AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2018: Question 1

✅ Correct answer

chlorine

💡 Key knowledge

• In electrolysis of copper chloride solution, chloride ions go to the anode.
• At the anode, chloride ions are oxidised to form chlorine gas.
• For AQA GCSE, with aqueous copper chloride and inert electrodes, the anode product is chlorine.

🧠 Exam technique

• Match the electrode name to the correct ion/product.
• The positive electrode is the anode.
• If you are unsure, think: chloride → chlorine.

❌ Common errors

• Choosing oxygen because students confuse aqueous electrolysis with water splitting.
• Mixing up anode and cathode.
• Writing chloride instead of chlorine — the mark is for the gas formed.

✅ Correct answer

copper is less reactive than hydrogen

💡 Key knowledge

• At the cathode, positive ions are reduced.
• If copper is deposited from solution, copper ions are being discharged instead of hydrogen.
• This shows copper is less reactive than hydrogen.

🧠 Exam technique

This is a reactivity series idea. If a metal is extracted during electrolysis from an aqueous solution, it means the metal is less reactive than hydrogen.

❌ Common errors

• Picking a statement about oxygen, carbon, or chlorine — these are not relevant here.
• Saying copper is more reactive because it is “being produced”. The correct comparison is with hydrogen.

✅ Correct answer

1.8 mg

📐 Calculation

Use the pattern in the table:

1. At 1 min, mean = 0.60 mg
2. At 2 min, mean = 1.20 mg
3. At 4 min, mean = 2.40 mg

The mean increases by about 0.60 mg per minute, so at 3 minutes:

1.20 + 0.60 = 1.80 mg

🧠 Exam technique

• For a missing mean, use the trend in the data if the table is regular.
• Write the answer with the correct unit: mg.
• Marks are usually for the final value, so a sensible estimate earns credit.

❌ Common errors

• Writing 1.8 g instead of 1.8 mg.
• Giving 1.2 mg because it is the previous table entry.
• Forgetting that the question asks for the mean, not one experiment value.

✅ Correct answer

X = 3.15 mg

📐 Calculation step-by-step

1. Use the mean formula:

(3.02 + 3.01 + X) ÷ 3 = 3.06

1. Multiply both sides by 3:

3.02 + 3.01 + X = 9.18

1. Add the known results:

6.03 + X = 9.18

1. Subtract:

X = 9.18 - 6.03 = 3.15 mg

🧠 Exam technique

• Always use the mean = total ÷ number of values idea.
• Check that all values are in the same unit before calculating.
• The mark scheme allows a full-mark method if the final answer is correct, even if you use another sensible approach.

❌ Common errors

• Adding the numbers but forgetting to divide by 3.
• Using 3.06 as the answer instead of the missing value.
• Arithmetic slips: 3.02 + 3.01 = 6.03, not 6.03? Double-check carefully.
• Writing the final answer without mg.

✅ Correct answer

15 g

📐 Calculation step-by-step

Given: 300 g per dm³

Need: mass in 50 cm³

1. Convert volume to dm³:

50 cm³ = 50 ÷ 1000 = 0.05 dm³

1. Use proportion:

0.05 × 300 = 15

1. State the final answer with unit:

15 g

💡 Key knowledge

• 1 dm³ = 1000 cm³
• Concentration in g per dm³ means “grams in 1 dm³”.
• To find the mass in a smaller volume, multiply the concentration by the fraction of a dm³.

🧠 Exam technique

• Always convert volumes before multiplying.
• Use ratio thinking: 50 cm³ is 1/20 of 1000 cm³, so the mass is 1/20 of 300 g.
• Show the unit in the final line: g, not dm³ or cm³.

❌ Common errors

• Using 50 ÷ 1000 but then forgetting to multiply by 300.
• Confusing cm³ with dm³.
• Writing 0.15 g or 150 g from place-value slips.
• Leaving out the unit: the answer must be 15 g.

💡 What top answers did well

• Used the exact product name: chlorine.
• Linked copper deposition to the reactivity series: less reactive than hydrogen.
• Showed working clearly in calculations, especially for the missing value and the mass from concentration.
• Included correct units: mg and g.

❌ Where students lost marks

• Wrong gas at the anode, especially oxygen.
• Confusing reactivity with charge or electrode polarity.
• Arithmetic slips in the mean calculation.
• Not converting cm³ to dm³ in the concentration question.

💡 Remember

• Anode = positive, cathode = negative
• Chloride ions → chlorine gas
• Copper deposited means copper is less reactive than hydrogen
• Mean = total ÷ number of values
• 1 dm³ = 1000 cm³

🧠 Full-marks habit

Always write the exact scientific term, show clear working, and finish with the correct unit. That is the simplest way to secure the marks in this question.