AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2024: Question 2

Total marks: 8

Topics: Salts, crystallisation, thermal decomposition, graph skills

Demand: Standard GCSE practical chemistry

✅ Correct answer

H₂SO₄

💡 Key knowledge

• To make copper sulfate, the acid must supply sulfate ions.
• That means the acid is sulfuric acid.
• Hydrochloric acid would make a chloride, not a sulfate.
• Nitric acid would make a nitrate, not a sulfate.

❌ Common errors

• Choosing HCl because it is a common acid.
• Choosing HNO₃ without linking the acid to the name of the salt.
• Writing the name sulfuric acid when the question asks for the formula.

🧠 Exam technique

Match the ending of the salt to the acid: sulfate → sulfuric acid → H₂SO₄. For a 1-mark question, one precise formula is enough.

✅ Correct answer

So all the acid reacts / so all the acid is used up.

💡 Key knowledge

• Copper carbonate is an insoluble solid.
• If it is added in excess, there will be more than enough to react with all the acid.
• Any extra unreacted copper carbonate can then be filtered off.
• This leaves a solution without leftover acid.

🧠 Exam technique

The mark scheme wants the idea that the acid is fully used up. Keep your answer focused on that exact point.

❌ Common errors

• Saying only to make more copper sulfate — this is too vague.
• Saying so all the copper carbonate reacts — this is the opposite of what happens.
• Talking only about filtration without explaining why excess is used.

✅ Correct answer

• Heat the solution to remove some of the water.
• Leave it to cool so crystals form / allow it to crystallise.

💡 Key knowledge

• Beaker N already contains copper sulfate solution.
• To get crystals, you first make the solution more concentrated by evaporating some water.
• Then on cooling, the copper sulfate crystallises out.
• The mark scheme says to ignore references to filtration here, because the question starts from beaker N after filtration.

🧠 Exam technique

For 2-mark method questions, give two ordered steps. Top answers are short and in the right sequence: heat, then cool.

❌ Common errors

• Writing only evaporate the water and forgetting the cooling/crystallisation step.
• Saying boil until dry — this would not be the correct method for obtaining crystals.
• Describing filtration again instead of starting from the copper sulfate solution already in beaker N.

✅ Correct answers

• 2 marks for all points correctly plotted.
• 1 mark for a suitable line of best fit.
• The mark scheme allows a tolerance of ± ½ a small square.
• If only 3 or 4 points are correctly plotted, that can gain 1 mark for plotting.

🧠 Exam technique

1. Check the x-axis: mass of copper carbonate before heating.
2. Check the y-axis: mass after heating.
3. Plot each point carefully with a small neat cross.
4. Draw a single straight line of best fit through the overall trend.
5. Do not join the points dot-to-dot.

💡 Key knowledge

The data show that as the starting mass increases, the mass after heating also increases. The points should make a clear upward trend, so the line of best fit should slope upwards.

❌ Common errors

• Swapping the axes and plotting the coordinates the wrong way round.
• Using large blobs instead of precise crosses.
• Joining points one by one rather than drawing a best-fit line.
• Missing one plotted point and losing the full plotting mark.

✅ Correct answer

A gas escapes from the test tube / carbon dioxide escapes.

💡 Key knowledge

• Thermal decomposition of copper carbonate produces copper oxide and carbon dioxide.
• Equation: CuCO₃ → CuO + CO₂
• The CO₂ gas leaves the test tube, so the mass measured afterwards is lower.

🧠 Exam technique

The key idea is not just that a reaction happened, but that something leaves the apparatus. Mentioning the escaping gas is what gets the mark.

❌ Common errors

• Saying only copper carbonate decomposes — this is not enough.
• Saying the mass decreases because the solid melts or burns away.
• Forgetting to mention that the gas escapes from the test tube.

🧠 What stronger answers did well

• Used the exact formula H₂SO₄ rather than just naming an acid.
• Explained excess copper carbonate in terms of using up all the acid.
• Gave the crystallisation method in the correct order: heat, then cool.
• Plotted graph points accurately and drew a real line of best fit.
• Linked the mass decrease to gas escaping, not just to decomposition.

❌ Where students often lost marks

• Confusing which acid makes which salt.
• Giving vague answers like to help the reaction.
• Missing one of the two crystallisation steps.
• Careless graph plotting or drawing a zig-zag line through the points.
• Not stating that CO₂ leaves the test tube.