AQA A-Level Chemistry Paper 3, June 2018: Question 25
1 mark · Medium difficulty · Multiple Choice
Identify which Period 3 element forms an ionic oxide that reacts with strong alkalis.
Practise this questionQuestion
Question text
25 Which element forms an ionic oxide that reacts with strong alkalis?
[1 mark]
A Aluminium
B Magnesium
C Sodium
D Sulfur
Mark scheme
Show the mark scheme
25 A
How to answer it
Period 3 Oxides: Structure, Bonding & Acid-Base Character
This question assesses knowledge from AQA A-Level Chemistry Section 3.2.4 (Properties of Period 3 Elements and their Oxides):
- Classifying bonding and crystal structures of Period 3 oxides (ionic, giant covalent, simple molecular).
- Acid-base nature of oxides: distinguishing between basic, amphoteric, and acidic behavior.
- Reactions of Period 3 oxides with strong acids and strong alkalis.
Question 25 Analysis
Multiple Choice Question (1 Mark)
✅ Correct Option: A (Aluminium)
Reasoning: Aluminium forms aluminium oxide (Al₂O₃), which has ionic bonding with additional covalent character. Because it is amphoteric, it reacts with both strong acids and strong alkalis.
Reaction with a strong alkali (e.g. concentrated NaOH):
Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq)
Or in ionic form:
Al₂O₃(s) + 2OH⁻(aq) + 3H₂O(l) → 2[Al(OH)₄]⁻(aq)
💡 Key Knowledge: Period 3 Oxides Summary
You must know the bonding and acid-base nature across Period 3:
| Element Oxide | Bonding & Structure | Acid-Base Character | Reacts with Alkalis? |
|---|---|---|---|
| Na₂O | Ionic lattice | Basic | ❌ No |
| MgO | Ionic lattice | Basic | ❌ No |
| Al₂O₃ | Ionic with covalent character | Amphoteric | ✅ Yes |
| SiO₂ | Macromolecular (covalent) | Acidic (very weak) | ✅ Yes (hot, conc. NaOH) |
| P₄O₁₀ | Simple molecular (covalent) | Acidic | ✅ Yes |
| SO₂ / SO₃ | Simple molecular (covalent) | Acidic | ✅ Yes |
🧠 Exam Technique: Two-Condition Filtering
Break the question stem into two essential criteria:
- Condition 1: "Ionic oxide"
• Sodium (Na₂O): Ionic ✅
• Magnesium (MgO): Ionic ✅
• Aluminium (Al₂O₃): Ionic ✅
• Sulfur (SO₂/SO₃): Covalent molecular ❌ (Eliminate D immediately!) - Condition 2: "Reacts with strong alkalis"
• Na₂O and MgO are basic oxides—they only react with acids, not alkalis ❌
• Al₂O₃ is amphoteric—it reacts with both acids and strong bases ✅
Therefore, only Aluminium meets both conditions.
❌ Common Misconceptions & Errors
- Picking Sulfur (D): Many students focus only on "reacts with strong alkalis" and recall that sulfur oxides are acidic and react with bases. However, sulfur oxides (SO₂ and SO₃) have covalent bonding and simple molecular structures, violating the first condition.
- Confusing Amphoteric with Neutral: Amphoteric does not mean unreactive or neutral; it means the substance can act as an acid (reacting with bases) or as a base (reacting with acids).
- Assuming Aluminium Oxide is purely covalent: AQA explicitly classifies Al₂O₃ as ionic with covalent character due to the high charge density of the small Al³⁺ ion polarizing the oxide ion's electron cloud. In multiple-choice questions, it is counted under the "ionic" group of Period 3 oxides.
• 1 mark for shading bubble A. No intermediate working marks for Section B multiple-choice questions.
Topics
Inorganic Chemistry · 3.2.4 Properties of Period 3 Elements and Their Oxides
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.