Bulk and Surface Properties of Matter Including Nanoparticles [chemistry only]

Lesson 2.09 · Chemistry only · Paper 1

Bulk & Surface Properties — Nanoparticles

Sizes, surface area to volume ratio, uses, risks. Spec 4.2.4.1–4.2.4.2.

What You Need to Know

Spec 4.2.4 has just two parts. Both are chemistry only (not on combined science).

4.2.4.1 — SIZES & PROPERTIES

Define nanoparticles, fine particles (PM2.5), coarse particles (PM10)

Compare sizes using standard form (1 nm = 1 × 10⁻⁹ m)

Calculate surface area to volume (SA:V) ratios for cubes

Recognise: as cube side ÷ 10, SA:V × 10

Explain how high SA:V leads to different properties from bulk

4.2.4.2 — USES & RISKS

List uses: medicine, electronics, cosmetics & sun creams, deodorants, catalysts

Evaluate the use of nanoparticles for a specified purpose (advantages and disadvantages)

Explain possible risks of nanoparticle use

EXAM PRESENCE

This topic appears mostly as 1–4 mark questions on Paper 1 — testing definitions, SA:V reasoning, and AO3 evaluation. The 2-mark Q10.1 from Jun 2023 is the most-cited example: about a third of students scored one mark, far fewer scored both — usually for the same comparative-language slip.

Three Particle Size Categories

The spec defines three categories by diameter. Memorise the size ranges in standard form.

⚠ NOT TO SCALE — actual nanoparticle is ~100× smaller than coarse particle (next slide shows true ratios)

SPEC DEFINITIONS (verbatim)

Nanoparticles: structures of 1–100 nm — "of the order of a few hundred atoms"

Fine particles (PM2.5): diameters 100 nm – 2500 nm (1 × 10⁻⁷ m to 2.5 × 10⁻⁶ m)

Coarse particles (PM10): diameters 2.5 × 10⁻⁶ m to 1 × 10⁻⁵ m — often called dust

Nanoscale — In Perspective

The spec asks you to "compare 'nano' dimensions to typical dimensions of atoms and molecules". A nanoparticle contains a few hundred atoms.

SCALE INTUITION

A typical atom is ~0.1 nm across. So a 1 nm nanoparticle is about 10 atoms across each side → roughly 10³ = 1000 atoms total ("a few hundred atoms" — the spec's wording fits the smallest end of the range). A 100 nm nanoparticle is about 1000 atoms across each side → ~10⁹ atoms.

Compare to a fine particle (PM2.5, up to 2500 nm across): hundreds of times wider than the largest nanoparticle. Compared to dust (PM10, up to 10 µm): another order of magnitude larger again.

Surface Area to Volume Ratio (SA:V)

The single concept that drives every nanoparticle question. Define it; calculate it for a cube; understand why it matters.

DEFINITION

Surface area to volume ratio = surface area ÷ volume.

For a cube of side L: SA = 6L², V = L³, so SA:V = 6/L. As L gets smaller, SA:V gets larger.

⚠ Cubes shown not to scale — true ÷10 reductions would shrink each cube to a tenth of the previous linear size

SPEC RULE (4.2.4.1)

Verbatim: "As the side of a cube decreases by a factor of 10 the surface area to volume ratio increases by a factor of 10." So a 1 cm cube → 1 mm cube → 100 µm cube → … the SA:V multiplies by 10 each step. By the time you reach nano-scale, the SA:V is enormous compared to bulk.

Why High SA:V Changes Properties

The spec line that gives the whole topic its purpose: "Nanoparticles may have properties different from those for the same materials in bulk because of their high surface area to volume ratio."

THE LOGIC IN ONE CHAIN

High SA:V → a much greater fraction of the atoms are on the surface (not buried inside)

Surface atoms are the ones that react, absorb, scatter, or interact with the surroundings

So nanoparticles can be more reactive, more catalytically active, more effective at blocking UV — for the same mass of material

SPEC CONSEQUENCE

Verbatim: "It may also mean that smaller quantities are needed to be effective than for materials with normal particle sizes." A thinner sun-cream layer of TiO₂ nanoparticles blocks the same UV as a much thicker layer of bulk TiO₂.

REMEMBER — SAME MASS

The advantage is always relative to the same mass of bulk material. For the same mass, dividing into smaller pieces creates more surface — so more atoms to do the work. This "for the same mass" idea is the most-tested phrase in this lesson.

Uses of Nanoparticles

Spec 4.2.4.2 lists the application areas. The spec is explicit: students "do not need to know specific examples or properties other than those specified" — so memorise the application areas, not particular nanomaterials.

MEDICINE

Drug delivery — nanoparticles can carry drugs deeper into tissue and target specific cells. Smaller quantities of active drug needed.

ELECTRONICS

Smaller, faster components for phones and computers. Nanoscale components allow more transistors per chip.

COSMETICS & SUN CREAMS

Common example: TiO₂ and ZnO nanoparticles in sun creams block UV light effectively. Because they are nanoscale, they appear transparent on skin rather than the thick white paste of bulk TiO₂. (Specific examples are not required by the spec.)

DEODORANTS

Antibacterial nanoparticles can be added to reduce bacterial growth that causes odour.

CATALYSTS

High SA:V means more surface atoms per gram of catalyst — same activity from much less material.

RESEARCH

Spec note: "New applications for nanoparticulate materials are an important area of research." Expect questions on novel/given applications you haven't seen before — apply the SA:V reasoning.

PEE Worked Example — Real Exam Question

The single most-cited nanoparticle question: P1 Jun 2023 Q10.1 (2 marks, AO3). Verbatim from the paper:

QUESTION (P1 JUN 2023 Q10.1) — VERBATIM

"Self-cleaning windows are coated with a layer of nanoparticles of titanium dioxide. Titanium dioxide:

helps sunlight break down dirt particles

attracts water, so dirt is washed away by rain.

Nanoparticles of titanium dioxide are used instead of fine particles of titanium dioxide for coating self-cleaning windows. Suggest two reasons why."

Spec ref: 4.2.4.2 · Marks: 2 · AO: AO3

VERBATIM MARK SCHEME — ANY TWO FROM

"have a higher surface area to volume ratio"

"less (material) needed (for the same effect)"

"more light gets through"

"Allow converse arguments for fine particles. Allow 'a thinner coating is needed'. References to cost were ignored."

MODEL ANSWER — TWO REASONS

Reason 1: Nanoparticles have a higher surface area to volume ratio than fine particles, so for the same mass of TiO₂ a greater proportion of atoms is on the surface — so less material is needed to achieve the same self-cleaning effect (sunlight breaks down dirt; water washes it away).

Reason 2: A thinner coating of nanoparticles is needed, so more light passes through the window.

Examiner Alert — Two Traps that Lose the Mark

The Jun 2023 examiner report on Q10.1 reveals two specific, repeated errors that cost most students the second mark. About a third of students scored one mark; far fewer scored two.

EXAMINER ALERT — P1 JUN 2023 Q10.1 (verbatim)

"About a third of students scored one mark, but far fewer scored a second mark. Many students did not give comparative answers. Many referred to nanoparticles having a larger surface area, but failed to realise that this depends on the amount used. A higher surface area to volume ratio, or a higher surface area for the same mass of material was creditworthy and was the most common correct answer."

TWO ERRORS, TWO FIXES

Wrong (no mark)Right (scores)

"Nanoparticles have a larger surface area""Higher surface area to volume ratio " OR "larger surface area for the same mass "

"Nanoparticles are more reactive because they're small" (no comparison)"... compared to bulk / fine particles " (always comparative)

WHY "LARGER SURFACE AREA" ALONE FAILS

A 1 kg block of TiO₂ has plenty of surface area too — more in absolute terms than a few grams of nanoparticles. The point isn't "more surface". It's more surface per gram. Always anchor your answer to ratio or same mass.

Risks of Nanoparticles

Spec 4.2.4.2 requires you to "explain that there are possible risks associated with the use of nanoparticles." The risks come from the same property that makes them useful — being very small.

FOUR RISKS TO REMEMBER

Health effects not fully known — nanoparticles are recent enough that long-term human health data are limited

Can be absorbed into the body — through skin (cosmetics, sun cream), lungs (inhalation), or digestive system

May cross the cell membrane — once inside cells, effects on DNA, organelles and immune response are still being studied

Environmental impact uncertain — when products containing nanoparticles are washed away or disposed of, the long-term effects on water and soil ecosystems are unclear

EXAM FRAMING

Risk questions are usually 1–2 marks. State a specific risk, not just "they could be dangerous". Best phrasing: "the long-term health effects of [absorption / inhalation] are not fully known" — this earns the mark because it's specific and accurate.

"Evaluate" Questions — How to Answer

The spec demands: "given appropriate information, evaluate the use of nanoparticles for a specified purpose." "Evaluate" is the AQA command word that asks you to weigh evidence and reach a judgement.

WHAT EVALUATE MEANS (AQA COMMAND WORD)

"Evaluate" requires you to review the information given, then identify advantages and disadvantages, and arrive at a judgement. The judgement is the part most students miss.

EVALUATE TEMPLATE FOR NANOPARTICLE QUESTIONS

Advantages grounded in SA:V — pull from the question stem and from the SA:V principle (e.g. "less material needed for the same effect", "more effective per gram", "transparent on skin if cosmetic")

Disadvantages grounded in risk (e.g. "long-term health effects of absorption are not fully known", "environmental effects unclear")

Reasoned conclusion — weigh them up against the specific use given. e.g. "For sunscreen, the UV protection benefit may outweigh the risks because untreated UV exposure causes skin cancer; but caution is warranted because skin absorption is possible."

DON'T LIST PROS AND CONS WITHOUT A JUDGEMENT

For higher-mark "evaluate" questions (especially extended response with banded marking), listing pros and cons without a clear judgement caps the response at the lower bands. Always end with "Overall I think..." or "On balance..." followed by a position rooted in the question's specific context.

Calculation Practice — SA:V Ratios

The spec lists this as an opportunity for MS 5c (calculate areas and volumes of cubes) and MS 1b (standard form). Work the formulas confidently for any cube.

Q1: SA:V FOR A CUBE OF SIDE 2 cm

SA = 6 × (2)² = 6 × 4 = 24 cm²

V = (2)³ = 8 cm³

SA:V = 24 ÷ 8 = 3 cm⁻¹ (or "3 to 1")

Q2: SA:V FOR A NANOPARTICLE CUBE OF SIDE 100 nm

Using SA:V = 6/L (in consistent units): SA:V = 6 ÷ (100 × 10⁻⁹ m) = 6 × 10⁷ m⁻¹

A 1 m cube of the same material has SA:V = 6 m⁻¹. So the nanoparticle's SA:V is 10⁷ (ten million) times larger than the bulk cube.

Q3: COMPARE — IF SIDE LENGTH ÷ 10, BY WHAT FACTOR DOES SA:V CHANGE?

SA:V for cube side L = 6/L. If L → L/10, then SA:V → 6/(L/10) = 60/L = 10 × original SA:V. Spec rule confirmed: factor of 10 reduction in side → factor of 10 increase in SA:V.

SHORTCUT

For any cube, SA:V simplifies to 6/L. Memorise this — it turns long calculations into one-liners.

Common Errors — Wrong vs Right

Drawn from the Jun 2023 examiner report and recurring patterns across nanoparticle questions.

WrongRightWhy It Matters

"Larger surface area""Higher SA:V ratio " or "larger surface area for the same mass "Absolute area depends on amount

"More reactive because they're small""Higher SA:V → greater fraction of atoms at surface → more available to react"Mechanism, not just size

"Nanoparticles are better" (no comparison)"Better than bulk / fine particles because..."Comparative needed

"They're dangerous" (vague)"Long-term health effects of [skin absorption / inhalation] are not fully known"Specific risk required

"PM2.5 = 2.5 nm""PM2.5 means 2.5 µm = 2500 nm"PM2.5 is the upper limit in micrometres

"Pros and cons" — no conclusion"Overall, [judgement] because [context]"Evaluate questions need a verdict

Key Definitions Summary

The mark-scheme phrasings you should be able to write under exam conditions.

NANOPARTICLE

A particle with dimensions between 1 nm and 100 nm (1 nm = 1 × 10⁻⁹ m), containing a few hundred atoms.

FINE PARTICLE (PM2.5)

A particle with diameter between 100 nm and 2500 nm (1 × 10⁻⁷ m to 2.5 × 10⁻⁶ m).

COARSE PARTICLE (PM10) — DUST

A particle with diameter between 2.5 × 10⁻⁶ m and 1 × 10⁻⁵ m (2500 nm to 10 µm). Often called dust.

SURFACE AREA TO VOLUME RATIO (SA:V)

The surface area divided by the volume. For a cube of side L: SA:V = 6/L. As particle size decreases, SA:V increases.

Lesson 2.09 — Recap

THE FIVE THINGS YOU MUST KNOW

Sizes: nano = 1–100 nm; fine (PM2.5) = 100–2500 nm; coarse (PM10) = 2.5 × 10⁻⁶ to 1 × 10⁻⁵ m. Memorise in standard form.

SA:V rule: for a cube, SA:V = 6/L. ÷10 in side → ×10 in SA:V.

Why nanoparticles differ: high SA:V → greater fraction of atoms at surface → different properties from the bulk material of the same mass.

Uses: medicine, electronics, cosmetics & sun creams (TiO₂, ZnO), deodorants, catalysts. Less material needed for the same effect.

Risks: long-term health and environmental effects not fully known; can be absorbed through skin or inhaled.

THE TWO PHRASES THAT WIN MARKS

"Higher surface area to volume ratio" — never just "larger surface area".

"For the same mass" or "compared to bulk / fine particles" — every nanoparticle answer must be comparative.

NEXT LESSON

Lesson 3.01 — Conservation of Mass and Balanced Chemical Equations. Topic 3 starts here.