Bulk & Surface Properties of Matter Including NanoparticlesEdexcel GCSE Chemistry: Revision notes
Section 1
How big are nanoparticles compared with atoms and molecules?
Nanoparticles are particles between 1 and 100 nanometres (nm) in diameter. This is much larger than single atoms and small molecules (which are typically around 0.1–1 nm across) but much smaller than everyday 'bulk' particles you can see (which are typically over 1000 nm, i.e. over 1 micrometre).
A nanoparticle contains a few hundred atoms — small enough that a much higher proportion of its atoms sit on the surface compared with a bulk sample of the same substance.
If asked to give the size range of nanoparticles, state both bounds precisely: 1 nm to 100 nm — a vague answer like 'very small' will not gain the mark.
Section 2
How does surface area to volume ratio explain nanoparticle properties and uses?
As particles get smaller, their surface area to volume ratio increases sharply. Nanoparticles have a very high surface area to volume ratio compared with the same mass of bulk material.
This means a much greater proportion of the particle's atoms are exposed on the surface, able to react or interact with their surroundings. This makes nanoparticles:
- More reactive/effective per gram than the bulk material
- Useful in smaller quantities to achieve the same effect
Sunscreens are a key example: nanoparticles of substances like titanium dioxide or zinc oxide are used in some sunscreens. Their high surface area to volume ratio makes them more effective at absorbing/reflecting UV light per gram than larger bulk particles, and being so small, they can also be transparent on the skin rather than leaving a visible white layer.
Titanium dioxide nanoparticles in sunscreen: high surface area to volume ratio makes them effective UV absorbers/reflectors while remaining invisible on skin, unlike larger bulk particles.
Section 3
What are the possible risks of nanoparticles?
Because nanoparticles are so small, they can behave differently in the body and environment compared with the bulk material of the same substance, raising possible risks:
- They may be able to pass through cell membranes or the skin more easily than larger particles, entering the bloodstream or body tissues
- The long-term health effects are not yet fully known/understood, as nanoparticle technology is still relatively new
- Their high reactivity (from high surface area to volume ratio) could cause unexpected reactions in living cells or the environment
Examiners expect you to be able to evaluate these risks against the benefits of a nanoparticle use when given specific data.
Do not simply state 'nanoparticles are dangerous' — the specification requires you to explain the possible risk is uncertainty/lack of long-term evidence and their ability to penetrate cells, not a definite proven harm.
Section 4
How do glass, clay ceramics, polymers, composites and metals compare as bulk materials?
Different bulk materials have different physical properties suited to different uses. Typical comparisons (using data given in a question):
| Material type | Typical properties |
|---|---|
| Glass and clay ceramics | Hard, brittle (shatter rather than bend), good electrical insulators, high melting points |
| Polymers | Low density, can be flexible or rigid, generally poor conductors, lower melting points than ceramics/metals |
| Composites | Combine properties of two or more materials (e.g. a matrix + reinforcing fibres) to give strength and low weight together |
| Metals | Malleable, good conductors of electricity and heat, generally high melting points and high density |
In an exam question you will usually be given data (e.g. density, strength, cost, melting point) and asked to select the most suitable material for a specific use, justifying your choice using that data.
When selecting a material for a use, always refer explicitly to the given data (e.g. 'because it has the highest strength and lowest density of the options') rather than general knowledge.
Must Know
- Nanoparticles are 1–100 nm in diameter — larger than atoms/small molecules, smaller than bulk material particles
- Small size gives nanoparticles a very high surface area to volume ratio, making them more reactive/effective per gram
- Sunscreens use nanoparticles (e.g. titanium dioxide) because their high surface area to volume ratio makes them effective and transparent on skin
- Possible risks: nanoparticles may pass through cell membranes/skin more easily, and their long-term health/environmental effects are not fully known
- Glass/ceramics are hard and brittle; polymers are low density; composites combine properties of materials; metals are malleable and conduct electricity
- Always justify material choice using the specific data given in the question
That's the notes covered.
Carry on to the next subtopic.