AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2020: Question 2

✅ Correct Choices

• ☑ They form ions with different charges. (1 mark)
• ☑ They have high melting points. (1 mark)

💡 Key Knowledge

Compare transition metals to Group 1 alkali metals:

• Transition metals: High melting points, high densities, form coloured compounds, form multiple ions (e.g. Fe²⁺, Fe³⁺), useful as catalysts.
• Group 1 metals: Soft, low melting points, low densities, form +1 ions only, form white/colourless compounds.

❌ Common Errors

• Confusing transition metals with Group 1 (e.g. ticking "They are soft metals" or "They have low densities").
• Ticking more or fewer than the requested two boxes, which risks forfeiting marks.

✅ Correct Answer Breakdown

• Mark 1: The pale grey crystals forming are silver metal.
• Mark 2: The solution turns blue because copper ions (Cu²⁺) are formed (or copper nitrate solution forms).
• Mark 3: Copper displaces silver because copper is more reactive than silver (or silver is less reactive than copper).

🧠 Exam Technique: Link Observation to Species

When a question lists specific observations, you must explain both of them to get full marks:

• Observation 1 (grey solid) → What substance is it? (Silver)
• Observation 2 (blue solution) → What substance causes it? (Copper ions / copper nitrate)
• Conclusion → Why did this happen? (Displacement occurs because copper is more reactive)

❌ Common Errors

• Stating merely that "a displacement reaction happens" without identifying which substance makes the crystals or turns the solution blue.
• Confusing the colours: incorrectly thinking the grey crystals are copper and the blue liquid is silver nitrate.

✅ Model Method (Level 2: 3–4 Marks)

1. Setup: Measure an equal volume (e.g. 25 cm³) of dilute hydrochloric acid into three boiling tubes or polystyrene cups.
2. Control: Keep the concentration of acid, initial temperature, and the mass/surface area of metal samples identical.
3. Procedure: Add an equal sample of metal X, Y, and Z to separate tubes.
4. Measurement: Record the temperature change using a thermometer, OR compare the rate of bubbling/effervescence.
5. Deductions:
   • Copper: No reaction (no bubbles, 0 °C temperature change).
   • Magnesium: Most vigorous reaction (fastest bubbling / greatest temperature rise).
   • Iron: Slower reaction (gentle bubbling / smaller temperature rise).

🧠 How Examiners Grade This (Level of Response)

Level 2 (3–4 marks): The method leads to a valid outcome. Key steps are logically ordered, including:

• Adding acid to all three metals.
• A measurable dependent variable (temperature rise or bubbling rate).
• Expected observations linked to identifying each metal.
• At least one valid control variable.

Level 1 (1–2 marks): Steps are relevant but incomplete (e.g. mentions adding acid and observing bubbles, but fails to identify which metal produces which result or misses control variables).

❌ Common Traps to Avoid

• Forgetting that copper does not react with dilute acids because it is below hydrogen in the reactivity series.
• Forgetting control variables (mass of metal, volume/concentration of acid). Without them, the investigation is not a valid test.

📐 Step-by-Step Calculation

Formula:

Aᵣ = Σ(mass number × abundance) / 100

1. Multiply each mass by its percentage abundance:
   (203 × 30) = 6090
   (205 × 70) = 14350
2. Add values together:
   6090 + 14350 = 20440
3. Divide by total percentage (100):
   20440 / 100 = 204.4 [1 mark for correct method]
4. Round to required precision (1 d.p.):
   204.4 [1 mark for final value]

🧠 Sanity Check & Precision

• Reasonableness check: The answer must lie between 203 and 205. Since 70% of the sample is isotope 205, the value must be closer to 205 than 203. 204.4 fits perfectly!
• Instruction check: The question specifically asked for 1 decimal place. Writing 204 or 204.40 would risk losing the second mark.

❌ Common Errors

• Dividing by 2 instead of 100 (taking a simple average of the mass numbers rather than a weighted average).
• Swapping the abundances (e.g. 203 × 70 and 205 × 30).
• Adding units (e.g. grams). Note: Aᵣ is a relative ratio and has no units, though the mark scheme ignores units here.