AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Foundation), 2024: Question 7

💡 Key knowledge

• Copper sulfate is made by reacting copper carbonate with sulfuric acid.
• Excess copper carbonate is used so all the acid reacts.
• To make crystals: heat to concentrate, then cool to crystallise.
• When copper carbonate is heated, carbon dioxide gas escapes.

🧠 Exam technique

• For 1-mark recall questions, one exact correct word or formula is often enough.
• For method questions, give the steps in the correct order.
• For graph questions, every point must be plotted accurately and then joined with a sensible best-fit line.

❌ Common errors

• Choosing HCl or HNO₃ instead of H₂SO₄ .
• Saying excess copper carbonate is used “to make more crystals” rather than to use up all the acid.
• Writing “filter” for crystal preparation when the question is about crystallisation.
• Explaining mass loss without mentioning a gas leaving the test tube.

✅ Correct answer

H₂SO₄

💡 Key knowledge

• Copper sulfate is made using sulfuric acid.
• Formula: H₂SO₄

❌ Common errors

• HCl = hydrochloric acid
• HNO₃ = nitric acid
• Any spelling error in the formula loses the mark if it changes the acid.

✅ Correct answer

So all the acid reacts.

💡 Key knowledge

• Using an excess of an insoluble reactant makes sure none of the acid is left over.
• This is useful because any unreacted copper carbonate can be filtered off later.

🧠 Exam technique

• Say the purpose of the excess, not just what it is.
• The mark scheme wanted the idea that the acid is completely reacted.

❌ Common errors

• “So the reaction goes faster” — not the marking point.
• “So there is more copper sulfate” — too vague for this mark.
• “To make the solution saturated” — not what stage 1 is testing.

✅ Correct answer

1. Heat the solution to remove some of the water.
2. Leave it to cool so crystals form.

💡 Key knowledge

• Crystals form when a solution becomes more concentrated.
• Cooling helps the dissolved copper sulfate come out of solution as crystals.
• In this question, filtration is not the key step being marked.

🧠 Exam technique

• Use the correct sequence: heat → cool.
• “Evaporate some water” is accepted and means the same thing as concentrating the solution.
• For full marks, mention both steps.

❌ Common errors

• Stopping at “filter the solution” — filtering is not enough to make crystals.
• Saying “leave it to evaporate” without mentioning cooling/crystallisation.
• Writing “dry the crystals” too early in the process.

📐 Data to plot

• (1.2, 0.8)
• (2.4, 1.7)
• (3.6, 2.2)
• (4.8, 3.1)
• (6.0, 3.9)

🧠 Exam technique

• Plot all five points carefully.
• Use a sharp pencil and check the scale before plotting.
• Draw a smooth straight line of best fit, not dot-to-dot.
• The line should pass close to all points.

✅ How marks are awarded

• 2 marks for all points correctly plotted.
• 1 mark for a clear line of best fit.
• You could still gain some credit if 3 or 4 points are correct, but full marks need careful plotting.

❌ Common errors

• Mixing up x- and y-values.
• Forgetting units on the axis reading.
• Joining the points with straight segments instead of a best-fit line.
• Drawing a line that goes through every point exactly when the data is meant to show a trend.

✅ Correct answer

A gas escapes from the test tube.

💡 Key knowledge

• Copper carbonate decomposes on heating.
• One product is carbon dioxide gas, which leaves the test tube.
• When a gas leaves, the measured mass decreases.

🧠 Exam technique

• Focus on the measured mass: if gas escapes, the contents weigh less afterwards.
• A short, direct statement is enough for 1 mark.

❌ Common errors

• Saying “the copper carbonate disappears” without explaining why mass changes.
• Only writing “it decomposes” — this is true, but not the marking point.
• Not mentioning a gas or carbon dioxide.

💡 Must-know answers

• Acid used: H₂SO₄
• Why excess copper carbonate? So all the acid reacts
• Crystals from solution: heat to remove some water, then cool
• Mass decreases because a gas escapes

🧠 Final exam tip

In GCSE chemistry questions like this, marks often come from using the exact process words: reacts, heats, cools, crystallises, gas escapes. Keep answers short, clear, and linked to the question.