AQA GCSE Chemistry Chemistry Paper 1 (Foundation), November 2021: Question 5

✅ Correct Answer

H⁺ (Tick box 1)

💡 Key Knowledge

• Acids produce hydrogen ions ( H⁺ ) in aqueous solutions.
• Alkalis produce hydroxide ions ( OH⁻ ) in aqueous solutions.

❌ Common Errors

Confusing H⁺ with OH⁻ (alkalis) or oxide ions ( O²⁻ ).

✅ Correct Answer

Neutralisation (Tick box 3)

💡 Key Knowledge

When an acid reacts with an alkali, they neutralise each other to form a salt and water:

H⁺(aq) + OH⁻(aq) → H₂O(l)

🧠 Exam Technique

Look at the reactants: Acid + Alkali is always a neutralisation reaction at GCSE level.

✅ Correct Answer

H₂SO₄ + 2 KOH → K₂SO₄ + 2 H₂O

Both numbers must be 2.

❌ Common Errors

Writing formulas instead of balancing numbers, or forgetting to balance both sides (e.g. putting a 2 in front of KOH but leaving H₂O blank).

✅ Correct Answer

14 (Tick box 4)

💡 Key Knowledge

• pH 1: Strongly acidic (Red)
• pH 4: Weakly acidic (Orange/Yellow)
• pH 7: Neutral (Green)
• pH 14: Strongly alkaline (Purple)

KOH is a strong alkali, so universal indicator turns purple at pH 13–14.

❌ Common Errors

Selecting 7 (neutral) assuming the reaction has already taken place, rather than reading that this is pure potassium hydroxide solution.

✅ Correct Answer

Pipette (Tick box 3)

💡 Key Knowledge

A volumetric pipette measures one fixed volume (e.g. 25.0 cm³) with high precision and low percentage uncertainty, much more accurately than a measuring cylinder or beaker.

❌ Common Errors

Selecting "Beaker" or "Test tube" — these have very poor accuracy and are not volumetric instruments.

✅ Model Answer (5 Marks)

1. Add alkali to flask: Measure 25 cm³ of potassium hydroxide solution and transfer it into the conical flask. [1 mark]
2. Add indicator: Add a few drops of indicator (e.g. phenolphthalein / methyl orange) to the flask. [1 mark]
3. Add acid from burette: Fill the burette with sulfuric acid and slowly add it into the conical flask (swirling continuously). [1 mark]
4. Observe endpoint: Stop adding acid when the indicator changes colour permanently. [1 mark]
5. Take reading: Read and record the final volume on the burette (subtract initial reading to calculate volume added). [1 mark]

❌ Common Pitfalls & Lost Marks

• Swapping apparatus: Saying acid is measured in the cylinder and alkali goes in the burette. Look carefully at Figure 7: the cylinder has KOH and the burette contains sulfuric acid.
• Omitting the indicator: Forgetting to state that indicator is added to the conical flask.
• Vague endpoint: Simply saying "until it neutralises" without stating how you know (i.e. the indicator changes colour).
• Missing the final measurement: Ending the response at the colour change without stating that the volume of acid added must be read from the burette.