AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2025: Question 7

✅ Correct Matches

• Mixture of compounds → 3rd box down
  Shows two different types of compound molecules (some made of 1 black atom bonded to 2 white atoms, others made of 2 black atoms bonded to 1 white atom).
• Mixture of a compound and an element → 5th box down (bottom box)
  Shows triatomic compound molecules (1 black atom bonded to 2 white atoms) alongside diatomic element molecules (2 white atoms bonded together).

💡 Key Knowledge

• Element: Contains only one type of atom (e.g. diatomic pairs of matching circles).
• Compound: Contains two or more different types of atoms chemically bonded together in fixed ratios.
• Mixture: Different substances together that are not chemically bonded.
• Box Breakdown:
  • Box 1: Pure diatomic element
  • Box 2: Pure compound
  • Box 3: Mixture of compounds
  • Box 4: Mixture of elements
  • Box 5: Mixture of a compound & element

🧠 Exam Technique

Scan every individual particle in the box:

• Are all clusters identical? If yes, it is a pure substance. If no, it is a mixture.
• Inside the clusters, are the circles different shades? If all circles are one shade, it's an element. If mixed shades are joined, it's a compound.
• The rubric states "Draw one line from each type of mixture". If you draw two lines from one box, you forfeit that mark!

❌ Common Errors

• Confusing Box 4 with a mixture of compounds (Box 4 contains molecules of two different elements: pairs of black atoms and pairs of white atoms).
• Missing that Box 2 shows a pure compound, not a mixture, because every single molecule in the box has the exact same formula.

✅ Model Method (Full 6 Marks)

1. Dissolve the salt: Place the salt-sand mixture into a beaker. Add water from the bottle. Place the beaker on a tripod and gauze and heat gently using a Bunsen burner, stirring with the glass rod to completely dissolve the salt.
2. Filter the mixture: Set up the filter funnel lined with filter paper over a clean beaker or flask. Pour the mixture into the funnel.
3. Obtain pure, dry sand: Sand remains on the filter paper (residue). Rinse the sand with a small amount of distilled water to remove residual salt solution, then leave it on filter paper or in a warm oven to dry.
4. Obtain pure, dry salt: Pour the filtrate (salt solution) into an evaporating basin. Heat over a Bunsen burner until crystals start to form (or to reduce volume by half), then allow to cool and crystallise. Pat dry with filter paper (or heat gently to complete dryness).

🧠 Level of Response Breakdown (6 Marks)

This is an extended response question marked across three levels:

• Level 3 (5–6 marks): A coherent, logically sequenced method that clearly explains how to obtain both pure dry sand and pure dry salt. All relevant apparatus named and used correctly.
• Level 2 (3–4 marks): Most steps are identified (e.g. dissolving, filtering, evaporating), but steps may lack sequencing, or the drying/washing stages are omitted.
• Level 1 (1–2 marks): Fragmented steps mentioned without logical order or missing one whole component (e.g. only mentions filtering to get sand).

❌ Common Misconceptions & Lost Marks

• Forgetting to obtain DRY samples: Students often stop at "filter the sand" and "evaporate the water", without stating how to wash and dry the sand or dry the salt crystals.
• Ignoring provided equipment: Figure 11 explicitly gives you apparatus (glass rod, beaker, evaporating basin, tripod and gauze). Top answers specifically integrate these named items into their steps.
• Overheating to dryness: While heating to complete dryness is credited, heating to the crystallisation point and letting crystals form yields larger, purer crystals without spitting.

📐 Step-by-Step Calculation

Formula provided:

1. Identify the values from the question:
   • Mass of salt obtained (actual yield) = 13.6 g
   • Mass of salt in mixture (maximum theoretical yield) = 16.0 g
2. Substitute values into the equation [1 mark]:
   % yield = (13.6 / 16.0) × 100
3. Calculate final percentage [1 mark]:
   % yield = 0.85 × 100 = 85%

❌ Calculation Traps

• Inverting the fraction: Calculating (16.0 / 13.6) × 100 = 117.6% . Remember that actual yield cannot exceed the initial amount present (you cannot produce salt out of thin air!). The smaller value goes on top.
• Forgetting to multiply by 100: Leaving the answer as 0.85 instead of converting to a percentage.
• Rounding errors: Always check if a specific number of decimal places or significant figures is requested. Here, 85% is an exact integer.