Edexcel A-Level Chemistry AS Paper 1, June 2016: Question 4

✅ Correct Answer

C (potassium chloride)

💡 Key Knowledge

• Potassium ions ( K⁺ ) produce a lilac flame.
• Sodium ions ( Na⁺ ) produce a yellow/orange flame.
• Barium ions ( Ba²⁺ ) produce a pale green flame.

✅ Correct Answer

B

💡 Key Knowledge

• Heat energy causes electrons to be promoted from the ground state to an excited state (absorption).
• When electrons fall back down from an excited state to the ground state, energy is released as electromagnetic radiation (light).
• Diagram B correctly shows an arrow starting at the upper excited state and pointing downwards to the lower ground state, with the label Energy Released .

✅ Correct Answer

B (magnesium sulfate)

💡 Key Knowledge

• To test for sulfate ions ( SO₄²⁻ ), add acidified barium chloride solution ( BaCl₂ ).
• A positive result is the formation of a white precipitate of barium sulfate ( BaSO₄ ).
• Magnesium sulfate is the only solution provided containing sulfate ions.

✅ Correct Answer

Observation: White precipitate (or white solid)

Solutions: Hydrochloric acid and potassium chloride (Accept HCl and KCl , or A and C)

🧠 Exam Technique

Both solutions contain chloride ions ( Cl⁻ ). When reacted with acidified silver nitrate ( AgNO₃ ), they form silver chloride, which is a white precipitate.

❌ Common Errors

Students often lose marks by writing "off-white" or describing a cloudy solution instead of explicitly stating white precipitate. Make sure to use standard terminology.

✅ Correct Answer

Observation: Fizzing / bubbles / effervescence

Ionic equation: 2H⁺ + CO₃²⁻ → CO₂ + H₂O

💡 Key Knowledge

Acids react with carbonates to produce a salt, water, and carbon dioxide gas. The visible sign of carbon dioxide gas being evolved is effervescence (fizzing).

❌ Common Errors

Do not write "solid dissolves" or refer to hydrogen gas ( H₂ ). The gas evolved is carbon dioxide from the carbonate ion.