AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2022: Question 1

✅ Correct answers

• Green solid / green powder is present.
• Fizzing / effervescence / bubbles of gas.
• The green solid disappears or gets smaller.
• A blue solution forms.
• A colourless solution may be seen before the blue copper sulfate solution forms.
• At the end, some solid is left behind because copper carbonate is in excess.
• The container gets warm / the temperature increases.

Any three of the above score full marks.

💡 Key knowledge

• Copper carbonate is a green solid.
• Metal carbonates + acids produce a salt, water, and carbon dioxide.
• The carbon dioxide causes fizzing.
• The blue colour comes from copper sulfate solution.

🧠 Exam technique

• Write clear observations, not explanations.
• Use simple visible changes: colour, fizzing, solid disappearing, temperature change.
• Each observation should be distinct to gain separate credit.

❌ Common errors

• Saying “gas is produced” is weaker than “fizzing” or “bubbles”.
• Writing “copper carbonate reacts” is an explanation, not an observation.
• Confusing the colours: the solid is green, the solution becomes blue.
• Only giving one observation in different words.

✅ Correct answer

Filtration / use a filter.

💡 Key knowledge

• The unreacted copper carbonate is an insoluble solid.
• Filtration separates the solid residue from the salt solution.

🧠 Exam technique

• If asked “how can it be removed?”, name the separation method directly.
• “Filter it off” is a good GCSE answer.

❌ Common errors

• Saying “evaporation” — this is for getting crystals from solution, not removing excess solid.
• Saying “decanting” without making it clear the solid is being removed.

✅ Correct answer

pH = 7

💡 Key knowledge

• The acid is fully used up because copper carbonate is added in excess.
• The solution is therefore neutral at the end.
• Neutral solutions have a pH of 7.

🧠 Exam technique

• If the question says “at the end of the reaction”, think about which reactant is left over.
• Excess carbonate means no acid remains, so the final solution is neutral.

❌ Common errors

• Giving pH 1–6 because acid was used at the start.
• Saying “alkaline” — the final answer expected here is neutral.

✅ Correct answer

Neutralisation

💡 Key knowledge

• Acid + carbonate → salt + water + carbon dioxide
• This is classed as a neutralisation reaction in GCSE chemistry.
• The mark scheme also allowed exothermic.

🧠 Exam technique

• Learn the reaction type words exactly: neutralisation, precipitation, displacement, etc.
• Even if you know the equation, the question is asking for the type.

❌ Common errors

• Saying “acid-carbonate reaction” without the named type.
• Saying “combustion” or “oxidation” — incorrect here.

✅ Correct answer

51 g

The mark scheme accepted graph readings of about 83 g at 80 °C and 32 g at 20 °C.

📐 Calculations — step by step

1. Read the solubility at 80 °C: about 83 g per 100 cm³ of water.
2. Read the solubility at 20 °C: about 32 g per 100 cm³ of water.
3. Find the difference: 83 − 32 = 51 g

Answer: 51 g

💡 Key knowledge

• When a solution cools, the solubility usually decreases.
• The extra dissolved solute forms crystals.
• The question is based on 100 cm³ of water, so use the graph values directly.

🧠 Exam technique

• Always check the axis units: here it is grams per 100 cm³ of water.
• Use the correct temperature values from the graph.
• Show your subtraction clearly for the final mark.
• If your graph reading is slightly off, you can still get the final calculation mark if the subtraction is correct.

❌ Common errors

• Using the wrong graph scale or reading the curve inaccurately.
• Subtracting in the wrong order or forgetting to subtract at all.
• Ignoring the fact that the solubility values are for 100 cm³.
• Writing the answer without units. The answer should be in g.

Key facts to remember

• Metal carbonate + acid → salt + water + carbon dioxide
• Copper carbonate is green
• Copper sulfate solution is blue
• Excess solid is removed by filtration
• Neutral pH = 7
• Neutralisation is the reaction type

How to score well

• Use short, clear observations.
• Learn practical method names: filtration, evaporation, crystallisation.
• For graph questions, read values first, then calculate the difference.

Calculation trap alert

• Do not confuse solubility with mass crystallised.
• The crystals formed = solubility at 80 °C − solubility at 20 °C.
• Always include g in the final answer.