AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2023: Question 6

✅ Correct answer (what to write)

• Filter the mixture to remove the excess magnesium.
• Then obtain the salt by crystallisation (allowing water to evaporate from the magnesium chloride solution).

💡 Key knowledge

• After the reaction, you have: MgCl₂(aq) (dissolved) + solid Mg (leftover) + water.
• Filtering separates an insoluble solid (Mg) from a liquid (salt solution).
• Crystallisation means concentrating the solution and letting crystals form as water evaporates.

🧠 Exam technique

• For 2-mark “how to obtain solid salt” questions, aim for a clear two-step method in the correct order.
• Use process verbs: filter → crystallise/evaporate.
• Be specific: state what you are removing (excess magnesium) and what you are making (solid MgCl₂ crystals).

❌ Common errors (why marks are lost)

• Saying “evaporate to dryness” with no mention of crystallisation—can lose the method mark because salts may decompose / it’s not the standard practical wording.
• Missing filtering (you must remove the unreacted magnesium first).
• Writing “decant” instead of filter: decanting may leave solid Mg behind, so it’s less secure than the mark-scheme method.

✅ Correct answer (2 marks)

• Magnesium atoms are oxidised.
• Because magnesium atoms lose two electrons (forming Mg²⁺ ions).

💡 Key knowledge

• Oxidation = loss of electrons (OIL).
• Magnesium becomes a 2+ ion, so it must lose 2 electrons.
• In metal + acid reactions, the metal is usually oxidised and hydrogen ions are reduced to H₂.

🧠 Exam technique

• For “explain” redox questions, always include both:
  • the redox word (oxidised / reduced)
  • the electron idea (loss/gain + number of electrons)
• Link charge to electrons: Mg²⁺ means “lost 2 electrons”.

❌ Common errors

• Writing “magnesium is reduced” (wrong direction).
• Saying “gains electrons” or missing the number of electrons.
• Confusing oxidation with “gain of oxygen” only—here you must use the electron definition to score.

📐 Calculations (step-by-step, with marks)

1. Work out moles of Mg (1 mark):
   moles = mass ÷ Ar
   moles Mg = 0.72 ÷ 24 = 0.03 mol
2. Use the equation ratio (1 mark):
   From Mg + 2HCl → MgCl₂ + H₂ , the ratio is 1 mol Mg : 1 mol MgCl₂
   So moles MgCl₂ = 0.03 mol
3. Convert moles of MgCl₂ to mass (1 mark):
   mass = moles × Mr
   mass MgCl₂ = 0.03 × 95 = 2.85 g
4. Convert volume into dm³ (1 mark):
   100 cm³ = 0.100 dm³
5. Concentration in g/dm³ (1 mark):
   concentration = mass ÷ volume
   = 2.85 ÷ 0.100 = 28.5 g/dm³

💡 Key knowledge

• If a reactant is in excess, it does not limit the amount of product. Here, Mg limits.
• Always read coefficients: Mg : MgCl₂ is 1 : 1.
• g/dm³ uses volume in dm³. Convert: 1 dm³ = 1000 cm³.

🧠 Exam technique (how to secure the 6 marks)

• Write the key formula each time: moles = mass ÷ Mr/Ar , mass = moles × Mr , conc = mass ÷ volume .
• Show units clearly: g, mol, cm³, dm³, g/dm³.
• Keep 2–3 significant figures through the working, then round at the end. (28.5 is fine.)

❌ Common calculation traps

• Forgetting cm³ to dm³: using 100 instead of 0.100 makes the answer 1000× too small.
• Using Mr of Mg (24) instead of Mr of MgCl₂ (95) when finding the product mass.
• Wrong ratio from the equation: some students think “2 HCl” means “2 MgCl₂”. It doesn’t—coefficients show the reacting ratio for each substance.
• Writing the correct number but missing g/dm³ can lose the final mark in many mark schemes—always include units.

🧠 What top responses do

• Use the standard salt-prep sequence: filter then crystallise.
• State redox with electron detail: oxidised because lose 2e⁻.
• Set out calculations in a clean chain with conversions and units.

💡 Key facts to memorise

• Oxidation = loss of electrons.
• 100 cm³ = 0.100 dm³.
• Concentration (g/dm³) = mass (g) ÷ volume (dm³).

✅ Final answers summary

• 6.1 Filter (remove excess Mg); then crystallisation/evaporation to get solid MgCl₂.
• 6.2 Mg is oxidised; Mg atoms lose 2 electrons.
• 6.3 Concentration = 28.5 g/dm³.