AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Foundation), June 2025: Question 5

✅ Any TWO Acceptable Improvements

• Measure the volume of alkali (e.g. using a burette or measuring cylinder instead of counting drops).
• Repeat the experiment and calculate a mean (excluding anomalies).
• Stir or swirl the mixture continuously while adding alkali.
• Use universal indicator solution (or a digital pH probe / meter) instead of paper.

🧠 Exam Technique & Examiner Insight

The original method counted "drops" from a dropping pipette into a stationary beaker with paper at the bottom. This is inaccurate because:

• Drop sizes vary; use a burette for precise volume measurements.
• Unmixed liquids create local pH pockets; always state "stir or swirl".
• If suggesting repetitions, always write "repeat AND calculate a mean" to guarantee the mark.

❌ Common Errors to Avoid

• Writing safety precautions: The mark scheme explicitly states: "ignore references to safety" (e.g. wearing goggles or gloves gets 0 marks).
• Vague repetition: Just writing "repeat" without mentioning calculating a mean or identifying anomalies.

✅ Correct Selections

• 05.2: 7 (Neutral pH when indicator turns green) [1 mark]
• 05.3: Neutralisation [1 mark]
• 05.4: H⁺ (Hydrogen ion) [1 mark]

💡 Key Knowledge Recall

• pH Scale: pH 1–6 = Acidic (red/orange/yellow), pH 7 = Neutral (green), pH 8–14 = Alkaline (blue/purple).
• Reaction Type: Acid + Alkali → Salt + Water is a neutralisation reaction.
• Ionic Theory: All acids release H⁺ ions in aqueous solution. All alkalis release OH⁻ ions.

❌ Common Misconceptions

• Ticking Cl⁻ for "all acids": Only hydrochloric acid contains chloride; sulfuric and nitric do not.
• Confusing combustion (burning in oxygen) or displacement (more reactive element kicking out a less reactive one) with neutralisation.

✅ Correct Answer

nitric acid (allow chemical formula: HNO₃ )

💡 Naming Salts Rulebook

• Hydrochloric acid → makes chlorides
• Sulfuric acid → makes sulfates
• Nitric acid → makes nitrates

Base: Sodium hydroxide provides the sodium part; Acid provides the nitrate part.

✅ Correct Labels

• A: overall energy change [1 mark]
• B: progress of reaction [1 mark]

💡 How to Read Energy Profiles

• Vertical arrow from reactants to products (A): Represents the overall energy change (ΔH). Since products are lower than reactants, this is an exothermic reaction.
• Horizontal axis (B): Represents the time / course of the process, formally named the progress of reaction.
• Note: The arrow from the reactants line up to the peak of the curve represents activation energy (not labelled here).

✅ Mark Scheme Requirements

1. Mark 1: Filter funnel with filter paper clearly shown inside it.
2. Mark 2: A suitable container to collect the liquid (filtrate), such as a conical flask or beaker underneath the funnel.

📐 How to Draw Standard Scientific Apparatus

✅ Correct Answer

zinc (allow chemical symbol: Zn )

💡 Word Equation Deduction

metal + acid → salt + hydrogen

• Hydrochloric acid + Zinc → Zinc chloride + Hydrogen
• Zn + 2HCl → ZnCl₂ + H₂

❌ Common Error: Adding Oxides or Carbonates

• If Solid X were zinc oxide or zinc hydroxide, the products would be zinc chloride and water (no hydrogen gas!).
• If Solid X were zinc carbonate, carbon dioxide gas would also be produced.
• Because the products are strictly salt + hydrogen, Solid X must be the uncombined pure metal: zinc.