AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2024: Question 8

✅ Correct Answer

• Test: Flame test [1 mark]
• Result: Yellow flame [1 mark]

Alternative accepted: Flame emission spectroscopy (FES) [1 mark] ; result: lines match sodium spectrum [1 mark] .

💡 Key Knowledge

Key flame test colours you must know for AQA GCSE:

• Sodium (Na⁺): Yellow
• Lithium (Li⁺): Crimson
• Potassium (K⁺): Lilac
• Calcium (Ca²⁺): Orange-red
• Copper(II) (Cu²⁺): Green

🧠 Exam Technique

Keep your answer concise. The question specifically asks for the "test" and the "result" in separate lines. Writing "flame test" followed by "yellow" scores both marks instantly.

❌ Common Errors

• Writing "orange" instead of "yellow" — GCSE examiners specifically look for "yellow" for sodium. Orange-red is calcium.
• Confusing flame tests with sodium hydroxide solution tests (sodium ions do not form a precipitate with NaOH).

✅ Correct Answer

• Test: Add (dilute nitric acid and) silver nitrate solution [1 mark]
• Result: White precipitate [1 mark]

💡 Key Knowledge

Halide tests rely on precipitation reactions with aqueous silver ions (Ag⁺):

• Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (white precipitate)
• Bromide (Br⁻) gives a cream precipitate (AgBr).
• Iodide (I⁻) gives a yellow precipitate (AgI).
• Acidifying with dilute nitric acid (HNO₃) removes carbonate or sulfite impurities that would give false precipitates.

🧠 Exam Technique

Always state the state/appearance clearly: write "white precipitate" or "white solid", not simply "it turns white" or "white solution".

❌ Common Errors

• Confusing reagents: using barium chloride (the test for sulfate ions, SO₄²⁻) instead of silver nitrate.
• Suggesting hydrochloric acid (HCl) to acidify the test — adding HCl adds chloride ions, ruining the test!

✅ 08.3: Why Heat Until Constant Mass?

Answer: To ensure that all the water has evaporated. [1 mark]

Examiner Insight: Saying "to make sure it's completely dry" is acceptable, but the precise scientific reasoning is that water continues to be driven off until only pure anhydrous salt remains.

✅ 08.4: Calculation of Dry Salt Mass

Correct Box to Tick:

☑ Mass of evaporating dish and dry contents − mass of empty evaporating dish [1 mark]

(The 4th option in the list).

💡 Experimental Technique: Constant Mass

When determining the mass of a dissolved residue:

• Initial weighing: Mass of empty clean dry dish (m₁).
• After evaporation: Mass of dish + salt after heating (m₂).
• Heating, cooling, and reweighing is repeated until m₂ remains unchanged between two consecutive weighings.
• Mass of dry salt = m₂ − m₁.

❌ Common Errors in 08.3 & 08.4

• In 08.3: Vague answers such as "to make the test fair" or "to get accurate results" score 0 marks.
• In 08.4: Choosing options that involve the initial 25 cm³ liquid solution rather than the dried dish and contents.

📐 Step-by-Step Calculation

💡 Alternative Valid Approaches

Alternative 1:

Find total Na⁺ first: 140 × (39.3 / 100) = 55.02 g/dm³.
Then divide by 4: 55.02 / 4 = 13.8 g/dm³.

Alternative 2:

Calculate Na⁺ concentration for each individual trial first:
Trial 1: 13.83 | Trial 2: 13.60 | Trial 3: 14.31 | Trial 4: 13.28
Mean = (13.83 + 13.60 + 14.31 + 13.28) / 4 = 13.8 g/dm³.

🧠 Exam Technique & Anomalies

• Look closely at the data: values range from 33.8 to 36.4. None of them are obvious outliers, so all four trials must be included in the mean.
• Notice the mark scheme allows an error-carried-forward (ECF) mark if you made an arithmetic slip in Step 1 but multiplied correctly by 39.3% in Step 2. Always show your working clearly!

❌ Common Errors

• Dividing by 3 instead of 4 (mistakenly treating one result as an anomaly without good reason).
• Inverting the percentage: e.g., dividing by 0.393 instead of multiplying by 0.393.
• Premature rounding: round only your final answer to avoid rounding errors.