MgO + H₂ [1]; burns with a bright white flame, or white solid forms [1] (c) Na loses one electron per atom, but Mg must lose two [1]; the total energy needed to form Mg²⁺ is much greater, so Mg reacts more slowly [1] Examiner insight: Swapping the products (MgO for cold water, Mg(OH)₂ for steam) loses both equation marks, even when each equation is balanced.

4. (a) 4Al + 3O₂ → 2Al₂O₃ [1]; P₄ + 5O₂ → P₄O₁₀ [1] (b) +4 in SO₂ and +6 in SO₃ [1]; a catalyst (vanadium(V) oxide) and heat, with more oxygen [1] (c) The surface is covered by a thin protective layer of Al₂O₃ that stops further reaction [1] Examiner insight: Oxidation states need the sign; “4” and “6” without “+” may not be accepted.

5. (a) SiO₂ is macromolecular, with strong covalent bonds throughout [1]; SO₃ is simple molecular, held by weak van der Waals forces between molecules [1]; much more energy is needed to break the many covalent bonds in SiO₂ than to overcome the intermolecular forces in SO₃ [1] (b) Both are giant ionic lattices [1]; Mg²⁺ has a higher charge and a smaller radius than Na⁺ [1]; so the attraction between Mg²⁺ and O²⁻ is stronger and needs more energy to overcome [1] (c) Al³⁺ is small and highly charged, so it polarises the oxide ions [1]; the bonding has some covalent character, so it is not fully ionic [1] Examiner insight: In (a), saying covalent bonds break in SO₃ contradicts your answer and costs the SO₃ mark.

6. (a) P₄O₁₀ + 6H₂O → 4H₃PO₄ [1]; pH 0 to 1 [1] (b) SO₂ + H₂O → H₂SO₃ [1]; pH 2 to 3 [1] (c) Na₂O + H₂O → 2NaOH [1]; pH 13 to 14 [1] Examiner insight: A pH outside a reasonable range (for example pH 5 for SO₂) loses the mark even if the equation is right.

7. Na₂O is ionic and contains O²⁻ ions [1]; oxide ions react with water to form OH⁻ ions, O²⁻ + H₂O → 2OH⁻ [1]; SO₃ is simple molecular covalent [1]; it reacts with water to form H₂SO₄, which releases H⁺ ions [1]; SiO₂ is macromolecular with strong covalent bonds, so it is insoluble and the pH stays 7 [1] Examiner insight: Name the bonding type for each oxide; describing reactions alone misses the “in terms of bonding” marks.

8. (a) Correct structure: P with one P=O and three P–O–H bonds [1]

         O
         ‖
  H–O – P – O–H
         |
         O–H

(b) S with one S=O and two S–O⁻ bonds [1]; one lone pair shown on S and overall charge 2− [1]

          O
          ‖
   ⁻O – S – O⁻     (one lone pair on S)

(c) Tetrahedral [1] Examiner insight: SO₃²⁻ drawn with three double bonds and no charges, or without the lone pair, loses the second mark in (b).

9. (a) Reacts with (or behaves as) both an acid and a base [1] (b) Al₂O₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂O [1]; Al₂O₃ + 2OH⁻ + 3H₂O → 2Al(OH)₄⁻ [1] (c) Mr Al₂O₃ = 102.0, so moles = 1.53 / 102.0 = 0.0150 mol [1]; moles NaOH = 2 × 0.0150 = 0.0300 mol [1]; volume = 0.0300 / 2.00 × 1000 = 15.0 cm³ [1] Examiner insight: With a wrong mole ratio in (c), the volume step can still score as error carried forward if it follows from your ratio.

10. (a) P₄O₁₀ + 12NaOH → 4Na₃PO₄ + 6H₂O [1] (b) Mr P₄O₁₀ = 284.0, so moles = 1.42 / 284.0 = 0.00500 mol [1]; moles NaOH = 12 × 0.00500 = 0.0600 mol [1]; mass = 0.0600 × 40.0 = 2.40 g [1] Examiner insight: Take the 1 : 12 ratio from the P₄O₁₀ equation, not 1 : 3 from H₃PO₄ + 3NaOH.

11. (a) X is Al₂O₃ [1]; Y is SiO₂ [1]; Z is P₄O₁₀ [1] (b) Y: macromolecular (giant covalent) [1]; Z: simple molecular [1] (c) SiO₂ + 2NaOH → Na₂SiO₃ + H₂O [1] (d) P₄O₁₀ + 6MgO → 2Mg₃(PO₄)₂ [1] (e) X reacts with both an acid and a base, but Y reacts only with a base [1] Examiner insight: In (a), give formulas or full names; “aluminium” or “phosphorus” without “oxide” names the element, not the oxide.

12. (a) 60.0 / 1000 × 0.100 = 6.00 × 10⁻³ mol [1] (b) 6.00 × 10⁻³ / 2 = 3.00 × 10⁻³ mol [1] (c) Mr = 0.240 / 3.00 × 10⁻³ = 80.0 [1]; this matches SO₃ (Mr 80.1), not SO₂ (Mr 64.1), so the oxide is SO₃ [1] (d) H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O [1] (e) H₂SO₃ (from SO₂) has the higher pH [1]; it is a weak acid that only partly dissociates, so it releases fewer H⁺ ions [1] Examiner insight: In (c) the identification must be justified by comparing your Mr with both possible values; a bare “SO₃” scores only the calculation mark.

Where marks are usually lost

• Giving MgO as the product of magnesium with cold water.
• Writing H₂SO₄ instead of H₂SO₃ for the product of SO₂ and water.
• Saying Al₂O₃ and SiO₂ “dissolve to give a neutral solution” rather than “are insoluble”.
• Describing melting of P₄O₁₀ or SO₃ as breaking covalent bonds.
• Missing the three water molecules when Al₂O₃ reacts with hydroxide ions.
• Drawing sulfate(IV) without the lone pair on sulfur, or with the wrong charge.

Next steps

• Period 3 oxides revision notes
• Period 3 oxides study guide
• Periodicity practice questions
• AQA A-Level Chemistry exam preparation
• AQA A-Level Chemistry hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.2.4 Properties of Period 3 elements and their oxides (A-level only).