AQA GCSE Chemistry Chemistry Paper 1 (Higher), June 2025: Question 2

✅ Correct Answer

CaS

💡 Key Knowledge

The empirical formula is the simplest whole-number ratio of atoms or ions in a compound.

• Calcium is in Group 2 → forms Ca²⁺
• Sulfur is in Group 6 → forms S²⁻
• Ratio is 1:1, giving the formula CaS .

🧠 Exam Technique

Always write chemical symbols with correct case (capital C , lowercase a , capital S ). Do not include charges in an empirical formula (e.g. do not write Ca²⁺S²⁻).

❌ Common Errors

• Counting the total spheres in the diagram and writing something like Ca₁₄S₁₃ .
• Incorrect capitalisation like CAS or cas .

✅ 4-Mark Model Answer

1. Calcium (atom) loses electrons [1]
2. Sulfur (atom) gains electrons [1]
3. There is a transfer of two electrons [1]
4. Calcium forms a positive ion (or Ca²⁺ ) and sulfur forms a negative ion (or S²⁻ ) [1]

💡 Key Knowledge

• Metals lose electrons from their outer shell to form positive cations.
• Non-metals gain electrons into their outer shell to form negative anions.
• Group 2 elements have 2 outer electrons to lose. Group 6 elements need 2 electrons to complete a full shell of 8.

🧠 Structure Your Answer

When an exam prompt says "in terms of electrons and ions", check off both:

• Electrons: Who loses? Who gains? Exactly how many?
• Ions: Name each ion and state its charge (positive/negative or Ca²⁺/S²⁻).

❌ Common Errors

• Saying calcium and sulfur "share" electrons (confusing ionic with covalent bonding).
• Saying electrons are transferred, but failing to specify two electrons.
• Calling the negative ion "sulfate" instead of "sulfide" or "sulfur ion".

✅ Model Answer (Level 2: 4 Marks)

"Graphite has a giant covalent structure composed of carbon atoms arranged in layers of hexagonal rings. Each carbon atom forms three strong covalent bonds by sharing pairs of electrons. There are no covalent bonds between the layers, only weak forces. Each carbon atom has one unused outer electron, resulting in delocalised electrons that can move through the layers."

🧠 Level of Response Criteria

• Level 2 (3–4 marks): Relevant points from both structure and bonding are identified and linked clearly to form a detailed, coherent account.
• Level 1 (1–2 marks): Isolated points stated without clear distinction or incomplete detail (e.g. mentions "layers" and "covalent bonds" only).

✅ Correct Answer (Any one of the following)

• The molecules do not have an overall electric charge.
• There are no free ions.
• There are no delocalised electrons.

💡 Key Rule for Conduction

For any substance to conduct electricity, it must contain charged particles that are free to move:

• Metals/Graphite → delocalised electrons
• Molten/Aqueous Ionic → mobile ions
• Small molecules → neutral molecules, no free charges!

❌ Insufficient Answers

• "They have strong covalent bonds" (Irrelevant to conductivity).
• "Electrons can't move" (Too vague — must specify that there are no delocalised electrons or no free ions).

🧠 Exam Tip

Always state the specific particle! Don't just write "no charge"; write "the molecules have no overall charge" or "there are no free ions".

✅ Correct Choice

[ ✓ ] Poly(ethene) has stronger intermolecular forces to overcome than methane.

💡 Why is this true?

Poly(ethene) molecules are very large polymer chains compared to tiny methane ( CH₄ ) molecules.

As molecular size increases, the strength of the intermolecular forces increases, meaning more energy is needed to overcome them and melt the substance.