AQA Chemistry 4.2.4 (chemistry only) · Quantitative
Nanoparticles
Gold is yellow and unreactive. Grind it into particles a few nanometres across and it turns red and becomes a catalyst. Same atoms. What changed, and can you calculate it?
Triple Science · Higher tier
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A red glass window, coloured with gold.
Medieval glassmakers turned windows ruby-red by stirring in a trace of gold. The gold spreads through the glass as particles around 25 nm across, too small to see. A gold ring and the red glass contain exactly the same kind of atom.
Commit first
Why does gold behave so differently at this size?
Nanoscience is about structures 1 to 100 nm across: 1 nm = 1 × 10⁻⁹ m, a billionth of a metre. Nanoparticles are of the order of a few hundred atoms; an atom has a radius of about 0.1 nm. Nanoparticles are smaller than fine particles (PM2.5), which are 100 to 2500 nm across, and much smaller than coarse particles (PM10), the dust of 2.5 × 10⁻⁶ m to 1 × 10⁻⁵ m. Sorting a particle means getting every size into the same unit first.
Sort by size
Coarse, fine or nano?
Convert each size to nanometres first. 1 µm = 1000 nm.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
The cube splitter
Make the particle smaller. Watch the surface win.
1 of 4 sizes viewed
Commit first. A cube's side shrinks from 100 nm to 10 nm, ten times smaller. What happens to its surface area to volume ratio?
Think again
“Red nanogold must be a different substance from yellow gold.”
Spot the flaw
ExplainWhat really changes when gold becomes nanoparticles?
Equation
Derived, not on any sheetSA : V = 6L² ÷ L³ = 6 ÷ L
For a cube of side L: six faces of area L², volume L³.
L = 6 ÷ (SA : V)
Rearranged, to find the side from a ratio.
Units
L in nm gives the ratio in nm⁻¹ (per nanometre). Convert every length to nm before you insert it.
Uses and risks · AQA 4.2.4.2
Nanoparticles are used in medicine, electronics, cosmetics and sun creams, deodorants and as catalysts. Because the particles are so small, and their properties so different, there are possible risks: they may be absorbed through skin or lungs, and their long-term effects on health and the environment are not yet fully known. An “evaluate” question wants a benefit, a risk and a judgement.
Extended response
Evaluate4 marksTitanium dioxide nanoparticles are used in sun creams. Evaluate their use.
Command words in this lesson
- Calculate
- Show the working. Convert, formula, insert, fine-tune, answer with a unit.
- Compare
- Describe the similarities or differences; with ratios, say how many times bigger by dividing one by the other.
- Evaluate
- Benefit, risk, then a judgement with a reason.
Key fact
When the side of a cube decreases by a factor of 10, its surface area to volume ratio increases by a factor of 10. That is why nanoparticles behave differently from the same material in bulk.
Examiner tip
Key note · AQA 4.2.4 (chemistry only)
Nanoparticles
- Nanoparticles are 1 to 100 nm across. 1 nm = 1 × 10⁻⁹ m. They are of the order of a few hundred atoms.
- Fine particles (PM2.5): 100 to 2500 nm. Coarse particles (PM10): 2.5 × 10⁻⁶ to 1 × 10⁻⁵ m.
- For a cube: SA : V = 6 ÷ L. Convert lengths to nm first.
- Side ÷ 10 means SA : V × 10.
- Nanoparticles are the same substance as the bulk material; their high SA : V gives them different properties.
- Uses: medicine, electronics, sun creams, deodorants, catalysts. Risks: possible harm to health and the environment, not yet fully known.
Photograph this card. Tomorrow, cover it and say each line first.
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Stuck converting µm and m into nanometres?
Particles are modelled as perfect cubes, the GCSE convention; real nanoparticles are irregular, so real ratios differ but follow the same pattern. The cube splitter draws a cube of each size sliced into smaller cubes of the same total volume; the number of slices drawn is schematic, not to scale. Coarse (PM10) and fine (PM2.5) size ranges follow AQA 4.2.4.1.