AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2022: Question 7

✅ Correct Answer

Tick box 1: Coarse particle

💡 Key Knowledge

Order of particle sizes from largest to smallest:

• Coarse particles (PM₁₀): 2500 nm to 10 000 nm (often called dust).
• Fine particles (PM₂.₅): 100 nm to 2500 nm.
• Nanoparticles: 1 nm to 100 nm (only a few hundred atoms across).

📐 Step-by-Step Calculation

Step 1: Calculate volume

Volume = length × width × height

Volume = 2 nm × 2 nm × 2 nm = 8 nm³

Step 2: Set up the initial ratio

Surface area : Volume = 24 : 8

Step 3: Simplify to the form [n : 1]

Divide both sides by 8:

24 ÷ 8 = 3

Simplest ratio = 3 : 1

✅ Mark Breakdown

• Mark 1: Formula or substitution seen: 2³ or 2 × 2 × 2
• Mark 2: Correct volume calculated: 8 (nm³)
• Mark 3: Unsimplified ratio set up: 24 : 8 (allow error carried forward from incorrect volume)
• Mark 4: Fully simplified ratio: 3 (given as 3 : 1 on the dotted line)

❌ Common Errors

• Confusing Area and Volume: Adding the dimensions (2 + 2 + 2 = 6) instead of multiplying them.
• Incomplete Simplification: Leaving the answer as 24 : 8 or 12 : 4 . Notice the exam answer line already ends in : 1 . You must give a single simplified number on that line!

🧠 Exam Technique

Always inspect the answer line carefully. When the question asks for the simplest ratio in the format ______ : 1 , the right-hand side is already set to 1. Simply calculate:

Surface Area ÷ Volume = 24 ÷ 8 = 3

✅ Correct Answer

Compared with normal sized particles, the surface area to volume ratio of nanoparticles is higher / larger. [1 mark]

This means that the mass of a nanoparticle catalyst needed to have the same effect as the same catalyst made of normal sized particles is lower / less / smaller. [1 mark]

💡 Key Knowledge

As particle size decreases by a factor of 10, the surface area to volume ratio increases by a factor of 10. Because catalytic reactions take place on the surface, nanoparticles have a huge percentage of their atoms exposed at the surface.

This makes them far more reactive, meaning you need a significantly smaller mass compared to bulk materials to achieve the same catalytic effect.

✅ Acceptable Answers

Advantage (any one):

• Stops / reduces unpleasant smells.
• Can stop / reduce (foot) infections.
• Kills bacteria.

Disadvantage (any one):

• High cost of socks / silver is very expensive.
• Could be harmful / toxic if breathed in.

🧠 Exam Technique: Use the Provided Stem

The prompt provides five bullet points. The examiner is explicitly looking for you to link those facts to a practical benefit and a drawback:

• "Bacteria can break down sweat to produce unpleasant smells" → Advantage: stops bad smells.
• "Silver is very expensive" → Disadvantage: increases product cost.
• "Small enough to be breathed in" → Disadvantage: potential lung hazard if particles shed.

✅ Correct Answer

Tick box 3: 100 times

📐 Working with Powers of Ten

Divide the nanoparticle radius by the atomic radius:

Scale factor = (1 × 10⁻⁸) ÷ (1 × 10⁻¹⁰)

10⁻⁸ ÷ 10⁻¹⁰ = 10^(-8 - (-10)) = 10² = 100

The nanoparticle radius is 100 times larger than the atom radius.

❌ Common Errors

• Selecting "2 times": Subtracting the exponents (-8 - (-10) = 2) and mistaking the power of 2 for a direct factor of 2.
• Selecting "10 times": Miscounting the difference in powers of ten. Remember, each step in exponent is a factor of 10 (10¹ = 10, 10² = 100).