AQA A-Level Chemistry Paper 1, June 2025: Question 1

✅ Correct Answers

• Equation: TiCl₄ + 2Mg → Ti + 2MgCl₂
• Role of Magnesium: Reducing agent (or electron donor)

💡 Key Knowledge

• Titanium is extracted by reduction of titanium(IV) chloride ( TiCl₄ ) using a more reactive metal like magnesium.
• Magnesium changes oxidation state from 0 in Mg to +2 in MgCl₂ . Because it loses electrons (is oxidized), it acts as the reducing agent.

🧠 Exam Technique

Always balance the chlorine atoms first! TiCl₄ has 4 chlorines, so you need 2 molecules of MgCl₂ on the right, which in turn requires 2 atoms of Mg on the left.

❌ Common Errors

• Writing magnesium chloride as MgCl instead of MgCl₂ . Magnesium is in Group 2 and forms Mg²⁺ ions!
• Stating the role of magnesium is "oxidation" instead of "reducing agent". Oxidation is the process it undergoes, not its role.

✅ Correct Answers

• Decrease acidity of soil: Ca(OH)₂ (Calcium hydroxide / Slaked lime)
• Remove SO₂ from flue gases: CaCO₃ (Calcium carbonate) OR CaO (Calcium oxide)

💡 Key Knowledge

• Agricultural Lime: Ca(OH)₂ is alkaline and neutralizes acidic soils to optimize crop growth.
• Flue Gas Desulfurization: Power stations burn fossil fuels containing sulfur, producing acidic SO₂ gas. This is neutralized by spraying it with a slurry of basic calcium oxide ( CaO ) or calcium carbonate ( CaCO₃ ).

🧠 Exam Technique

Read the prompt carefully! The question explicitly asks for the formula of a suitable calcium compound, not the name. Writing "calcium hydroxide" instead of Ca(OH)₂ risks losing the mark.

❌ Common Errors

• Writing incorrect formulas like CaOH (forgetting the brackets and the 2+ charge of calcium).
• Confusing the two uses and swapping the compounds.

✅ Correct Answers

Magnesium hydroxide is insoluble (or sparingly/less soluble than sodium hydroxide).
OR
Magnesium hydroxide is not corrosive (whereas sodium hydroxide is highly corrosive).

💡 Key Knowledge

• Solubility of Group 2 hydroxides increases down the group. Mg(OH)₂ is extremely sparingly soluble.
• Because it is sparingly soluble, it does not dissolve fully in the stomach to release a high concentration of OH⁻ ions. This keeps the pH safe and non-corrosive, unlike highly soluble NaOH .

🧠 Exam Technique

Focus on physical properties (solubility) or safety hazards (corrosiveness). The mark scheme explicitly states to ignore references to base strength or pH, as these do not directly address why one is safer to ingest in terms of chemical availability.

❌ Common Errors

• Vague statements like "it is a weaker base" or "it has a lower pH".
• Saying "it is non-toxic" without explaining why (i.e., its low solubility prevents a high concentration of corrosive hydroxide ions).

✅ Correct Answers

• Group 2 Cation: Mg²⁺ (allow Ca²⁺ )
• Ionic Equation: Mg²⁺ + 2OH⁻ → Mg(OH)₂ (or Ca²⁺ + 2OH⁻ → Ca(OH)₂ )

💡 Key Knowledge

• Adding NaOH(aq) to a solution containing Mg²⁺ forms a white precipitate of magnesium hydroxide because Mg(OH)₂ is sparingly soluble.
• Further down the group, hydroxides become highly soluble, so Ba²⁺ or Sr²⁺ would not form a precipitate with NaOH .

🧠 Exam Technique

An ionic equation should only show the reacting ions that form the precipitate. Do not include spectator ions like Na⁺ . State symbols were not explicitly requested here, but it is always good practice to write them if you are sure.

❌ Common Errors

• Writing a full molecular equation instead of an ionic equation.
• Forgetting to balance the charges in the ionic equation: Mg²⁺ requires 2OH⁻ to form neutral Mg(OH)₂ .

✅ Correct Answers

• Anion: Br⁻ (or bromide)
• Ionic Equation: Ag⁺(aq) + Br⁻(aq) → AgBr(s)
• Explanation: To remove carbonate ions ( CO₃²⁻ ) which would otherwise react with silver nitrate to form an interfering white precipitate of silver carbonate ( Ag₂CO₃ ).

💡 Key Knowledge

• Silver halides have distinct colors: AgCl is white, AgBr is cream, and AgI is yellow.
• Acidifying with nitric acid ( HNO₃ ) destroys carbonate impurities: CO₃²⁻ + 2H⁺ → CO₂ + H₂O .

🧠 Exam Technique

Read carefully! The question explicitly asks for state symbols in this ionic equation. You must include (aq) for the aqueous ions and (s) for the solid precipitate to get the mark.

❌ Common Errors

• Using hydrochloric acid (HCl) to acidify: This is a fatal error! HCl introduces chloride ions ( Cl⁻ ), which immediately react with Ag⁺ to form a false-positive white precipitate of AgCl .
• Vague explanations like "to clean the solution" or "to remove impurities" without specifying carbonate ions or that they form an interfering precipitate.

✅ Correct Answers

Insoluble
OR
Absorbs / blocks / scatters X-rays (radiopaque)

💡 Key Knowledge

• Barium ions ( Ba²⁺ ) are highly toxic to humans.
• However, barium sulfate ( BaSO₄ ) is completely insoluble. When swallowed as a "barium meal", it is not absorbed into the bloodstream and safely passes through the digestive tract.
• Its high electron density allows it to absorb X-rays, outlining the digestive tract on an X-ray scan.

🧠 Exam Technique

The question asks for a property. Do not just write "barium meal" or "X-rays" without qualifying them. You must state the physical property (insoluble) or its physical behavior with radiation (absorbs X-rays).

❌ Common Errors

• Writing "it is non-toxic" without stating that its insolubility is what makes it safe.
• Confusing the solubility trend: Group 2 sulfates become less soluble down the group, making BaSO₄ the least soluble sulfate.