Edexcel A-Level Chemistry Paper 1, June 2024: Question 3

✅ Correct Answer (Full Marks Scheme)

1. Use a nichrome or platinum wire (or silica rod).
2. Dip the wire into concentrated hydrochloric acid (HCl).
3. Dip the wire into the solid sample and place it in a blue / roaring / non-luminous Bunsen flame.

💡 Key Knowledge

• Why concentrated HCl? It cleans the wire and converts metal salts into volatile metal chlorides, which vaporise readily in the flame.
• Why nichrome/platinum? High melting points and unreactive, producing no interfering flame colour.
• Why a roaring blue flame? It is hot enough to excite electrons and its faint colour does not mask the emission colour.

🧠 Exam Technique

Always specify the type of wire and the acid used. Never just say "acid" or "metal loop". Specify "blue flame" or "non-luminous flame"—writing just "put in the flame" can lose the third mark.

❌ Common Errors

• Writing sulfuric acid or nitric acid instead of HCl (sulfates are non-volatile and won't vaporise).
• Stating "yellow flame" or "safety flame" (yellow flames leave soot and obscure the colour).
• Referring to a wooden splint (examiner specifically instructs: do not award splint).

✅ Expected Observations

• calcium bromide: orange-red / brick-red
• calcium iodide: orange-red / brick-red
• potassium sulfate: lilac

❌ Common Pitfalls

• Calling potassium purple, violet, or mauve: Edexcel specifically requires lilac.
• Confusing the red flames: Calcium is brick-red / orange-red, whereas Strontium is red / crimson and Lithium is carmine / magenta red.
• Giving different colours for calcium bromide and calcium iodide (anions do not alter the flame test colour!).

💡 Solubility in Ammonia Trend

• AgCl (white): Dissolves in dilute NH₃(aq).
• AgBr (cream): Insoluble in dilute NH₃(aq), but dissolves in concentrated NH₃(aq).
• AgI (yellow): Insoluble in both dilute and concentrated NH₃(aq).

🧠 Exam Technique

Notice the column heading asks for Formula, not name! Writing "silver bromide" instead of AgBr will forfeit the mark for that box.

✅ Correct Reagents & Observation

• Reagent: Add dilute hydrochloric acid (HCl) or dilute nitric acid (HNO₃), followed by barium chloride (BaCl₂) solution or barium nitrate (Ba(NO₃)₂) solution. [1 mark]
• Observation: White precipitate forms. [1 mark]

💡 Why add acid first?

Acidifying the solution removes carbonate (CO₃²⁻) or sulfite (SO₃²⁻) ions that would otherwise react with barium ions to form competing white precipitates:
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

❌ Common Errors

• Saying "add sulfuric acid and barium chloride": Adding H₂SO₄ adds sulfate ions, producing a false positive!
• Writing "acidified" without naming an acceptable acid (examiners reject just "acidified").
• Calling the result a "white powder" or "cloudy solution" instead of a white precipitate.

📐 Step-by-Step Calculation

Step 1: Calculate moles of K₂SO₄ required:

Volume = 250 cm³ = 0.250 dm³

n = concentration × volume

n = 0.0450 mol dm⁻³ × 0.250 dm³ = 0.01125 mol

Step 2: Calculate mass from moles:

mass = n × Molar Mass

mass = 0.01125 mol × 174.3 g mol⁻¹ = 1.960875 g

mass ≈ 1.96 g (to 3 sig figs)

Correct Option: A (1.96 g)

❌ Deciphering the Distractors

• B (7.84 g): Forgot to divide volume by 4 (calculated for 1.00 dm³ instead of 250 cm³).
• C (19.6 g): Misplaced the decimal point / used 0.450 mol dm⁻³.
• D (31.4 g): Multiplied volume by molar mass incorrectly ( 174.3 × 0.18 ).