AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2021: Question 6

✅ Correct Answer

Tick box C.

💡 Key Knowledge

• Metals have delocalised electrons that are free to move throughout the structure, meaning they conduct electricity in both solid and liquid states.
• They also typically have high melting and boiling points due to strong electrostatic attraction between positive ions and delocalised electrons.

🧠 Deductions for All Substances

• A: Low mp/bp, never conducts → Small covalent molecule.
• B: High mp, conducts only when molten → Ionic compound.
• C: High mp/bp, conducts as solid and liquid → Metal.
• D: High mp/bp, does not conduct → Giant covalent structure.

❌ Common Errors

Confusing substance B (ionic) with a metal. Remember: ionic substances have ions fixed in position in a solid lattice, so they cannot conduct electricity as solids.

✅ Model Answer (3/3 Marks)

1. In an alloy, the added atoms are of different sizes [1 mark].
2. This distorts the regular layers of atoms [1 mark].
3. Therefore, the layers cannot slide over each other as easily [1 mark].

💡 Key Knowledge

• Pure metals: Atoms are all identical in size and arranged in regular, uniform layers. When force is applied, these layers slide easily (malleable & soft).
• Alloys: A mixture of a metal with at least one other element. The different atomic radiuses disrupt the lattice regularity.

❌ Common Errors & Misconceptions

• "Alloys have stronger bonds": Do not claim metallic bonds are "stronger" or "tighter". Hardness is about layer movement, not bond breaking.
• Referring to "molecules": Metals and alloys consist of atoms/ions in a giant metallic lattice; there are no molecules.
• Incomplete answers: Stating that "layers can't slide" without explaining why (different sized atoms causing distortion) will forfeit marks 1 and 2.

🧠 Exam Technique: The 3-Step Chain

Always write alloy hardness explanations in this logical cause-and-effect sequence:

Different sizes → Distorted layers → Layers slide less easily

📐 How Marks are Awarded (Mark Scheme Breakdown)

• Mark 1 (AO1): Measure temperature change (or measure temperature before and after reaction).
• Mark 2 (AO1): When each metal is added to silver nitrate solution.
• Mark 3 (AO2): Control variable specified — same concentration/volume of silver nitrate solution OR same mass/moles of metal powder OR same initial temperature.
• Mark 4 (AO3): Valid comparison — the greater the temperature change, the more reactive the metal.

🧠 Examiner Insight: Ensuring Validity

• Why use temperature change? Displacement reactions are exothermic. A more reactive metal releases heat energy more rapidly to the surroundings when displacing silver.
• Validity: The question explicitly demands: "Your method should give valid results." This requires at least one explicitly named control variable.
• Insulation (e.g., using a polystyrene cup with a lid) reduces heat loss to make temperature readings more accurate.