AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2025: Question 9
11 marks · Standard Demand difficulty · Extended Answer
Determine the empirical formula and describe ionic bonding in calcium sulfide, describe the structure and bonding of graphite, explain why small molecules do not conduct electricity, and compare the melting points of poly(ethene) and methane.
Practise this questionQuestion
Question text
09 This question is about structure and bonding.
Calcium sulfide is an ionic compound.
Figure 13 represents the structure of calcium sulfide.
Figure 13
09.1 Determine the empirical formula of calcium sulfide.
[1 mark]
Empirical formula
09.2 Calcium is in Group 2 of the periodic table.
Sulfur is in Group 6 of the periodic table.
Describe what happens when a calcium atom reacts with a sulfur atom.
Answer in terms of electrons and ions.
[4 marks]
Figure 14 shows the structure of graphite.
Figure 14
09.3 Describe the structure and bonding of graphite.
*34* [4 marks]
Extra space
09.4 Substances that consist of small molecules do not conduct electricity.
Give one reason why.
*35* [1 mark]
09.5 Methane consists of small molecules.
Poly(ethene) is a polymer.
Why does poly(ethene) have a higher melting point than methane?
[1 mark]
Tick ( ) one box.
Poly(ethene) has more covalent bonds to break than methane.
Poly(ethene) has no intermolecular forces to overcome.
Poly(ethene) has stronger covalent bonds to break than methane.
Poly(ethene) has stronger intermolecular forces to overcome
than methane.
Mark scheme
Show the mark scheme
Question 9
AO /
Question Answers Extra information Mark
Spec. Ref.
09.1 CaS 1 AO2
4.2.1.3
AO /
Spec. Ref.
09.2 calcium (atom) loses electrons 1 AO2
4.2.1.1
4.2.1.2
sulfur (atom) gains electrons 1
reference to transfer of two 1
electrons
calcium forms positive ions and allow Ca2+ (ions) and S2- (ions) 1
sulfur forms negative ions are formed
allow calcium ions and sulfide
ions are formed
AO /
Question Answers Mark
Spec. Ref.
09.3 Level 2: Scientifically relevant facts, events or processes are 3–4 AO1
identified and given in detail to form an accurate account. 4.2.1.1
4.2.1.4
4.2.2.6
Level 1: Facts, events or processes are identified and simply 1–2 4.2.3.2
stated but their relevance is not clear.
No relevant content 0
Indicative content
• giant structure
• layers
• of hexagonal rings
• each (carbon) atom forms three bonds
• has no covalent bonds between layers
• with delocalised electrons
• one delocalised electron from each carbon atom
• the bonds are covalent 25
• with shared (pairs of) electrons
• the (covalent) bonds are strong
AO /
Spec. Ref.
09.4 the molecules do not have an allow there are no ions 1 AO1
(overall electric) charge 4.2.2.4
allow there are no delocalised
electrons
AO /
Spec. Ref.
09.5 poly(ethene) has stronger 1 AO1
intermolecular forces to 4.2.2.4
overcome than methane 4.2.2.5
Total Question 9 11
How to answer it
Structure, Bonding, and Properties of Matter
This question assesses fundamental concepts across AQA Chemistry Topic 2 (Bonding, Structure, and Properties of Matter):
- Deducing an empirical formula from a giant ionic lattice representation.
- Explaining ionic bond formation via electron transfer and ion formation (Group 2 with Group 6).
- Describing the bonding and giant covalent structure of graphite.
- Explaining why small molecular substances cannot conduct electricity.
- Comparing the melting points of polymers and small covalent molecules using intermolecular forces.
Empirical Formula of Calcium Sulfide
Deducing the ratio of ions in an ionic lattice
✅ Correct Answer
CaS
💡 Key Knowledge
- An empirical formula gives the simplest whole-number ratio of atoms or ions in a compound.
- Calcium forms a Ca²⁺ ion (Group 2, loses 2 electrons).
- Sulfur forms an S²⁻ ion (Group 6, gains 2 electrons).
- Because the charges balance at 1:1, the empirical formula is CaS.
❌ Common Errors
- Counting all spheres in the diagram: Trying to count each sphere in the unit cell (e.g. writing Ca₁₂S₁₄) instead of giving the simplest ratio.
- Incorrect case: Writing CAS or cas . Chemical symbols must always use correct upper and lower case letters.
Ionic Bonding: Reaction Between Calcium and Sulfur
Describing electron transfer and ion formation
✅ Model 4-Mark Response
When calcium reacts with sulfur:
- Calcium loses electrons. [1 mark]
- Sulfur gains electrons. [1 mark]
- There is a transfer of two electrons from each calcium atom to each sulfur atom. [1 mark]
- Calcium forms a positive ion (Ca²⁺) and sulfur forms a negative ion (S²⁻). [1 mark]
🧠 Exam Technique: Break It Down
The prompt asks to "Answer in terms of electrons and ions". Structure your answer using a four-step checklist:
- Who loses? → Calcium loses electrons.
- Who gains? → Sulfur gains electrons.
- How many? → Exactly 2 electrons transferred.
- What ions form? → Ca²⁺ (positive ion) and S²⁻ (negative ion / sulfide ion).
❌ Common Errors
- Vague sharing: Writing that atoms "share electrons" — this describes covalent bonding, not ionic bonding!
- Missing the number: Stating that electrons are transferred without specifying two electrons loses the specific quantitative mark.
- Confusing ion charges: Claiming calcium becomes negative or sulfur becomes positive. Remember: losing negative electrons makes an atom positive!
Structure and Bonding of Graphite
Extended response on giant covalent structures
✅ Level 2 (3–4 Marks) Model Answer
Graphite has a giant covalent structure arranged in flat layers of hexagonal rings.
- Each carbon atom forms three strong covalent bonds with neighbouring carbon atoms.
- Because carbon has four outer electrons, there is one delocalised electron per carbon atom that is free to move throughout the layers.
- There are no covalent bonds between the layers (only weak intermolecular forces), allowing the layers to slide over one another easily.
💡 Mark Scheme Breakdown
Level 2 (3–4 marks): Scientifically relevant facts are identified and linked in detail to give a complete, accurate account covering structure AND bonding.
Level 1 (1–2 marks): Isolated facts stated without clear structure (e.g. just mentioning "layers" or "covalent bonds").
❌ Common Errors
- Confusing graphite with diamond: Stating carbon forms four bonds. In graphite, carbon forms only three bonds; diamond forms four.
- Calling the bonds between layers covalent: There are NO covalent bonds between the layers, only weak intermolecular forces. Covalent bonds only exist within the layers.
- Omitting bonding: Describing only the shape (layers of hexagons) while forgetting to state that the bonds are covalent and that delocalised electrons exist.
Electrical Conductivity of Small Molecules
Why simple molecular substances are electrical insulators
✅ Accepted Answers (Any ONE)
- The molecules do not have an overall electric charge.
- There are no ions.
- There are no delocalised electrons (or free electrons).
🧠 Exam Golden Rule for Conduction
To conduct electricity, a substance must have charged particles that are free to move. Always name the specific particle:
- In metals & graphite → delocalised electrons.
- In molten/aqueous ionic compounds → ions.
- Small molecules have neither free ions nor delocalised electrons!
❌ Common Errors
- Writing just "electrons cannot move" without stating that there are no delocalised / free electrons.
- Writing "it is a non-metal" — this is an observation, not a scientific explanation.
Melting Point: Poly(ethene) vs Methane
Multiple-choice question comparing polymers and small molecules
✅ Correct Checkbox Selection
☑ Poly(ethene) has stronger intermolecular forces to overcome than methane.
💡 Why Poly(ethene) Has a Higher Melting Point
- Both methane and poly(ethene) consist of covalently bonded molecules.
- When melting simple molecular substances or polymers, you overcome intermolecular forces, NOT covalent bonds.
- Poly(ethene) molecules are much larger (longer chains) than small methane (CH₄) molecules.
- Larger molecules have stronger intermolecular forces, requiring more thermal energy to overcome.
❌ The #1 Student Trap in GCSE Chemistry
Many students ticked "Poly(ethene) has stronger covalent bonds to break than methane". This is completely incorrect!
Key Rule: When simple molecular substances or polymers melt or boil, covalent bonds do not break . Only the weak intermolecular forces between molecules are overcome.
Topics
Chemistry · C2: Bonding, Structure and the Properties of Matter
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Foundation), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.