AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2021: Question 2

✅ Correct answer

Green to red

💡 Key knowledge

• Universal indicator shows pH by colour.
• Neutral water is usually green.
• Adding an acid makes the solution more acidic, so the colour moves towards red.

🧠 Exam technique

• Only one box should be ticked.
• Think: acid + neutral solution → lower pH.
• For AQA, universal indicator in neutral water is a common recall point.

❌ Common errors

• Choosing blue to red or red to purple.
• Mixing up universal indicator with litmus.
• Thinking the indicator goes purple because “an acid is added” — purple is alkaline.

✅ Correct answer

Decreases

💡 Key knowledge

• Acids have pH values below 7.
• Adding acid to water increases acidity, so the pH goes down.
• More acid particles in the solution = lower pH.

🧠 Exam technique

• If you see “adds acid”, ask yourself: more acidic or less acidic?
• pH is a scale where lower number = more acidic.

❌ Common errors

• Writing increases because “more stuff is added”.
• Saying stays the same because water is neutral — it is no longer neutral once acid is added.

✅ Correct answer

(aq)

💡 Key knowledge

• (aq) means aqueous: dissolved in water.
• Nitric acid in these reactions is usually used as an aqueous solution.

🧠 Exam technique

• State symbols are short but important.
• If a chemical is in solution, the safest choice is often (aq).

❌ Common errors

• Writing just HNO₃ without a state symbol.
• Writing (l) — nitric acid in this question is not pure liquid nitric acid.
• Writing the full word aqueous instead of the symbol.

✅ Correct answers

• White solid disappears / gets used up.
• Fizzing / bubbles / effervescence (gas produced).
• Also acceptable: fizzing stops, or white solid left at the end.

💡 Key knowledge

• Zinc carbonate + acid produces carbon dioxide.
• Carbon dioxide causes the fizzing.
• As acid is used up, the reaction stops; if carbonate is in excess, some solid remains.

🧠 Exam technique

• For observation questions, describe what you would see.
• Good observation words: fizzing, bubbles, solid disappears, solid left.
• You do not need to explain the chemistry here.

❌ Common errors

• Writing vague statements like it changes or a reaction happens.
• Including irrelevant colour changes — the mark scheme does not need them.
• Saying the solution becomes colourless — this was ignored, so it does not earn the mark.

✅ Correct answer

Zn(NO₃)₂

💡 Key knowledge

• Zn²⁺ has a 2+ charge.
• Nitrate, NO₃⁻, has a 1− charge.
• Two nitrate ions are needed to balance one zinc ion.

🧠 Exam technique

• Always balance charges when writing ionic formulas.
• Use brackets if more than one nitrate ion is needed: Zn(NO₃)₂.

❌ Common errors

• ZnNO₃ – charges are not balanced.
• Zn₂NO₃ – wrong ratio.
• Zn₂(NO₃)₂ – extra zinc atoms.
• Forgetting brackets around nitrate.

✅ Correct answer: a full method

1. Put hydrochloric acid in a suitable container, such as a beaker.
2. Warm the acid gently.
3. Add copper oxide a little at a time while stirring.
4. Add more copper oxide until some solid remains / the oxide is in excess.
5. Filter to remove the excess copper oxide.
6. Pour the filtrate into an evaporating basin.
7. Heat gently using a water bath or electric heater to partially evaporate the water.
8. Leave the solution to cool and crystallise.
9. Dry the crystals if required.

💡 Key knowledge

• This is the standard method for making a soluble salt from an acid and an insoluble base.
• Using an excess of copper oxide makes sure all the acid reacts.
• Filtering removes the unreacted solid so the salt solution is pure.
• Heating gently prevents spitting and avoids overheating the product.

🧠 Exam technique

• To reach the top level, include the steps in the right order.
• Use the exam language: warm, stir, filter, evaporating basin, crystallise.
• State the correct reactants clearly: hydrochloric acid + copper oxide.
• The question asks for a pure, dry sample, so you need both separation and crystallisation/drying.

❌ Common errors

• Using the wrong acid, e.g. nitric acid instead of hydrochloric acid.
• Forgetting to use excess copper oxide.
• Not filtering off the unreacted solid.
• Missing the crystallisation step.
• Mixing up copper oxide with copper metal.

💡 Facts to memorise

• Universal indicator: neutral = green, acid = red.
• Acids make pH decrease.
• Nitric acid in solution is (aq).
• Zn²⁺ + 2NO₃⁻ → Zn(NO₃)₂.

🧠 How to score full marks

• Use precise science words.
• For formula questions, balance charges.
• For method questions, write clear steps in order.
• For observation questions, describe visible changes.