AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2023: Question 5

✅ Correct Answer

Tick: Coarse particle [1 mark]

💡 Key Knowledge

• Coarse particles (PM₁₀): Diameters between 2.5 × 10⁻⁶ m and 1 × 10⁻⁵ m (2500 nm to 10 000 nm). Often referred to as dust.
• Fine particles (PM₂.₅): Diameters between 1 × 10⁻⁷ m and 2.5 × 10⁻⁶ m (100 nm to 2500 nm).
• Nanoparticles: Diameters between 1 × 10⁻⁹ m and 1 × 10⁻⁷ m (1 nm to 100 nm).

✅ Correct Answer

Tick: 20 times [1 mark]

📐 Calculation

To find how many times larger the coarse particle is than the fine particle:

Scale factor = Coarse diameter ÷ Fine diameter

Scale factor = 4000 nm ÷ 200 nm = 20

❌ Common Errors

• Miscounting zeros: selecting 2 or 200 instead of 20.
• Subtracting values (4000 − 200) instead of dividing to find a ratio/scale factor.

📐 Step-by-Step Calculation

1. Area of one face:
   3 nm × 3 nm = 9 nm²
2. Total surface area:
   A cube has 6 faces:
   6 × 9 = 54 nm² [2 marks: 1 for 6 × 3², 1 for 54]
3. Write unsimplified ratio:
   Surface area : Volume = 54 : 27 [1 mark]
4. Simplify to the form n : 1:
   Divide both sides by 27:
   54 / 27 = 2
   Simplest ratio = 2 : 1 [1 mark]

✅ Final Values for Mark Scheme

• Surface area of cubic nanoparticle = 54 nm²
• Simplest whole number ratio of surface area : volume = 2 : 1

🧠 Exam Technique

• Notice the question already provided the volume ( 27 nm³ ) and formula ( 6 × surface area of one face ), so don't re-calculate volume!
• Always check the final answer blank carefully: the ": 1" was already printed, so only write 2 on the dotted line.

❌ Common Errors

• Multiplying 3 × 6 to get 18 instead of finding area first (3 × 3 = 9) then multiplying by 6.
• Reversing the ratio (writing 1 : 2 instead of 2 : 1). The question asks for surface area : volume.
• Writing "2 : 1" on the line when ": 1" is already printed, making it look like 2:1:1.

✅ Correct Answer

Tick: A smaller mass of nanoparticles is needed to be effective. [1 mark]

💡 Key Knowledge

• Because nanoparticles have a very high surface area to volume ratio, they are far more reactive and offer better coverage than normal-sized bulk particles.
• This means less mass/quantity is required to provide the same UV protection.
• They also give better, clearer skin coverage (they don't leave white chalky marks like traditional creams).

❌ Why the other options are wrong

• "Nanoparticles cost more..." — This is a disadvantage, not an advantage.
• "...have a lower surface area to volume ratio" — Incorrect science: nanoparticles have a much higher surface area to volume ratio.

✅ Correct Answer

Tick: TiO₂ [1 mark]

📐 Balancing Ionic Charges

Total charge of an ionic compound must equal zero:

• Titanium ion: Ti⁴⁺ (charge = +4)
• Oxide ion: O²⁻ (charge = -2)
• Number of oxide ions required = +4 ÷ 2 = 2
• Overall: (+4) + 2 × (-2) = 0
• Ratio is 1 Ti to 2 O → TiO₂

❌ Common Errors

• Swapping charges directly without simplifying: writing Ti₂O₄ instead of TiO₂ (always simplify ionic empirical formulae).
• Mistaking the charge superscript for subscript directly (e.g. writing TiO₄ or Ti₄O₂).