AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2020: Question 4

✅ Correct answer

ZnO(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l)

Full marks: 2 marks. You needed the correct state symbols. The equation is already balanced.

💡 Key knowledge

• Zinc oxide is a solid, so (s) .
• Hydrochloric acid is in aqueous solution, so (aq) .
• Zinc chloride is soluble, so (aq) .
• Water is a liquid, so (l) .

🧠 Exam technique

• Check all 4 state symbols carefully.
• Remember: aq means dissolved in water.
• Even if the chemistry is right, missing symbols can lose marks.

❌ Common errors

• Writing (l) for hydrochloric acid.
• Writing (s) for zinc chloride.
• Forgetting the 2 in front of HCl.
• Using the wrong symbol for water, such as (aq) .

✅ Correct answers

Any one of these gains the mark:

• Warm / heat the mixture
• Increase the concentration of hydrochloric acid

💡 Key knowledge

• Higher temperature means particles move faster and collide more often.
• Higher concentration means more acid particles in the same volume, so more collisions.

🧠 Exam technique

The mark scheme accepted warm the mixture or increase concentration. Be precise: “add a catalyst” is not correct here.

❌ Common errors

• “Add more zinc oxide” — not accepted.
• “Increase surface area” — not accepted for this mark.
• “Stir it” — ignored by the mark scheme.
• “Add a catalyst” — ignored.

✅ Correct answer

Zinc oxide remains / solid remains.

💡 Key knowledge

Hydrochloric acid is the limiting reactant, so once it is used up, some zinc oxide is left over as a solid.

🧠 Exam technique

The mark scheme allowed “zinc oxide remains” or “solid remains.” The key idea is that there must be some visible unused solid.

❌ Common errors

• Saying the solution “turns colourless” — colour was ignored.
• Saying “all reactants are used up” — not true here.
• Saying “bubbles stop” — this is not the mark scheme answer.

✅ Correct answer

Filtration / filter.

💡 Key knowledge

Filtration separates an insoluble solid from a liquid. Here, the excess zinc oxide stays on the filter paper and the zinc chloride solution passes through.

🧠 Exam technique

If asked for a method, one word is enough: filtration. If explaining in a longer answer, mention the solid residue and the filtrate.

❌ Common errors

• Writing “evaporation” here — that is for making crystals, not separating the solid first.
• Writing “decanting” — not the expected answer.

✅ Correct answer

Heat the solution, then leave it to cool / crystallise.

💡 Key knowledge

• Gently heat to evaporate some water.
• Do not heat to dryness.
• Leave the solution to cool so crystals form.

🧠 Exam technique

For 2 marks, include both stages: heat/evaporate some water and cool/leave to crystallise.

❌ Common errors

• Saying “heat to dryness” — specifically not accepted.
• Only saying “leave to cool” — this gives only part of the answer.
• Forgetting that some water must be removed first.

✅ Correct answers

• pH at start: 12–14
• pH at end: 0–3

💡 Key knowledge

• Sodium hydroxide is an alkali, so it starts with a high pH.
• Sulfuric acid in excess makes the mixture acidic, so pH becomes low.
• The values had to be in this order: start high, end low.

🧠 Exam technique

The mark scheme accepted a range, not one exact number. As long as your answers are in the correct ranges and in the right order, you score.

❌ Common errors

• Mixing up acid and alkali values.
• Giving the same pH twice.
• Forgetting that excess acid makes the final pH low.

✅ Correct answer

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

💡 Key knowledge

• Sodium hydroxide neutralises sulfuric acid.
• Products are a salt, sodium sulfate, and water.
• Balance the atoms on both sides.

🧠 Exam technique

The mark scheme allowed 1 mark for getting Na₂SO₄ and H₂O correct. To get both marks, the full balanced equation must be correct.

❌ Common errors

• Forgetting the 2 in front of NaOH.
• Writing NaSO₄ instead of Na₂SO₄.
• Putting the wrong number of water molecules.

📐 Calculation / reasoning

Original hydrogen ion concentration = 1.0 mol/dm³

1. A rise in pH by 1 means the hydrogen ion concentration goes down by a factor of 10.
2. So divide 1.0 by 10 .
3. 1.0 ÷ 10 = 0.10 mol/dm³

✅ Correct answer

0.10 mol/dm³

💡 Key knowledge

• Each increase of 1 in pH means a tenfold decrease in hydrogen ion concentration.
• Keep the units: mol/dm³ .
• The correct option was 0.10 mol/dm³.

❌ Common calculation traps

• Choosing 10 mol/dm³ or 100 mol/dm³ instead of dividing by 10.
• Writing 0.01 mol/dm³ — that would be a pH increase of 2, not 1.
• Forgetting that pH and hydrogen ion concentration change in opposite directions.

🧠 What top answers did well

• Used the exact chemistry wording from the specification.
• Included correct state symbols.
• Gave practical methods in the right order: filter first, then crystallise.
• Understood pH changes in terms of acid/alkali and concentration.

❌ Where students lost marks

• Missing or incorrect state symbols.
• Wrong practical method for separating a salt solution.
• Saying “heat to dryness” when crystals were needed.
• Confusing pH values and concentration changes.