AQA GCSE Chemistry Chemistry Paper 1 (Foundation), November 2021: Question 7

✅ Correct Answer

Tick: By filtration [1 mark]

💡 Key Knowledge

• Silver metal is an insoluble solid.
• Silver nitrate solution is an aqueous liquid.
• An insoluble solid is separated from a liquid or solution using filtration (filter paper and funnel).

❌ Common Errors

• Crystallisation: Used to obtain a soluble salt from its solution, not an insoluble solid.
• Distillation: Used to separate liquids with different boiling points or collect solvent.
• Chromatography: Used to separate mixtures of soluble coloured substances.

✅ Marking Breakdown (4 Marks Total)

• Scale (1 mark): Sensible linear scale for the x-axis, using at least half the grid. Specifically: 10 minutes per 2 cm (or 5 minutes per 1 cm ).
• Plotting (2 marks): All 6 points plotted correctly within ± ½ small square. (Award 1 mark if 3 or 4 points are correct).
• Line of Best Fit (1 mark): A single, straight, ruled line passing through (0, 0) and the plotted points.

🧠 Exam Technique: How to Plot Perfectly

• Scale rule: The x-axis must go from 0 to at least 25 minutes. With 10 large squares across, label every 2 large squares as 10 (e.g. 0, 5, 10, 15, 20, 25, 30).
• Use neat crosses (×): Dots can get lost; large blobs lose accuracy marks. Keep points within half a small square of their true position.
• Use a clear ruler: Line of best fit must be drawn with a ruler. Do NOT sketch freehand or join point-to-point (dot-to-dot).

❌ Common Pitfalls

• Non-linear scale: Writing only the table numbers (0, 5, 10, 15, 20, 25) at irregular spacings.
• Thick or doubled lines: A 'fuzzy' or multiple-traced line of best fit loses the line mark.
• Forgetting (0, 0): The data clearly starts at (0, 0.00); the line must start at the origin.

📐 Step-by-Step Interpolation

1. Locate 12 minutes on the horizontal x-axis (2 small squares past 10 minutes).
2. Draw a dashed vertical line straight up to hit your line of best fit.
3. Read across horizontally to the vertical y-axis.
4. Value = 0.14 g (allow ± ½ small square, or error carried forward from candidate's graph).

🧠 Examiner Insight

Examiners love seeing construction/tie lines drawn on the graph paper. It guarantees you are using your own line of best fit and allows for "Error Carried Forward" (ecf) even if your line was slightly misdrawn in 07.2.

✅ Completed Table 6

💡 Cathode Rules in Aqueous Solutions

• Water dissociates into H⁺ and OH⁻ ions.
• At the cathode (-), metal ions compete with H⁺ ions.
• If the metal is more reactive than hydrogen (e.g. Sodium): Hydrogen gas forms → seen as bubbles / fizzing.
• If the metal is less reactive than hydrogen (e.g. Copper): The metal forms → seen as a red/brown/orange solid coating.

❌ Common Errors

• Giving names instead of observations: The question asks what you would observe (see). Writing just "copper" or "hydrogen" misses the primary observation marks.
• Saying sodium is produced: Sodium is far too reactive to form in aqueous solution; H₂ forms instead.

✅ Correct Answer

Bromine (fumes) is toxic / poisonous [1 mark]

(Also accept: harmful / corrosive fumes)

❌ Answers That Score 0 Marks

• "It is dangerous" (too vague).
• "It is deadly / lethal" (unscientific; AQA explicitly rejects "deadly").
• "It smells bad" (insufficient justification for safety equipment).

🧠 Exam Technique

Whenever asked why a reaction involving halogens (chlorine, bromine, iodine) or sulfur dioxide must be done in a fume cupboard, the safest keyword is always toxic or poisonous.

✅ Completed Table 7

💡 Molten Salt Rule (No Water Present!)

• Because the compounds are molten, there are no H⁺ or OH⁻ ions present to interfere.
• Negative electrode (Cathode): Attracts positive metal ions → forms the pure metal (e.g. Zn²⁺ + 2e⁻ → Zn).
• Positive electrode (Anode): Attracts negative non-metal ions → forms the diatomic non-metal gas/element (e.g. 2Cl⁻ → Cl₂ + 2e⁻).

❌ Common Errors

• Writing chloride instead of chlorine (chloride is the ion; chlorine is the elemental product).
• Spelling: Must be potassium iodide as the compound, not iodine.