AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2020: Question 8

✅ Correct Answer

Green

(Also allowed: blue-green)

💡 Key Knowledge: Flame Test Colours

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

❌ Common Errors

• Writing "blue" alone without green (copper flame is green/blue-green; its solution is blue).
• Confusing copper (green) with barium (apple green) or calcium (orange-red / brick red).

🧠 Exam Technique

Flame test colours are pure recall. Link "Copper = Green" in your revision. Do not confuse the colour of the flame with the colour of copper sulfate aqueous solution (which is bright blue).

✅ Correct Answer (Both required for 2 marks)

• Point 1: The student did not clean the wire loop between tests (or copper sulfate is still present on the wire). [1 mark]
• Point 2: The colours are mixed / blended / masked. [1 mark]

🧠 Exam Technique: Two-Part Explanations

Notice the command phrase is "Explain why" for 2 marks:

1. Cause: What did the student physically fail to do? (Did not clean the wire).
2. Consequence: How did that cause the outcome? (The copper left on the wire masked the calcium colour).

💡 How to Clean a Flame Test Wire

To avoid contamination, dip a nichrome or platinum wire into concentrated hydrochloric acid ( HCl ) and place it into a hot Bunsen flame until it burns with no colour.

❌ Common Errors

• Saying "the wire got too hot" or "the Bunsen flame was yellow". The prompt specifies they left out a step before reusing the wire.
• Only stating that the wire wasn't cleaned without mentioning that the colours mask or mix with each other.

✅ Correct Answer

• Copper sulfate solution: Blue precipitate (allow: blue solid) [1 mark]
• Calcium iodide solution: White precipitate (allow: white solid) [1 mark]

💡 Key Knowledge: Insoluble Hydroxides

• Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) (blue precipitate)
• Ca²⁺(aq) + 2OH⁻(aq) → Ca(OH)₂(s) (white precipitate; does not dissolve in excess NaOH)
• Fe²⁺ forms a green precipitate.
• Fe³⁺ forms a brown precipitate.
• Al³⁺ forms a white precipitate that redissolves in excess NaOH.

❌ Common Errors

• Writing just "blue" or "white" without specifying precipitate or solid. Always write "precipitate" when testing with NaOH.
• Writing "blue solution"—copper sulfate starts as a blue solution, but adding NaOH forms an insoluble solid (precipitate).

🧠 Exam Technique: Be Precise with Terminology

Examiners routinely reject responses like "turns blue" or "goes cloudy" when the required mark scheme keyword is precipitate or solid.

✅ Correct Answer

Barium chloride (solution)

(Also allowed: barium nitrate solution)

💡 Key Knowledge: The Sulfate Test

To test for sulfate ions:

1. Add dilute hydrochloric acid ( HCl ) to eliminate carbonate ions (which would also form a white precipitate).
2. Add barium chloride solution ( BaCl₂ ).
3. A white precipitate of barium sulfate forms:

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

❌ Common Errors

• Writing just "barium" (barium is a metal element, not a solution).
• Confusing the reagent with silver nitrate (which is used for halides).

🧠 Exam Technique

Always name the full compound name or provide the correct chemical formula: write barium chloride or BaCl₂ .

✅ Correct Answer

• Solution: Silver nitrate (solution) [1 mark]
• Result: Yellow precipitate (allow: yellow solid / pale yellow precipitate) [1 mark]

💡 Key Knowledge: The Halide Test Summary

Acidify with dilute nitric acid ( HNO₃ ), then add silver nitrate ( AgNO₃ ):

• Chloride (Cl⁻): White precipitate (AgCl)
• Bromide (Br⁻): Cream precipitate (AgBr)
• Iodide (I⁻): Yellow precipitate (AgI)

Ag⁺(aq) + I⁻(aq) → AgI(s)

❌ Common Errors

• Mixing up halide colours: writing cream (bromide) or white (chloride) instead of yellow.
• Writing "silver" alone rather than silver nitrate.
• Forgetting the word precipitate (e.g., writing "yellow solution" loses the observation mark).

🧠 Memory Aid for Halides

Think alphabetically by halide ion vs colour brightness:

• Chloride → White (Milk)
• Bromide → Cream (Cream)
• Iodide → Yellow (Butter)